FNA Annual Report 2010 - Technische Universiteit Eindhoven

TU e 

Technische 

Universiteit 

Eindhoven 

University of Technology 

Group Physics of Nanostructures 

Department of Applied Physics 

Group Physics of Nanostructures 


TU/e 

Technische 

Universiteit 

Eindhoven 

University of Technology 

Annual Report 

2010 

Where innovation starts

Annual Report 2010 

Physics of Nanostructures group (FNA) 


Eindhoven University of Technology 

Physics of Nanostructures 

NLe 1.06 

Den Dolech 2 

P.O. Box 513 

5600 MB Eindhoven 

The Netherlands 

Tel.: 040 2475778 

Fax: 040 2475724 

E-mail: c.a.m.jansen@tue.nl 

Website: http://www.fna.phys.tue.nl

2 FNA Annual Report 2010 0.Table of Contents 

Editors: 

M. Hoeijmakers BSc. 

T. Weekenstroo BSc.

FNA Annual Report 2010 0.Table of Contents 3 

Table of Contents 

1 Introduction....................................................................................................................................... 4 

2 Acknowledgments ........................................................................................................................... 7 

3 Group Members ............................................................................................................................... 9 

3.1 List of Group Members ...................................................................................................................... 9 

3.2 Photos of Group Members ............................................................................................................... 11 

3.3 Group Photo ....................................................................................................................................... 12 

4 Equipment ....................................................................................................................................... 13 

4.1 EUFORAC deposition and in-situ analysis facility ...................................................................... 13 

4.2 Scanning Probe Microscopy and Laser laboratories .................................................................... 14 

4.3 Ex-situ structural, electrical, and magnetic characterization ...................................................... 16 

4.4 Nanostructuring ................................................................................................................................ 17 

5 Research Projects ............................................................................................................................ 19 

5.1 NanoMagnetism ................................................................................................................................ 19 

5.2 Spintronics .......................................................................................................................................... 20 

5.3 Ultrafast spin dynamics .................................................................................................................... 21 

6 Results .............................................................................................................................................. 23 

6.1 Nano Magnetism ............................................................................................................................... 24 

6.2 Spintronics .......................................................................................................................................... 34 

6.3 Ultrafast Spin Dynamics ................................................................................................................... 48 

7 Output .............................................................................................................................................. 61 

7.1 Publications ........................................................................................................................................ 61 

7.2 Presentations ...................................................................................................................................... 62 

7.3 Chapters .............................................................................................................................................. 65 

7.4 Guest Lectures ................................................................................................................................... 66 

7.5 Posters ................................................................................................................................................. 66 

7.6 PhD Theses ......................................................................................................................................... 67 

7.7 Master Theses..................................................................................................................................... 67 

7.8 Internship Reports ............................................................................................................................. 68 

7.9 Publicity .............................................................................................................................................. 69 

8 Social Events ................................................................................................................................... 75

4 FNA Annual Report 2010 0.Table of Contents

FNA Annual Report 2010 1.Introduction 5 

1 Introduction 

Dear colleagues and friends of FNA, 

This Annual report of our group Physics of Nanostructures (FNA) at the Eindhoven University of Technology 

covers facts & figures as well as highlights of the year 2010. In particular it provides a review over research 

progress in the fields we are active in, viz. nanomagnetism, spin-polarized transport and ultrafast magnetic 

processes. It has become a tradition that two of our master students take the lead in the editorial process – while 

all group members contribute to the contents. With the final results we hope to present you an overview of not 

only our scientific achievements, but also providing you a glimpse of the atmosphere in our group. 

Within the department of Applied Physics, the past year was a special one in which we celebrated our 50 th 

anniversary. Many activities were organized, such as a grand party for all members and students of the 

department, and an ‘open day’ to which many of our group members contributed. It attracted a large number of 

visitors from all ages to our department. Nano-quizzes in which enthusiastic kids were confronted with the 

amazing world of nanotechnology, demos of our electromagnetic launching-gear (‘coil-gun’) – shooting quite a 

few trays with high precision, and a nano-slide show accompanied by a 3 byte-harddisk demo where young 

kids took care of ultimate spinning speeds. Finally, the main star was our EUFORAC (Eindhoven University 

nano-Film depOsition Research and Analysis System). Lighted by disco lights (see cover) the audience was 

enjoying explanations about atom-by-atom deposition and its applications in spintronics. 

The year 2010 was also a year of several lively international meetings organized by the group. End of June we 

hosted the joint Dutch-Korean meeting on Spintronics. The lectures in the Guus Hiddink hall, and a trip to the 

catacombs of the PSV-stadium (where quite a few Korean players were starring at the exhibitions), may have 

been particularly memorable for our guests. Early September, we organized (co-chaired by Peter Bobbert from 

our partner group at the department) the 3 rd International Conference on Spins in Organic Semiconductors in 

the historical Trippenhuis along the canals in downtown Amsterdam. The conference marked the end of the 

first phase of our Vici-program on organic spintronics. It attracted a record number of participants from all over 

the world, and illustrated the enormous development in the new field of research – combining issues in 

spintronics and organic electronics. 

As to scientific progress, the year 2010 showed new exciting results in fields of research we are traditionally 

strong in over the past years, but also important breakthroughs in new areas. Here, in particular, I want to 

mention our work in domain wall motion, and first successes in writing truly ferromagnetic nanostructures 

using our focused electron-beam induced nano-pencil. Thus, students produced our group’s logo with a 300 nm 

wide font of free standing ferromagnetic letters (see cover); results obtained in intense collaboration with FEI 

company. Among other new research programs, the national FOM program on ‘Controlling Spin Dynamics in 

Magnetic Nanostructures’ got a real start, our collaboration with Hitachi Global Storage Technologies (San Jose, 

CA) got formalized in the framework of a joint research effort, and our collaboration with our TU/e 

distinguished professor Stuart Parkin (IBM Almaden Research Center at San Jose, CA) will gain momentum 

again with a new PhD project. Finally, after several years of preparation, we’re particularly pleased about 

having obtained new funding opportunities for nano-research at TU/e within the Dutch NanoLab, and to 

welcome the final start-up of the new Dutch nanotechnology research program NanoNextNL. The latter is the 

successor of the NanoNed program, which may be familiar to some of you. 

Looking back the year, we saw a lot of new PhD/master/bachelor students coming and leaving the group, each 

of them leaving social and scientific footprints. In particular we had three PhD students getting their doctoral 

degree, Wiebe Wagemans, Francisco Bloom, and Reinoud Lavrijsen (early 2011) – each of them producing really 

high quality PhD theses and impressive publications scores. Without having the opportunity mentioning all of 

the other students, researchers, non-scientific staff members, as well as external collaborators, I do want to 

acknowledge their important contributions to our successes.

6 FNA Annual Report 2010 1.Introduction 

Finally, the news of 2010/2011 that is going to have the most significant impact on the future of the group 

regards our housing. Early 2011, the final decision was announced that our department will move to an entirely 

new building that will be constructed in the framework of the ‘Campus 2020’ project. Although beyond doubt it 

is something that is going to ask a lot of efforts from all our group members, it will also provide us with unique 

opportunities. While you are reading this Annual report, we are shaping our group’s future infrastructure. We 

are looking forward continuing our stimulating interaction with you also within the future setting. 

Best regards, 

Bert Koopmans

FNA Annual Report 2010 2.Acknowledgments 7 

2 Acknowledgments 

Department of Applied Physics of TU/e (Peter Bobbert, René Janssen, Martijn Kemerink, Paul 

Koenraad, Ton van Leeuwen, Thijs Meijer, Leo van IJzendoorn, Menno Prins, Peter Nouwens) 

Department of Chemical engineering and Chemistry of TU/e (Martijn Wienk) 

Department of Electrical engineering of TU/e (Siang Oei, Erik Jan van Geluk, Barry Smalbrugge, Meint 

Smit, Tjibbe de Vries) 

AGH Krakow (Maciej Czapkiewicz, Tomasz Stobiecki) 

CEMES-CNRS Toulouse (Etienne Snoeck) 

CNRS Paris (Gregory Malinowski) 

FEI Company (Hans Mulders, Piet Trompenaars) 

FIAT Research (Daniele Pullini) 

Hitachi Global Storage Technologies, San Jose (Paul van der Heijden, Jeff Childress, Young-Suk Choi) 

Holst Centre (Herman Schoo, Jo De Boeck) 

IBM Almaden Research Center (Stuart Parkin, Luc Thomas) 

IFW Dresden (Sabine Würmehl) 

Indian Institute of Science Bangalore, India (Anil Kumar) 

Inha University (Chun-Yeol You) 

IMEC at Leuven (Wim Van Roy, Liesbet Lagae, Jan Genoe, Koen Weerts) 

ISMN-CNR Bologna (Alek Dediu) 

Kavli Institute of NanoScience at Delft (Emile van der Drift, Arnold van Run, Anja van Langen) 

Korea University (Kungwon Rhie) 

Kyoto University (Teruo Ono) 

Leeds University (Chris Marrows) 

LLG Micromagnetics Simulator (Mike Scheinfein) 

Max-Planck-Institut für metallforschung (Manfred Fähnle, Daniel Steiauf) 

MIT Cambridge (Jagadeesh Moodera) 

MESA+, University of Twente (Ron Jansen, Vishwas Gadgil) 

NXP (Friso Jedema, Michiel van Duuren, Fred Roozeboom) 

Omicron NanoTechnology GmbH (Marcus Maier, Dieter Pohlenz, Joerg Seifritz) 

Philips Research (Reinder Coehoorn, Hans Boeve, Menno Prins, Dago De Leeuw, Denis Markov) 

Polish Academy of Sciences at Warsaw (Tomasz Story, Grzegorz Karczewski) 

Radboud University at Nijmegen (Rob de Groot, Gilles de Wijs, Jisk Attema, Theo Rasing, Andrei 

Kirilyuk, Alexey Kimel) 

Royal Institute of Technology, Stockholm (Stefano Bonetti, Johan Åkerman) 

Russian Academy of Science, St. Petersburg (Yurii Vladimirovich Trushin) 

SmartTip Probe Solutions (Roeland Huijink, Daan Bijl) 

Sogang University (Myung-Hwa Jung) 

SPECS Nanotechnology (Ad Ettema) 

Tecnische Universität Dresden (Karl Leo) 

TNO Science and Industry, Delft (Emile van Veldhoven, Diederik Maas) 

University of Alabama (Patrick LeClair, Tim Mewes) 

University of Antwerp (Etienne Goovaerts, Hans Moons) 

University of Goettingen (Markus Muenzenberger) 

University of Iowa (Markus Wohlgenannt) 

University of Kaiserslautern (Burkard Hillebrands, Martin Aeschlimann, Mirko Cinchetti, Tobias Röth) 

University of Konstanz (Mathias Kläui, Philipp Möhrke, Ulrich Rüdiger) 

University of Leeds (Serban Lepadatu, Chris Marrows, Brian Hickey) 

University of Mainz (Claudia Felser, Gerhard Fecher, Benjamin Balke, Christian Blum) 

University of Zaragoza (Rosa Córdoba Castillo, José Maria De Teresa, Manuel Ricardo Ibarra García, 

Frank Schoenaker) 

Uppsala Unitversity (Björgvin Hjörvarsson) 

Utrecht University (Rembert Duine, Erik van der Bijl) 

Yacht (Veronica den Bekker-Tiba)

8 FNA Annual Report 2010 2.Acknowledgments

FNA Annual Report 2010 3.Group Members 9 

3 Group Members 

3.1 List of Group Members 

Scientific Staff 

Prof. dr. B. (Bert) Koopmans (group leader) 

Prof. dr. ir. H.J.M. (Henk) Swagten 

Dr. J.T. (Jürgen) Kohlhepp 

Dr. O. (Oleg) Kurnosikov 

Prof. dr. W.J.M. (Wim) de Jonge (emeritus) 

Guests 

Prof. S.S.P. (Stuart) Parkin (Distinguished professor at TU/e) 

Technical Staff and Operators 

G.W.M. (Gerrie) Baselmans 

Ir. J. (Jeroen) Francke 

Ing. J.J.P.A.W. (Jef) Noijen 

Dr. B. (Beatriz) Barcones Campo 

Secretary 

C.A.M. (Karin) Jansen 

Post-Doctoral Staff 

Dr. E. (Elena) Murè since 01-06-‘10 

Dr. D. (Daowei) Wang since 01-09-‘10 

PhD Students 

Ir. W. (Wiebe) Wagemans until 14-06-‘10 

F. (Francisco) Bloom MSc. until 22-11-‘10 

Ir. P. (Paul) Janssen 

Ir. K.C. (Koen) Kuiper 

Ir. R. (Reinoud) Lavrijsen 

A.J. (Sjors) Schellekens MSc. since 01-03-‘10 

J.H. (Jeroen) Franken MSc. since 15-03-‘10 

Master Students 

A.J. (Sjors) Schellekens until 11-02-‘10 

J.H. (Jeroen) Franken until 12-02-‘10 

F.J. (Frank) Schoenaker until 17-02-‘10 

C.O. (Can) Avci until 05-08-‘10 

M.J.M. (Mathijs) van Schijndel until 18-10-‘10 

R. (Rik) Paesen until 19-10-‘10 

P.E.D. (Paul) Soto Rodriguez MSc. until 03-11-‘10 

G.C.F.L. (Geerit) Kruis until 16-12-‘10 

T.H. (Tim) Ellis 

N. (Niels) de Vreede 

M. (Matthijs) Cox since 01-02-‘10 

S. (Sükrü) Hasdemir since 04-02-‘10 

M. (Mark) Hoeijmakers since 15-06-‘10 

T. (Tim) Weekenstroo since 01-09-‘10

10 FNA Annual Report 2010 3.Group Members 

Bachelor Students 

M. (Mark) Herps until 26-01-‘10 

F.H.A. (Frank) Elich from 22-04-’10 until 11-10-‘10 

W. (Wouter) Verhoeven from 11-05-‘10 until 07-12-‘10 

Ing. C. (Christiaan) Otten since 07-09-‘10 

J. (Jeroen) de Groot since 11-10-‘10

FNA Annual Report 2010 3.Group Members 11 

3.2 Photos of Group Members 

Beatriz Barcones Campo Gerrie Baselmans Francisco Bloom Jeroen Francke 

Jeroen Franken Karin Jansen Paul Janssen Wim de Jonge 

Jürgen Kohlhepp Bert Koopmans Koen Kuiper Oleg Kurnosikov 

Reinoud Lavrijsen Elena Murè Jef Noijen Henk Swagten

12 FNA Annual Report 2010 3.Group Members 

Sjors Schellekens Wiebe Wagemans Daowei Wang 

3.3 Group Photo

FNA Annual Report 2010 4.Equipment 13 

4 Equipment 

Research in the group Physics of Nanostructures is aimed at the engineering and investigation of functional 

nanostructures. Current emphasis is on structures and devices with application potential in the field of 

spintronics, (magnetic) data storage, and sensors. A state-of-the-art infrastructure for preparation and 

manipulation, as well as in-situ and ex-situ characterization of nanostructures, is available. 

4.1 EUFORAC deposition and in-situ analysis facility 

The group Physics of Nanostructures is equipped with a state-of-the-art deposition and analysis facility 

EUFORAC, the Eindhoven University nano-Film depOsition Research and Analysis Center. 

In EUFORAC a complementary cluster of ultra-high vacuum (UHV) deposition and analysis facilities are 

present, and exists of: 

 

 

 

 

 

 

 

MEMULA: Vacuum Generators V80 M MBE: The MEMULA (MEtallic MUltiLAyers) is a general purpose 

MBE for deposition of (magnetic) metallic multilayer systems. It features: a base pressure below 10 -11 mbar; 

7 deposition sources (4 Knudsen cells, 3 e-guns); and wedge growth and shadow mask evaporation 

(roughly < 50 micron resolution) 

CARUSO: Kurt J. Lesker UHV sputter facility: CARUSO, the Chamber for ARtificial Ultra-high vacuum 

Sputtered nanOstructures, is a dedicated, computer-controlled sputter coater manufactured at Kurt J. 

Lesker Co. Vacuum Products, and is connected to the Vacuum Generators V80 M MBE (MEMULA) as 

shown on the left side of the picture. The system is configured for sputter-down deposition, using oil-free 

diaphragm, molecular drag, and cryopumps. 

XPS, AES, RHEED, LEED: UHV chambers for in-situ analysis of the deposited layers with XPS, AES, 

RHEED, and LEED. 

Omicron-1 STM: A chamber for room-temperature scanning tunneling microscopy using the standard 

Omicron-1 system. 

Plasma oxidation: A chamber for DC/RF glow discharge oxidation of metal layers (such as Al), in 

particular to serve as a barrier in magnetic tunnel junctions. The system includes in-situ differential 

ellipsometry and an automized dosage system for reproducible oxidation of ultrathin films. 

Organic layer deposition: A UHV chamber for deposition and optical thickness control of hybrid 

nanostructured systems of metallic and organic materials. 

Glovebox: Facility for measuring (organic) samples in an oxygen and water free environment with options 

to measure with a modulated magnetic field and with temperatures down to 10 K.

14 FNA Annual Report 2010 4.Equipment 

4.2 Scanning Probe Microscopy and Laser laboratories 

A series of inter-communicating labs for low-temperature STM and UHV deposition, basic AFM and MFM, 

femtosecond pulsed laser set-ups covering a broad spectral range, and a variety of magneto-optical and other 

optical characterization techniques, including SNOM. 

Multiprobe LT STM surface analysis UHV system 

The UHV system is used for sample preparing, thin film deposition and in-situ analysis in low temperature 

STM. The LT-STM is also used for a magnetic characterization using spin-polarized tunneling as well as for 

single atom manipulation. It consists of: 

 

 

 

Omicron LT-STM: With a basic pressure of 5.10 -11 mbar and can reach temperatures down to 4.5 K, with a 

magnetic field at the sample position up to 200 Oe. Also, there is optical access to the sample. 

Omicron MBE chamber: Consisting of three e-beam evaporation cells for MBE deposition, LEED, and a 

manipulator with sample heating-cooling facilities. Options to equip the chamber with AES/XPS, RHEED, 

and MOKE are being investigated. 

Preparation chamber: The chamber consists of a tip preparation tool, an ion sputtering gun, and a 

manipulator with sample heating facility (up to 1200 K). In future, the chamber will be equipped with 

sputtering deposition sources and k-cells. 

TSUNAMI ultra-short pulse lab 

High power CW Spectra Physics Millennia V pump laser for pumping the: 

Spectra Physics TSUNAMI fs/ps Ti:Sapphire laser: Mode-locked Ti:Saphire pulsed laser (80 MHz, sub 50 

fs, and 700 - 850 nm optics set), with picosecond option. 

AvTech electrical pulse generator: Generator for pulses with risetime < 100 ps and repetition rate up to 80 

MHz. 

 

Set-ups for TR-MOKE: A controllable time delay between the pump (current/field) pulse and the probing 

laser pulse gives us a time-resolved measurement of the polarization change of the incident laser beam due 

to the change of magnetization of the reflecting ferromagnetic structure. The TR-MOKE consists of a 2x300 

mm mechanical delay line (delay up to 3 - 6 ns); 50 kHz Photo-elastic modulator; and double-modulation 

configurations for < 10 -7 rad polarization sensitivity.


TSUNAMI - OPAL lab 

High power (10 W) CW Spectra Physics Millennia X pump laser for pumping the: 

 

 

 

 

Spectra Physics TSUNAMI fs Ti:Sapphire laser: 700-1000 nm optics set and 70 fs pulse duration; lock-toclock 

option for active synchronization with OPAL or second TSUNAMI in Ne 0.05. 

Spectra Physics OPAL laser: Optical Parametic Oscillator with 1050 -1350 nm signal optics set. 

Spectra Physics Frequency Doubler: Extending the wavelength range to 350 - 1350 nm. 

TiMMS set-up: For measuring picosecond spin-dynamics in semiconductors, with a 100 mm mechanical 

delay line and a 50 kHz photo-elastic modulator. Experiments can be performed at 5 K when performed in 

Ne 0.09. 

MOKE lab 

 

 

 

 

Homemade RT MOKE magnetometer: 

For measuring the Magneto Optical Kerr 

Effect of thin magnetic films at room 

temperature. With the use of a microscope 

objective the measurement of magnetic 

structures less than 1μm in size is possible. 

Photo- and electroluminescence: 

Oriel spectrometer with cooled Andor 

CCD camera (- 70 o C). 

Oxford flow cryostat for variable 

temperature MOKE and TiMMS: 

Cryostat for performing experiments at 

temperatures down to 5 K. 

Evico Magnetics wide-field Magneto- 

Optical Kerr-Microscope: 

Room-temperature visualization of 

magnetic domains down to the resolution 

limit of optical microscopy. 

MFM/AFM 

Variable temperature for measuring the 

temperature dependence of magnetic domains. 

Magnetic field for applying an external 

magnetic field during MFM measurements. 

Wide-field Magneto-Optical Kerr-Microscope 

.


4.3 Ex-situ structural, electrical, and magnetic characterization 

Stand-alone equipment for ex-situ characterization, which includes X-ray diffraction, various cryostats (He3, 

He4) for electrical conductance and magnetoresistance measurements, SQUID and MOKE magnetometry, NMR, 

and Mössbauer spectroscopy. 

Sorption-pumped He-cryostat 

Oxford Heliox VL for current-voltage measurements at 0.3 K in fields up to 8 T. 

NMR and SQUID laboratory 

 

 

SQUID: Superconducting Quantum Interference Device (Quantum Design), to measure the magnetic 

moment of a sample for temperatures between 5 and 400 K at fields up to 5 T. 

NMR: The group FNA runs a sensitive, home-built Nuclear Magnetic Resonance (NMR) apparatus, 

dedicated to research the structural properties of ferromagnetic materials (especially cobalt). The set-up is 

phase-coherent, frequency tuned from 100-400 MHz, uses fields of 0-5.5 T, can measure at temperatures 

down to 2 K, and has a sensitivity of better than 0.1 monolayers of Co. Setup is comprised of two cryostats; 

one high magnetic field bath cryostat and one variable temperature flow cryostat. 

Magnetoresistance laboratory 

MR-setup: Home-built magnetoresistance setup operating at temperatures down to 2 K, fields up to 1.2 T, 

and suitable to measure a wide range of input impedances (up to 10 giga-ohms). 

 

Probe station: Easy access setup to perform room-temperature magnetoresistance measurements on nonstandard 

sample geometries. Adaptable to perform Current-In-Plane Tunneling (CIPT) experiments for 

quick characterization of (new) materials for MTJs. 

X-Ray laboratory 

The X-Ray Diffraction (XRD) setup is a commercial Philips X'Pert system, using Bragg diffraction of Cu K-alpha 

radiation for determination of the lattice parameters and texture of thin films. It is also used for calibrating film 

thicknesses by reflection of X-rays coming in at very low angles to the sample surface. 

ESR, Domain wall motion setup, and "mini-MR/MOKE" laboratory 

 

 

 

ESR: Variable temperature Electron Spin Resonance is used to investigate a variety of magnetic materials, 

such as thin film magnetic layers and magnetic insulators, focusing mostly on magnetic anisotropy. 

Domain wall motion setup: Variable temperature (2 – 400 K) and high frequency (up to 6 GHz) magneto 

resistance (MR) measurement setup equipped with an electromagnet capable of applying fields up to 1.2 T 

at a variable angle relative to the sample plane. The setup uses chip carriers which are wire bonded to 

structured samples. The setup is used to measure domain wall velocities in perpendicular magnetized 

samples and to perform MR measurements in structured samples. 

Mini-MR/MOKE: Simple fast-entry setup to quickly determine the resistance of a magnetic thin film 

device as a function of magnetic field at room temperature. This setup has been upgraded to measure the 

Magneto Optical Kerr effect of thin magnetic films at room temperature.


4.4 Nanostructuring 

The nanostructuring facilities consist of a dualbeam system (FEI Nova NanoLab 600i) with focused ion and 

electron beams and an Ion Beam Miller (Unilab IonSys 500). Both devices belong to the NanLab network. The 

group also has access to the Spectrum Cleanroom where other fabrication techniques are available. 

See http://web.phys.tue.nl/en/the_department/department_staff/clean_room/ for more information. 

FEI Nova 600i 

Inside the vacuum chamber of this system, characterization, fabrication and manipulation can be performed in 

the nanoscale regime. Among other typical experiments, the machine can perform x-sections for direct 

inspection or X-ray spectroscopy analysis, TEM lamella fabrication, circuit modification and mask repair. 

 

 

 

 

 

 

 

Scanning Electron Microscope: Field emission gun filament, resolution of 1.1 nm at 15 kV, accelerating 

voltage range 200V-30 kV, and maximum current 20 nA. The electron source can be either used for imaging 

or for deposition of different precursors (EBID). 

Focused Ion Beam: Ga liquid ion source, resolution of 7 nm, accelerating voltage range 2kV-30kV and 

maximum current 20 nA. The ion source is used for imaging, patterning and deposition of different 

precursors (IBID). 

Electron Dispersive X-Ray detector: Si(Li) type detector with SUTW. Energy resolution of 136 eV capable 

of detection of every element down to and including Be. Combined with the scanning unit, can be used not 

only to acquire single spectra but also to produce elemental mappings. 

Gas Injection Systems: Different gases can be introduced in the vacuum chamber for the purpose of 

deposition or to help ion sputtering. At the moment only deposition gases are installed in the system, 

mostly metal based for low resistivity pad fabrication. 

MeCpPt(IV)Me3 - Pt deposition 

W(CO6) - W deposition 

TEOS - insulator deposition for high resistivity pads and lines, device edit and electrical isolation 

C10H8 - C deposition for protective layers and large area coating 

Fe2(CO)9 - Fe deposition for magnetic device fabrication 

Kleindieck Nanomanipulators: Two small motors can be installed inside the vacuum chamber allowing 

small elements to be manipulated and insitu electrical measurements to be performed. 

Electron Beam Lithography: A RAITH system coupled to the dualbeam permits the use of the scanning 

electron microscope as an electron beam lithography tool. This system adds to the dual beam extra tools 

like a CAD software editor and alignment software which permits the writing of complicated features and 

the possibility to fabricate designs with multi lithographic steps. The added system can work either with 

the electron or the ion beam. 

Detectors: The interaction of the electron and ion beams with matter produce different products, in the 

system both secondary and backscattered electrons can be collected to produce an image. Furthermore, 

secondary ions can be directly imaged with a CDEM detector. It is also possible to study TEM lamellas with 

an STEM detector, producing Bright Field, Dark Field and High Angle Dark Field images.


Unilab IonSys 500 

With this system it is possible to etch by Ar ion milling or deposit SiO2 layers, transferring patterns previously 

defined with a mask by Electron Beam or UV Lithography. 

 

 

 

 

Ion beam source: For ion beam milling using inert gases. Maximum RF power up to 300 W. Homogeneity 

of collimated ion beam +/-10 % over 30 mm diameter. 

Magnetron sputter source: For SiO2 deposition. 

Cryo thermostat: Temperature range -25

FNA Annual Report 2010 5.Research Projects 19 

5 Research Projects 

The research at FNA can be categorized into three main fields: Nanomagnetism, Spintronics, and Ultrafast Spin 

Dynamics. Within each of these areas a number of projects are active, supported by a wide range of 

organizations. In this chapter a brief overview of the projects is listed. Some highlights of recent results can be 

found in Chapter 6. 

5.1 NanoMagnetism 

‚Focused electron-beam induced deposition of Fe‛ (Subproject under EMM.6474) 

TU/e research priorities 

PhD student: R. Lavrijsen EBID of ferromagnetic nanostructures 

‚Sensing and switching of 'hidden' nanomagnets‛ (09PR2715) 

FOM projectruimte 2009 

Staff member: O. Kurnosikov Nanomagnetism & STM 

PhD student: 

Vacancy 

‚Nanowire manipulation for sensing and spintronics‛ 

Nanonext NL 2010, Program advanced nano-electonic devices 

PhD student: Vacancy Collaboration with Holst Centre 

Student projects 

MSc projects: C.O. Avci Near surface quantum wells induced by buried nanoparticles in 

metals 

T.H. Ellis 

F.J. Schoenaker 

Electron Beam Induced Deposition of Iron (collaboration with FEI) 

Exploring the fabrication of ferromagnetic nanostructures by EBID 

(collaboration with FEI) 

T. Weekenstro NMR on Heusler alloys (collaboration with Hitachi: Global data 

storage)

20 FNA Annual Report 2010 5.Research Projects 

5.2 Spintronics 

‚Engineering of nanostructured magnetic multilayers for generic MR devices‛ (EMM.6474) 

Flagship NanoSpintronics with NanoNed / NanoImpulse 

PhD student: R. Lavrijsen Co/Pt based structures for Spin-Transfer Torque devices 

‚Engineering and integration of electrical domain wall memory devices‛ 

Nanonext NL 2010, Program advanced nano-electronic devices 

PhD. student: Vacancy Collaboration with IMEC Leuven 

‚Spin engineering in molecular devices‛ (ETF.6628) 

NWO-Vici 2005 (Koopmans) 

PhD students: F. Bloom Investigating organic magnetoresistance 

W. Wagemans Organic spintronics 

P. Janssen Novel device options in organic spintronics 

‘Chasing the spin in organic spintronics’ 

NWO - Nano 2011 

PhD student: Matthijs Cox Chasing spin in organic spintronics (starting April 2011) 

‚Novel methods to drive domain walls‛ (08SPIN10) 

FOM Program 109 - Controlling spin dynamics in magnetic nanostructures 

PhD student: J. Franken Novel methods to drive domain walls 

Student projects: 

MSc projects: A.J. Schellekens Exploring spin interactions in organic semiconductors 

M.J.M. van Schijndel Modelling spin transport through organic layers 

J. Franken Domain wall motion in perpendicularly magnetized ultrathin 

Pt/CoFeB/Pt films 

G.C.F.L. Kruis 

LLG simulations of racing domain walls 

N. de Vreede Spin torque oscillators 

P.E.D. Soto Rodriguez Nano-stencil devices for spin-transfer torque switching 

M. Cox Tuning spin interactions in organic semiconductors 

M. Hoeijmakers Domain wall resistance in perpendicular magnetized materials 

BSc projects: C. Otten The flip-chip: A novel way of fabricating layered organic 

devices 

W. Verhoeven Frequency dependence of organic magnetoresistance

FNA Annual Report 2010 5.Research Projects 21 

5.3 Ultrafast spin dynamics 

‚Femtosecond spin transfer‛ (08PR2654) 

FOM projectruimte 2008 

Postdoc: D.W. Wang Ultrafast magneto-optics and spin transfer 

PhD student: K.C. Kuiper Ultrafast magneto-optics and spin transfer 

‚Ultrafast spin dynamics‛ (08SPIN06) 

FOM Program 109 - Controlling spin dynamics in magnetic nanostructures 

PhD student: A.J. Schellekens Ultrafast spin-transfer torque dynamics in nanomagnets 

‚Dynamics of magnetic domain walls‛ 

Funded by EPFL Lausanne 

Postdoc: E. Murè Ultrafast domain wall dynamics 

‚Magnetic logic devices‛ 

External project at IBM Almaden Research Center, San Jose (CA) (supervisor Prof. Stuart Parkin) 

PhD student: R. van Mourik Magnetic logic (Starting march 2011) 

Student projects: 

MSc projects: R. Paesen Ultrafast domain wall dynamics 

BSc projects: M. Herps Gilbert damping in Ga irradiated Pt/CoFeB/Pt 

F. Elich Gilbert damping in Pt/Co/ 

J. de Groot Ultrafast magnetization dynamics in Pt/Co/Pt/Co/Pt multi-layers

22 FNA Annual Report 2010 5.Research Projects

FNA Annual Report 2010 6.Results 23 

6 Results 

6.1 Nano Magnetism ............................................................................................................................ 24 

6.1.1 Diffraction of internal electrons from an atomically-ordered nano-interface in Cu(110) 24 

6.1.2 Reduced DW pinning in patterned strips of ................................................. 26 

6.1.3 Controlling domain walls by anisotropy engineering using focused He and Ga beams . 28 

6.1.4 Focused electron beam induced deposition of Fe – domain wall pinning ......................... 30 

6.1.5 Local formation of a Heusler type structure in CoFe-Al current perpendicular to the 

plane GMR spin-valves ............................................................................................................. 32 

6.2 Spintronics....................................................................................................................................... 34 

6.2.1 Tunable Rashba effect: spin-orbit-torque-assisted field-driven domain wall creep ......... 34 

6.2.2 Spin–spin interactions in organic magnetoresistance probed by angle-dependent 

measurements ............................................................................................................................. 36 

6.2.3 Frequency dependence of organic magnetoresistance .......................................................... 38 

6.2.4 Exploring Organic Magnetoresistance: An investigation of microscopic and device 

properties – PhD Thesis ............................................................................................................. 40 

6.2.5 Plastic Spintronics – PhD Thesis .............................................................................................. 42 

6.2.6 Microscopic modeling of spin-dependent interactions in organic semiconductors ......... 44 

6.2.7 Tuning Spin Interactions in Organic Semiconductors .......................................................... 46 

6.3 Ultrafast Spin Dynamics ............................................................................................................... 48 

6.3.1 Theory of femtosecond laser-induced magnetization dynamics ......................................... 48 

6.3.2 Magnetism and dynamics of Pt / Co / for domain wall devices ................................ 50 

6.3.3 Experiments and simulations on femtosecond laser-induced magnetization dynamics . 52 

6.3.4 Towards ultrafast studies of current induced domain wall motion ................................... 54 

6.3.5 Resolving the genuine laser-induced ultrafast dynamics of exchange interaction in 

ferromagnet/antiferromagnet bilayers .................................................................................... 56 

6.3.6 Magnetization dynamics in racetrack memory ...................................................................... 58

24 FNA Annual Report 2010 6.Results 

6.1 Nano Magnetism 

6.1.1 Diffraction of internal electrons from an atomically-ordered nano-interface in Cu(110) 

O. Kurnosikov, H.J.M.Swagten, B. Koopmans and C.O. Avci 

General Introduction 

Recently, it has been shown that a bulk subsurface impurity can induce spatial oscillations of the 3-D electron 

local density of states (LDOS), as observed at a surface by scanning tunneling microscopy/spectroscopy (Fig. 1). 

In an analogy with an advanced 2-D system such as a quantum corral, it would be also expected that a 

structured ensemble of bulk scattering centers may enhance the LDOS oscillations in some locations or in some 

directions. A simplest ensemble of scattering centers forming buried nanocluscters can provide an atomically 

ordered interface which can induce a specific scattering of electron waves. Although, mainly a flat interface 

parallel to a surface is expected to contribute in the variation of LDOS at the surface, the electron wave 

scattering from the atomic structure at the inclined interface may also induce extra LDOS variation if the 

condition for electron diffraction is fulfilled. Even if the structured interface is buried several nanometers below 

the surface, the diffraction effect can lead to the observation of resonances similar to the resonances induced by 

reflections from a flat parallel nanofacet (Fig. 2). Such a possibility can be realized for a metallic system 

displaying particular bulk electronic properties and a specific interface structure. 

Results 2010 

We investigated the electron scattering from faceted Ar-filled nanocavities buried several nanometers below a 

Cu(110) surface. In this system we observed simultaneously two kinds of resonances: (i) QW resonances formed 

by the reflection of electrons from the upper parallel flat facet of the nanocavity, which is consistent with 

previous observations, and (ii) initially unexpected resonances originating from electron diffraction from the 

interface at the sides of the nanocavity. This inclined interface is intrinsically nanostructured by atomic chains 

(Fig. 3), inducing diffraction of electrons back to the probing point. The intensity and sharpness of the effect are 

greatly enhanced by exploiting the phenomenon of electron focusing, which in copper is very efficient along the 

direction. 

We developed a model including a realistic band structure of the Cu substrate and describing the process of 

internal electron diffraction from the faceted and atomically ordered nanocavities. Our simulations qualitatively 

reproduce all observed features, and elucidate the role of the diffraction process (Fig. 4). 

Output 

Internal electron diffraction from subsurface atomically-ordered nanostructures in metals 

O. Kurnosikov, H.J.M.Swagten, B. Koopmans 

Physical Review Letters 

(submitted 7 May 2010, under consideration)


Fig. 1 (a) scheme of experiment; (b) differential 

conductance map (46.5 × 42 nm 2 ) measured at 400 mV 

and showing many spots of deviating conductance 

across the surface induced by subsurface nanocavities; 

(c) schematic drawing of the group of the spots 

appearing together in (b). 

Fig. 2 (a) A typical example of the central and satellite spots in the SDC map of 20×20 nm 2 . The color of the satellite 

spots can be different from spot to spot and varies with the bias voltage; (b) the side view of the facetted 

nanocavity. The (110) facet (encircled) parallel to the surface induces the central spot. Other locations inducing the 

satellite spots are also encircled; (c) Normalized plots of differential conductance measured in the marked points of 

(a). Curves 2-4 are shifted for better visibility. 

Fig. 3 (a-c) Top parts: different shapes of a subsurface nanocavity, represented by the first atomic layer of Cu at 

the interface, top view; bottom parts: corresponding SDC maps (20×20 nm 2 ) simulated with the model. The 

dashed encircling in (b) indicates the specific location of the atomic arrangement inducing the corresponding 

satellite spot; (d-f) Top parts: (e) is a side view of the diffracting atomic structure corresponding to the encircled 

location in (b) and (d,f) are the same for slightly different shapes of the nanocavity. In the bottom: simulation of a 

satellite spot (one from the four) induced by the corresponding structure in the top, 4.5×4.5 nm 2 . 

Fig. 4 (a) Simulated SDC map (20×20 nm 2 ) 

induced by a nanocavity with a slight shape 

asymmetry and distorted from one side; (b) 

Simulated normalized differential 

conductance plots in the locations encircled 

in (a). The curves are shifted for better 

visibility.


6.1.2 Reduced DW pinning in patterned strips of 

R. Lavrijsen, M.A. Verheijen, B. Barcones, J.T. Kohlhepp, H.J.M. Swagten and B. Koopmans 


Magnetic domain walls (DW) in magnetic nanowires have attracted much attention due to their application in 

field- and current-induced DW logic and magnetic memory devices. More recently, the attention is shifting to 

materials with high perpendicular magnetic anisotropy (PMA) resulting in an out-of-plane easy-axis. This high 

PMA results in narrow, robust and simple Bloch DWs for which current-induced DW motion/depinning is 

predicted to be efficient and also reported to show pure current-induced DW motion and depinning. 

Fundamentally, these systems are very interesting since the recently observed Rashba effect can lead to large 

spin-orbit torques of the current. Furthermore, the first demonstrations of shift registers have appeared showing 

the prospect for devices based on high PMA materials. 


The robust, narrow and simple Bloch DWs, however, lead to a strong pinning strength of the DWs on structural 

imperfections (pattering induced and/or defects). This has lead to many studies concentrating on the depinning 

of a DW from a certain pinning site and the so-called creep motion of DWs at low drive fields or currents. A 

major challenge to use the ultrathin PMA films for applications therefore lies in the control of the intrinsic and 

extrinsic pinning site density and/or strength for DW's. In the past we have shown that by doping cobalt with 

boron a significantly decrease in DW pinning strength was obtained in homogenous films (non-patterned). In 

our latest work we have studied the DW velocity by a full electrical-transport measurement technique in 

patterned 900 nm wide strips of 

or 

where FM stands for ferromagnetic Co 

. In Fig. 1 we show a SEM image of the used devices including the measurement setup. 

In Fig. 2 we show the measured DW velocity as function of drive field in a pure Co and CoB film. As predicted, 

we observe that the decreased DW pinning strength as was observed in the homogenous films greatly increases 

the DW creep velocity in the patterned strips. This proved that the CoB films are excellent candidates for DW 

motion devices. 

Output 

Reduced DW pinning in patterned strips of Pt/Co68B32/Pt 

R. Lavrijsen, M.A. Verheijen, B. Barcones, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans 

Appl. Phys. Lett. (accepted 2011)


4 

2 2 

1 

AC/DC Current Source 

high 

low 

ref out 

A 

B 

A 

B 

Lock-In #1 

ref in 

Lock-In #2 

ref in 

out 

out 

20 µm 

3 

A 

B 

Lock-In #3 

ref in 

out 

Out Sync. 

Pulse generator 

Ch. 1 

Oscilloscope 

Ch. 2 

Trig. 

Ch. 3 

Fig. 1 SEM image of used devices and electrical connections. The 900 nm wide magnetic strip (1) is connected (2) 

to a AC/DC current source. The pulse line (4) is connected to the output of a pulse generator on one side and 

grounded on the other. The three Hall probes (3) are differentially connected to individual lock-in amplifiers 

that lock in to the reference frequency of the AC current source. The outputs of the lock-ins are connected to a 

oscilloscope where the data acquisition is triggered by the pulse generator. 

Fig. 2 (a) Average DW velocity 

versus applied field for patterned w = 

900 nm wide strips of Pt(4 

nm)/FM(0.6 nm)/Pt(2 nm) with 

. (b) ln(v) versus 

for the same data as 

presented in (a). The solid lines are a 

fit to the creep law which allow us to 

deduce the effect strength of the DW 

pinning as indicated by in (b).


6.1.3 Controlling domain walls by anisotropy engineering using focused He and Ga beams 

J.H. Franken, M. Hoeijmakers, R. Lavrijsen, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans, E. van Veldhoven 

and D.J. Maas 1 

1 

TNO Science and Industry, Delft, The Netherlands 


The motion of magnetic domain walls (DWs) in small magnetic wires is a very interesting topic, because of the 

prospect for new information storage devices such as the magnetic racetrack memory. In our group, DW 

dynamics is investigated in materials which have their magnetization direction perpendicular to the sample 

plane, such as Pt/Co/Pt. The interaction of current with DWs in these materials is not yet understood and leads 

to very interesting fundamental physics (section 6.1.2, R. Lavrijsen). However, before studying the motion of 

DWs, you first need to create a DW in a controllable way. Although there are ways to achieve this, e.g. by 

thermomagnetic writing with a laser spot or using the Oersted field generated by a nearby current pulse line, 

these methods pose restrictions to the experimental environment and sample design. We have developed a 

controlled DW injection scheme by tuning the critical parameter in these materials, the perpendicular magnetic 

anisotropy (PMA). This is achieved by irradiating a small part of a Pt/Co/Pt wire with a focused ion beam, 

which locally reduces the anisotropy. By applying a magnetic field, a domain is first created in the area with 

reduced anisotropy, which can then be moved into the non-irradiated region at a tunable field strength. 

Although first intended only as an experimental trick to create a DW, we made the interesting observation that 

the anisotropy boundary acts as a tunable pinning site for the DW. This leads to exciting new device options, for 

example in magnetic memory devices where discrete stopping positions for the DW are required. 


Using our recently acquired Kerr microscope, we are able to directly visualize the magnetic switching of 

irradiated Pt/Co/Pt strips. Snapshots of the magnetic domain structure upon increasing the external magnetic 

field from negative to positive saturation are shown in Fig. 1. We see that the left, irradiated part (indicated by 

the shaded area) switches first because of the reduced anisotropy. In the shown sample, surprisingly, the DW 

that is nucleated in the irradiated part is not able to move into the right part of the sample and remains pinned 

at the boundary (center image). By increasing the field strength further, the pinning is overcome and the DW 

swipes through the right part of the structure. 

10 µm 

M 

0 mT 

DW 

7.3 mT 

7.7 mT 

External 

field 

Fig. 1 Kerr images of a Ga irradiated strip at increasing field strength. A DW is nucleated at 7.3 mT, but remains 

pinned at the boundary between the irradiated (shaded) and non-irradiated areas. A higher field of 7.7 mT is needed 

to move the DW into the latter area, which then switches completely as the DW reaches the end of the strip.


An interesting behavior was found as a function of the amount of ion irradiation: with increasing ion dose, the 

switching field of the left part is decreased as expected, but interestingly, the pinning strength at the boundary 

is increasing. This is summarized in Fig. 2a (solid circles), where the injection field (needed to switch the right 

part), is plotted as a function of the ion dose in the left part. We see that the injection field can be tuned in two 

regimes. Especially interesting is the second, increasing regime, which tells us that the pinning strength at the 

boundary scales with the anisotropy difference between the two regions. This observation was well supported 

by a simple micromagnetic model. 

He + dose (10 13 ions/cm 2 ) 

0 500 1000 1500 2000 

(a) 25 

20 

He + optimal focus 

A 

C 

15 

Ga + optimal focus 

B 

10 

5 

Ga + beam blur 200 nm 

0 

0.0 0.5 1.0 1.5 2.0 2.5 3.0 

0 

H in 

(mT) 

Ga + dose (10 13 ions/cm 2 ) 

0.5 

(b) out-of-plane in-plane 

 

0.4 

K eff,0 

K eff 


6.1.4 Focused electron beam induced deposition of Fe – domain wall pinning 

R. Lavrijsen, T.H. Ellis, F.J. Schoenaker, B. Barcones, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans, J.M. 

DeTeresa, C. Magen, M.R. Ibaraa, P. Trompenaars and J.J.L. Mulders 


Focused-electron-beam-induced-deposition (FEBID) to locally create electrical contacts is widely used due to its 

versatile application as a mask-less technique with nanometer resolution. In contrast, the use of magnetic 

materials, is a relative new area of application due to the inherent low metallic content of FEBID deposits 

detrimental to the magnetic properties. Recently, reports have appeared on high-quality Fe deposits up to 95 

at.% Fe, however, without a thorough magnetic characterization. Another report shows high-quality FEBID Co 

nanowires up to 95 at.% Co content showing high quality magneto-transport properties for deposits with a Co 

content higher than 80 at.%. This has resulted in a successful device for a field-induced domain wall motion 

study [Pacheco et al. Appl. Phys. Lett. 94, 192509 (2009)] exemplifying the widespread application potential for 

FEBID fabricated magnetic nanowires in spintronics. 


We have investigated the magnetic properties of Fe nanowires grown by focused-electron-beam-induced 

deposition (FEBID) using a Fe2(CO)9 precursor on a FEI Nova Nanolab 600i dualbeam system. The Fe wires 

contain up to ~80 at.% Fe as measured by in-situ Energy Dispersive X-ray spectroscopy (EDX). The magnetic 

properties are investigated using the Anisotropic Magneto Resistance (AMR) effect and Magneto Optical Kerr 

Effect (MOKE) as can be seen in Fig. 1 where the magnetic behavior of a large FEBID Fe is investigated. 

Furthermore, we have performed the first pilot experiments using Fe-FEBID for domain wall motion 

manipulation in perpendicularly magnetized Pt/Co/Pt layers as can be seen in Fig. 2. 

This preliminary 

demonstration shows that it is feasible to use the stray fields of the nanomagnets grown by Fe-FEBID in 

devices/applications. The possibility to grow free-standing 3D structures in virtually any shape allows for many 

more possibilities to shape stable stray fields, such as horse-shoe magnets or other magnetic-flux closure 

arrangements. For instance, by simply engineering a structure on top or close to the strip a local DW pinning 

landscape can be engineered, and more importantly, without changing the properties of the magnetic strip. 

Other exciting applications might be in scanning probe microscopy instruments, magnetic-bead based biosensors, 

whenever and wherever local magnetic stray fields are required for nano-scale properties and 

functionality. 

Output 

Fe:O:C grown by focused electron beam induced deposition: Electric and Magnetic properties 

R. Lavrijsen, T.H. Ellis, F.J. Schoenaker, B. Barcones, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans, J.M. DeTeresa, C. 

Magen, M.R. Ibaraa, P. Trompenaars, J.J.L. Mulders 

Nanotechnology 22, 025302, (2011)


(a) 

Kerr Intensity (arb. u.) 

* 

M 

M 

-15 -10 -5 0 5 10 15 

H (mT) 

0 

Fig. 1 (a) Hysteresis loop obtained by measuring 

the light intensity of wide-field Kerr-microscopy 

(longitudinal sensitivity) images as function of field 

applied along thelong axis of the structure. The 

insets show the Kerr images taken when the sample 

is saturated in the applied field direction; the 

gray/black scale clearly indicates the magnetic 

contrast. (b) Longitudinal Kerr sensitivity image 

(dotted arrow) taken at remanence (* in (a)). We 

have drawn the flux-closure domain structure 

indicated by the solid arrows. (c) Transverse Kerr 

sensitivity image (dotted arrow) taken at 

remanence (* in (a)). 

(b) 

(c) 

Fig. 2 (a) SEM image of a 1 µm wide Pt/Co/Pt strip where Fe-FEBID pillars with different heights have been 

deposited on top indicated by the dashed circles. (b) - (e) Kerr microscopy images of the 1 μm wide Pt/Co/Pt strip 

as a function of increasing applied field. The box in (b) indicates the area shown in the SEM of (a). The position 

and height of the pillars are indicated by the vertical white dashed lines. An expanding domain can be seen 

propagating from the left to the right with increasing field. The DW is pinned due to the stray field of the pillars


6.1.5 Local formation of a Heusler type structure in CoFe-Al current perpendicular to the 

plane GMR spin-valves 

S. Wurmehl a , P.J. Jacobs, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans, S. Maat b , M.J. Carey b and J.R. 

Childress b 

a 

now at: Institute for Solid State Research, IFW Dresden, Dresden, Germany 

b 

San Jose Research Center, Hitachi GST, San Jose, USA 


Current perpendicular-to-the-plane giant magneto-resistance (CPP-GMR) read heads are considered as a 

follow-up technology for tunnel magneto-resistive (TMR) read heads. CPP-GMR heads exhibit low resistancearea 

products and no shot-noise which makes them attractive for sub-50 nm track widths required for recording 

densities of 1 Tbit/in 2 and beyond. A crucial issue for the success of such devices is to further enhance the GMR 

ratio particularly at room temperature. A successful application of spin-polarized materials in such CPP-GMR 

devices requires a detailed knowledge of the interplay between the structure and the magnetic and electronic 

properties. Recently a significant improvement of the magneto-transport properties of CPP-GMR devices 

consisting of ferromagnetic CoFe alloys was demonstrated by addition of up to 25% Al. In order to understand 

this improvement, we started a systematic investigation of the changes in the local atomic environment as a 

function of the Al content by means of nuclear magnetic resonance (NMR) measurements. NMR is an ideal tool 

for such studies because it probes the local hyperfine fields of the active atoms, which strongly depend on the 

local environments. 


Fig. 1 shows the NMR spectrum of a sample with roughly 25 at.% Al content. This is also 

the composition that was found to exhibit the highest GMR values. If the Co, Fe and Al would form an A2 

random bcc alloy, only one broad resonance line located around 190 MHz should be expected. Instead, one 

clearly observes six distinct lines with a clear substructure in addition to the expected broad resonance line, 

which points to the local formation of a higher degree of order than a pure A2 structure in the film. The distinct 

resonance peaks found are in good agreement with the peaks found in bulk single crystal B2 type ordered 

Co2FeAl Heusler compound samples, where the peak at 190 MHz corresponds to local environments with 

4Fe+4Al neighbors, while higher and lower frequency peaks belong to Fe rich and Al rich environments, 

respectively. However, a closer evaluation of the NMR data clearly shows that this alloy sample consists of a 

mixture of A2, B2, and L21 contributions, and that the additional substructure observed in the main resonance 

lines originates in higher order shell effects. 

In Fig. 2 the MNR spectra for samples with different Al contents are summarized, and the 

clear trend of decreasing NMR frequencies for increasing Al content up to 22 at.% is demonstrated. These 

spectra show again, that the addition of Al to CoFe leads to a drastically different local structure than bcc CoFe, 

and the bcc CoFe contributions very quickly become negligible. A clear fingerprint of the resonance lines of a 

Heusler type ordering is observed for the 22%, 25%, and 28% samples. Thus, with the addition of Al, CoFe has 

the tendency to form a Heusler compound. 

In Fig. 3 we demonstrate how the structural results found with our NMR investigations are correlated with the 

enhanced GMR values found using such alloys in devices. The CPP-GMR ratio and the formation of a highly 

spin-polarized Heusler compound seemingly follow a similar trend upon Al addition. In particular, the highest 

GMR ratios are obtained for those Al contents that also show high B2 and L21 type contributions. 

in the 

B2 type structure is predicted to also conserve the high spin polarization which is found for the L21 structure, 

and consequently, the bulk spin-scattering asymmetry in the CPP-GMR spin-valves.


Output 

Local formation of a Heusler structure in CoFe-Al alloys 

S. Wurmehl, P.J. Jacobs, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans, S. Maat, M.J. Carey, and J.R. Childress 

Applied Physics Letters 98, 012506 (2011) 

Fig. 1 Left: 59 Co NMR spectrum of a CoFe- 

Al sample with 25 at.% Al, also shown is 

the corresponding fit with Gaussian lines 

for the different next neighbor (Fe, Al) 

environments of the Co nuclei. 

Right: Comparison between a random 

atom model and the relative areas of each 

resonance line in the fit, hinting to the 

formation of the local formation of a 

Heusler type structure. 

Fig. 2 

59 

Co NMR spectra for several studied 

samples. 

Fig. 3 Comparison between the CPP-GMR properties and the 

contribution of the highly spin-polarized Heusler compound both 

as a function of the Al content of the 

samples.


6.2 Spintronics 

6.2.1 Tunable Rashba effect: spin-orbit-torque-assisted field-driven domain wall creep 

R. Lavrijsen, E. van der Bijl, R.A. Duine, J.T. Kohlhepp, H.J.M. Swagten and B. Koopmans 


Manipulation of the magnetization of ferromagnets through torques induced by spin-polarized currents is a 

rapidly evolving research field. This is due to the prospect of devices with reduced size and energy 

consumption. An actively investigated topic is current-induced magnetic domain-wall motion. Specifically, 

perpendicularly magnetized materials are of interest due to narrow and simple Bloch-type domain walls 

predicted to enhance the interaction with spin-polarized currents. Here, we demonstrate that current-assisted 

field-driven domain wall creep in Pt/Co/Pt is influenced by spin torques due to Rashba spin-orbit coupling, that 

primarily affect the domain-wall precession. Surprisingly, the Rashba effect can be tuned by simply changing 

the Pt layer thicknesses sandwiching the Co layer providing an alternative origin of the Rashba effect in 

Pt/Co/AlOx [Miron et al. Nature Materials 9, 230-234 (2010)]. Our findings may explain contradicting reports in 

literature. We expect that the tunability of the Rashba effect will pave the way for new experimental and 

theoretical spintronic concepts. 


We have shown that by simply changing the relative Pt layer thicknesses in a Pt/Co/Pt stack we can tune the 

efficiency and direction of the current induced DW velocity as seen in Fig. 1. This can be explained using the 

current distribution in the stack which shows an asymmetry due to the different Pt layer thicknesses leading to 

an effective Rashba field as shown in Fig. 2. The presence of the Rashba effect is evidenced by measuring the 

perpendicularly applied-field-driven current-assisted DW creep velocity by varying an in-plane field. 

Output 

Tunable Rahba effect: spin-orbit-torque-assisted field-driven domain wall creep 

R. Lavrijsen, E. van der Bijl, R.A. Duine, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans 

(In Preparation)


Fig. 1 (a) Average DW velocity based on 20 measurements plotted as a function of the applied field and , 

(bottom and top x-axis, respectively) and current density (J, the error bars indicate the standard deviation. a Results obtained 

on Pt(4 nm)/Co(0.6 nm)/Pt(2 nm), (b) Results obtained on Pt(2 nm)/Co(0.6 nm)/Pt(4 nm). The direction of current relative to 

the DWM direction is indicated by the arrows. Please note that the top x-axis scale is non-linear, the lines are a guide to the 

eye. The inset in b shows a typical device which are used to measure the DW velocity. All results are obtained at a controlled 

constant temperature of T = 300 K. 

Fig. 2 Origin of the Rashba field (a) – (b) The direction and magnitude of the effective Rashba field is determined 

by the local current density asymmetry at the top and bottom interface of the Co layer. The direction switches 

sign between Pt(4 nm)/Co(0.6 nm)/Pt(2 nm) in (a) and Pt(2 nm)/Co(0.6 nm)/Pt(4 nm) in (b).


6.2.2 Spin–spin interactions in organic magnetoresistance probed by angle-dependent 

measurements 

W. Wagemans, A.J. Schellekens, M. Kemper, F.L. Bloom, P.A. Bobbert and B. Koopmans 


In organic devices, considerable changes in the current have been observed when applying a magnetic field, an 

effect called organic magnetoresistance (OMAR). OMAR is generally believed to originate from spin 

correlations of interacting charge carriers. The spin character of such pairs is mixed by the random hyperfine 

fields, which can be suppressed by an external magnetic field. Gaining a better understanding of the physics of 

OMAR will improve knowledge of (spin) transport in organic semiconductors and could help towards possible 

applications, for instance in adding the possibility of sensing magnetic fields to cheap organic electronic 

devices. 

So far, in literature, it has been claimed that OMAR is independent of the orientation of the applied magnetic 

field. Indeed, the effect is not just observed for a specific angle between the magnetic field and the current (like 

the Hall effect). The models suggested for OMAR, like the electron–hole (e–h) pair model and the bipolaron 

model, do not predict any angle dependence of OMAR. We show that changing the orientation of the applied 

magnetic field, with respect to the applied electric field, results in a small but systematic change in the 

magnitude of OMAR. We show that both anisotropic spin–spin interactions and anisotropic hyperfine fields can 

explain the observed effects. 


We performed experiments on typical OLED-like devices with tris-(8-hydroxyquinoline) aluminum ( ) as 

the active layer. The magnetoconductance (MC) observed for parallel alignment of the magnetic field 

with respect to the sample normal is smaller than for the perpendicular case (Fig. 1a). The MC for 

intermediate angles shows an oscillation as a function of θ on top of a slowly increasing signal (Fig. 1b). 

Vertically plotted is , which was obtained from fitting the curves with a typical `non-Lorentzian' that 

is commonly seen in OMAR measurements: , where is the MC at infinite 

magnetic field and B0 is the half width at quarter maximum. Within the accuracy of the fits, no change in is 

observed. The data can be accurately fitted with a dependence. Additional measurements confirm the 

dependence and exclude measurement artifacts to be responsible. It is shown that the angle 

dependence is intrinsic to OMAR. By changing the angle of the magnetic field, the processes responsible for 

OMAR are apparently modified. 

Fig. 1 (a) MC(B) curves measured at 12 V for θ = 0° and 90°, with θ as indicated. (b) MC at infinite field, from fitting 

MC(B) curves with a non-Lorentzian, as a function of angle, fitted with


Fig. 2 Simulated MC for the e–h model with a preferential orientation of the pairs. (a) MC for two orientations of 

B with dipole-energy prefactor and exchange-energy prefactor (b) Angle 

dependence of the MC at infinite field with and without exchange coupling, fitted with – 

The combination of two spin-½ particles has four total spin states: one singlet and three triplets. At zero applied 

field, the random hyperfine fields allow mixing between all four states. However, on applying a large external 

field, the triplets Zeeman-split in energy and the singlet and only one of the triplet states can mix. This reduced 

mixing results in the change in current that we call OMAR. As we observe an angle dependence at large fields, 

this mixing between the singlet and one triplet state has to be affected. The mixing at high fields can be altered 

in two ways. First, the strength of the hyperfine fields can be different for different orientations of the external 

field. Second, there could be a small angle-dependent energy difference between the singlet and triplet state, 

caused by spin–spin interactions. The two relevant spin–spin interactions are dipole coupling and exchange 

coupling. 

By modeling OMAR and including the two different origins (energy splitting or hyperfine coupling), we 

showed that both can explain the experimentally observed angle dependence. In Fig. 2, we show the results 

using the e–h model with angle-dependent energy splitting. Correct line shapes (Fig. 2a) are obtained. With 

only an energy splitting from dipole coupling (Fig. 2b, 

), additional features in the angle dependence, 

absent in the experiments, are observed, which are removed when also a small exchange coupling is included. 

These experiments confirm that OMAR is caused by spin–spin interactions. Two possible origins have been 

suggested, which could be distinguished by a smart choice of materials. The investigation of the angle 

dependence could provide another way of investigating the spin–spin interactions in organic materials and 

might help to further understanding of OMAR. 

Output 

Spin–spin interactions in organic magnetoresistance probed by angle-dependent measurements 

W. Wagemans, A.J. Schellekens, M. Kemper, F.L. Bloom, P.A. Bobbert, and B. Koopmans 

(Submitted to Phys. Rev. Lett.)


6.2.3 Frequency dependence of organic magnetoresistance 

P. Janssen, W. Wagemans, E.H.M. van der Heijden, W. Verhoeven, M. Kemerink and B. Koopmans 


Due to their relative ease of processing, chemical tunability and possible low costs, organic semiconductors 

provide exceptional promise for (future) electronic applications. Recently, it was discovered that the current 

through an organic semiconductor, sandwiched between two non-magnetic electrodes, can be changed by 

applying a small (~10 mT) magnetic field. This large (up to 20%) magnetoresistance effect is called organic 

magnetoresistance (OMAR). The effect can be both positive and negative depending on operating conditions. 

Up to now, several models have been proposed to explain OMAR, but the exact origin is still debated. Therefore 

novel measurements are needed to discriminate between models. 

In this work we investigate the frequency dependence of OMAR on Alq3 (small molecule) and MDMO-PPV 

(conjugated polymer) based devices. By this we aim to identify the timescales of the processes involved in 

OMAR. Doing so, we ultimately hope to be able to distinguish between the different models proposed for 

OMAR, because the processes that are used in the models each occur on different timescales. 


To study the frequency dependence of OMAR, we use a superposition of an AC and DC magnetic field. The 

magnetoconductance 

defined as the relative change in current when applying an external 

magnetic field, decreases when the frequency of the AC magnetic field is increased, as is illustrated in Fig. 1. 

In addition to the response of the current to an AC magnetic field, we measured the response to an AC voltage, 

which is the admittance , where is the conductance and the capacitance. The results for the 

capacitance as a function of frequency for different DC voltages are shown in Fig. 2. For a negative 

capacitance is obtained for a certain frequency range. A negative contribution to the capacitance is attributed to 

the presence of minority carriers. This can be more clearly visualized by plotting the normalized differential 

susceptance – th the geometrical capacitance, as can be seen in the inset of Fig. 

2. A clear peak in the differential susceptance, shifting to higher frequencies with increasing voltage, is 

observed. The position of this peak, and point where the negative capacitance disappears, are related to the 

inverse transit time of the minority carriers. 

To get a quantitative measure, we fit the frequency dependence of the MC with an empirical function 

, where is the characteristic cut-off frequency. In Fig. 3, the cut-off frequency for the 

, the frequencies where the capacitance is 95% of its low voltage value ( ) and the peak in differential 

susceptance ( are plotted as a function of voltage. A clear correlation between the frequencies is observed, 

indicating that the (transit time of the) minority carriers play(s) a crucial role in the frequency dependence of 

OMAR.


Normalized MC 

10 0 12 V 

11 V 

10 V 

10 -1 

10 -2 

9 V 

8 V 

Capacitance (nF) 

3 

7 V 

95% 

10 0 10 1 10 2 10 3 

7 V 8 9 10 11 12 V 

2 

Frequency (Hz) 10 0 10 1 10 2 10 3 10 4 10 5 

6 

5 

4 

< 7 V 

f C 

(8 V) 

Normalized B (-) 

1.0 

0.5 

0.0 

7 V 

Frequency (Hz) 

8 

9 

10 11 12 V 

10 0 10 1 10 2 10 3 10 4 


Fig 1. Normalized MC as a function of frequency for 

different voltages for a 100 nm thick Alq3 device. The 

lines are a fit to 

,, where 

represents the cut-off frequency for the MC. 

Fig 2. Capacitance as a function of frequency for 

different dc bias voltages. The frequency where C is 

95% of the 0V value is indicated for the 8V 

measurement. The inset shows the normalized 

differential susceptance as a function of frequency 

for different voltages. 


10 3 

10 2 

10 1 

10 0 

10 4 Alq 3 

(100 nm) 

f C 

f 0 

f B 

MC decreases 

"single carrier" 

"double carrier" 

7 8 9 10 11 12 

Voltage (V) 

MC (arb.u.) 

10 

1 

200 nm Alq 3 

Measurement 

Model 

10 0 10 1 10 2 10 3 


Fig 3. Characteristic cut-ff frequency (squares), the 

frequency where the capacitance is 95% of its low 

voltage value, (triangles), and the frequency for the 

peak in differential susceptance (circles) as a 

function of voltage for the 100 nm Alq3 device. 

Fig 4. A typical measurement for a 200 nm thick 

Alq3 device at 16V (open squares) and the simulation 

results of our device model (solid line). 

In conclusion, we have shown frequency-dependent OMAR measurements using a superposition of a DC and 

AC magnetic field. We observe a decrease of MC with increasing frequency. Using admittance spectroscopy we 

show that this decrease is related to a transition from a double carrier to single carrier device. Currently device 

simulations are performed to gain further insight in the transport in these devices. A typical result for a 

simulation where the magnetic field causes a decrease in minority carrier mobility is shown in Fig. 4, yielding a 

qualitative match between experiment and model. 

Output 

Frequency dependence of organic magnetoresistance 

W. Wagemans, P. Janssen, E. H. M. van der Heijden, M. Kemerink, and B. Koopmans 

Appl. Phys. Lett. 97, 123301 (2010)


6.2.4 Exploring Organic Magnetoresistance: An investigation of microscopic and device 

properties – PhD Thesis 

Francisco Bloom 


Recently there has been much interest in combining the fields of organic electronics and spintronics. This has 

been motivated by the fact that low atomic mass of organic materials are predicted to have long spin lifetimes. 

Also, spintronic devices could benefit from the chemical tunability, ease of fabrication, and mechanical 

flexibility of organic semiconductors. The nascent field of organic spintronics has already presented many new 

phenomena which must be explained with novel physics, here we explore one of these phenomena, organic 

magnetoresistance (OMAR). 

OMAR is a room temperature spintronic effect in organic devices without any magnetic materials. OMAR is a 

large change in resistance (up to 25%) at low magnetic fields (20mT). OMAR represents a scientific puzzle since 

no traditional magnetoresistance mechanisms can explain the combination of properties listed above. Another 

one of the remarkable properties of OMAR is that the sign of the MR can change based operating conditions of 

the device, like temperature and voltage. In this dissertation we focused in particular on resolving the origin of 

the sign change since understanding this unique property should be a major step in unraveling the microscopic 

origin of OMAR. 

Results 

We have explored the properties of the sign change experimentally with bipolar semiconducting small molecule 

and polymer devices, in which we observed sign changes as functions of voltage and temperature. These 

devices showed a strong correlation between the sign change and the onset of minority charge carrier injection 

and we could describe the lineshape and MR(V) behavior as a superposition of two MR effects of opposite sign. 

From this work we concluded the separate MR effects were from the mobilities of holes and electrons having 

different responses to magnetic fields, which is best described by the bipolaron model for OMAR. 

To test this conclusion, we employed analytical and 

numerical device models assigning separate 

magnetomobilities to holes and electrons. The models 

show, counter-intuitively, that in the case when the 

minority charge carrier contact is injection limited, a 

decrease in minority charge carrier mobility increases the 

current. This is a result of the minority carrier contact 

acting like a constant current source, and of the 

compensation of the majority carrier space charge by the 

oppositely charged minority carriers. We show that these 

models describe the observed MR(V) behavior very well, 

and if one assumes the magnetic field acts to reduce the 

mobility of electrons and holes, we observe that our 

models can reproduce all the sign changes observed in 

literature. The device model also predicts how different 

device parameters affect the observed MR, to test its 

predictions we performed experiments in which we 

increased the charge recombination by dye doping the 

organic active layer, we also observed how changing the 

charge injection by altering the organic semiconductor/ 

metal contacts experimentally compared with the device 

model.


The fact that the current can increase when the minority carrier mobility decreases may explain the fact that in 

experiments the magnitude of the negative MR features has been much larger than the positive MR features, 

even though, microscopically, the bipolaron model predicts the opposite. Therefore, the presence of both signs 

of magnetoresistance may be related only to the device physics and not to the microscopic mechanism which 

causes OMAR. 

Output 

Organic Magnetoresistance –An investigation of microscopic and device properties 


PhD thesis, November 2010


6.2.5 Plastic Spintronics – PhD Thesis 

Wiebe Wagemans 


In this thesis, both theoretical and experimental results on organic magnetoresistance (OMAR) and spin 

polarized transport have been presented. Contributions have been made to a new model for OMAR and new 

type of experiments have been performed that have added further insights to the puzzle of OMAR. The limiting 

role of the hyperfine fields on spin-polarized transport has been investigated theoretically, providing an 

explanation for the experimentally observed magnetoresistance curves of organic spin valves and providing 

suggestions for future experiments. 

Results 

We investigated the use of organic semiconductors in spintronics applications, where next to the charge of the 

electrons, also their spin is utilized. Organic semiconductors have several advantages that make them 

interesting for these applications. They are relatively cheap, are easy to process, and can be chemically adapted. 

Two different, but related, topics that combine organic materials with spintronics have been studied both 

experimentally and theoretically. 

The first topic is the recently discovered OMAR effect. OMAR is observed in a wide range of organic materials, 

from small molecules to polymers. It is believed that OMAR originates from the interactions of a pair of charge 

carriers (for instance, electron–electron, hole–hole, or electron–hole). More specifically, from the relative 

orientation of the spins of these two charges. Without an external magnetic field, small intrinsic magnetic fields 

in the organic layer (resulting from hyperfine coupling to nuclear spins) randomize the orientations of the two 

spins. This allows a change from a spin configuration that is less favorable for the current into a more favorable 

configuration. However, applying a magnetic field larger than these hyperfine fields results in a strong 

reduction of this spin randomization or spin mixing, causing a pair to remain locked in a less favorable spin 

configuration. 

Although there is agreement on the crucial role of hyperfine fields, the exact mechanisms behind OMAR are 

still heavily debated. Several models were proposed in literature explaining OMAR in terms of different charge 

pairs. We investigated a model based on pairs of equal carriers, called the bipolaron model. We used an 

elementary model of two neighboring sites, where, depending on the spins, one carrier might be preventing 

another one to pass. With this theoretical model we were able to successfully reproduce several characteristics 

of OMAR. Both a decrease and an increase in current, as found in experiments, could be obtained and also the 

universal shapes of the experimental OMAR curves could be reproduced. 

Additionally, we performed new types of experiments to gain better understanding of OMAR. We showed that 

when an oscillating magnetic field is applied, OMAR is reduced beyond a certain frequency threshold. This 

occurs when the slowest charges can no longer follow the oscillations, as we showed by measuring the 

frequency dependence of the capacitance. These findings are in agreement with recent interpretations in which 

these slowest carriers are expected to induce the largest OMAR effect. 

In literature, it was claimed that OMAR is independent of the orientation of the magnetic field. However, via 

sensitive measurements we demonstrated a small but systematic dependence on the angle between the 

magnetic field and the sample. We showed theoretically that this angle dependence can be explained in the 

different models by including an interaction between the spins. This interaction has to be direction dependent 

in order to explain the angle dependence. We identified dipole–dipole coupling or an anisotropy in the 

hyperfine fields as the most likely candidates. 

Furthermore, we outlined a first exploration of an alternative approach to describe OMAR curves. We 

introduced a function that allows us to extract information both about the hyperfine fields and about an


additional broadening of the curves. Thereby, this approach could allow for a more quantitative analysis of 

changes in the OMAR curves resulting from changes in the operating conditions or the material properties. 

In the second topic, we investigated spin-polarized 

transport through an organic layer, for instance in an 

organic spin valve. 

The main mechanism for loss of 

polarization in most inorganic semiconductors, which is 

related to spin–orbit coupling, is negligible in organic 

materials. However, there might still be other 

mechanisms that cause a smaller but non-zero loss of spin 

polarization. We conjectured that the hyperfine fields are 

the main source of polarization loss in organic materials, 

which results from mixing between the spin-up and spindown 

electrons by precession of spins about these 

random fields. 

We theoretically investigated this effect of the hyperfine 

fields on spin polarization. We explicitly included the 

hopping transport characteristic for organic 

semiconductors. Due to spatial and energetic disorder, 

the charges hop from one localized site to another. The 

longer the time they spend on a site, the larger the loss of 

spin polarization. We showed that an external magnetic 

field larger than the typical hyperfine-field strength 

reduces the loss of spin polarization. Hence, such an 

external field causes the polarization to persist over a larger distance, leading to a magnetic-field dependent 

increase of the spin-diffusion length. Using these findings, we could very accurately fit experimental data on the 

magnetoresistance of organic spin valves reported in literature. Moreover, we made predictions about the effect 

of changing the orientation of the magnetic field, thereby manipulating the spins during transport. 

Output 

Plastic Spintronics; spin transport and intrinsic magnetoresistance in organic semiconductors 

Wiebe Wagemans 

PhD thesis, June 2010


6.2.6 Microscopic modeling of spin-dependent interactions in organic semiconductors 

A.J. Schellekens, W. Wagemans, S.P. Kersten, P.A. Bobbert and B. Koopmans 


An applied magnetic field can alter the current in organic semiconductors. This relatively large effect (> 10%) is 

dubbed ‘Organic Magnetoresistance’ (OMAR) and can be measured even in small applied fields (≈ 10 mT) and 

at room temperature. Since its discovery, multiple models to explain the large magnetoresistance have been 

proposed by various authors. However, none of them unambiguously explain all the experimental results, 

making the origin of OMAR a heavily debated topic in the scientific community. 


Although the models for OMAR are based on different reactions, there is also a strong similarity between them. 

In the proposed models an applied magnetic field alters the spin-dependent reactions between two particles, 

thereby changing the current through the organic devices. Because of this similarity it is possible to perform 

microscopic calculations on the different models using a single mathematical framework. To do this we have 

used the following master equation: 

called the Stochastic Liouville equation (SLE). This master equation governing the system dynamics has been 

introduced with considerable success in different fields of research, ranging from delayed fluorescence in 

organic semiconducting crystals to laser theory. 

As an example of how the SLE works, we show a flow diagram for the so called bipolaron model in Fig. 1 In the 

bipolaron model spin pairs are continuously being added to the density matrix by, which is a matrix 

proportional to the identity matrix. When a spin pair has a singlet component there is a possibility that a 

bipolaron is formed, removing the pair from the density matrix by the projection operator Ʌ. Pairs dissociate by 

spin-independent hopping to the environment. The magnetic field dependence of the model enters through the 

Hamiltonian 

, which determines the coherent evolution in time of the spin states. The applied field suppresses 

singlet-triplet mixing by precession around random hyperfine fields, hereby changing the bipolaron formation 

rate and the current. 

polaron pair formation 

Г 

coherent interactions 

H bipolaron formation 

ρ 

hopping to environment 

Fig. 1 Flow diagram for microscopic calculations on the bipolaron model using the SLE. When a polaron pair is 

created by , the spins in the pair states evolve in time according to the Hamiltonian . The pairs can be 

removed from by bipolaron formation or hopping to the environment.


With the SLE it is possible to study the influence of the ratio of the typical hyperfine precession and hop 

frequencies , unlike with simple rate-equation models proposed in literature. In Fig. 2 the magneto 

conductance (MC) in large applied fields, i.e. – , is plotted as a function of for 

different values of the branching ratio b, which is the ratio between the bipolaron formation rate and hopping 

rate to the environment. What can be observed is that the MC vanishes when hopping is much faster than the 

hyperfine precession frequency. In Fig. 3 MC line-shapes are shown for various hop rates. Here it can be 

concluded that not only the magnitude of the MC decreases on increasing the hop rate, but also the line widths 

broaden. 

0 b = 0 

0 

hop / hf 

= 50 

MC (%) 

-20 

-40 

-60 

-80 

-100 

b = 10 

b = 100 

10 -1 10 1 10 3 

hop 

hf 

MC (%) 

-20 

hop 

/ hf 

= 15 

-40 

-60 

hop 

/ hf 

= 5 

-80 

-100 

hop 

/ hf 

= 0.5 

-20 -10 0 10 20 

B (mT) 

Fig. 2 MC in large applied fields as a function of ωhop / ωhf 

for different values of the branching ratio b. 

Fig. 3 MC as a function of B for different values of the hop 

rate ωhop / ωhf. 

To illustrate the power of the SLE for interpreting experimental data, a measurement of the MC in an Alq3 

OLED under illumination is show in Fig. 4. It can be observed that two contributions to the MC are present; a 

positive one in small applied fields and a negative one in large applied fields. From a large series of 

measurements for varying operating conditions it is concluded that triplet-charge interactions are likely to be 

responsible for the effects in large applied fields, while the interactions between electron and holes are likely to 

dominate the MC in small fields. This conclusion is supported by the line-shapes in Fig. 5, where the SLE 

equation is used to calculate both the effect of an applied field on e-h pair recombination as on the detrapping of 

charges by triplet excitons. Line-shapes similar to the experimental ones are obtained from the microscopic 

calculations. 

Measurement 

Calculation 

150 

150 

e-h pairs 

MC (%) 

100 

50 

MC (%) 

100 

50 

total 

0 

0 

-50 

-400 -200 0 200 400 

B (mT) 

-50 

triplet - charge 

-400 -200 0 200 400 

B (mT) 

Fig. 4 Example of a measurement of the photo-generated 

current in an Alq3 OLED. 

Fig. 5 Calculated MC line-shape due to e-h pair 

recombination and triplet-charge interactions.


6.2.7 Tuning Spin Interactions in Organic Semiconductors 

P. Janssen, M. Cox, M. Kemerink, M.M. Wienk and B. Koopmans 


This work focuses on the combination of two new emerging fields of electronics, namely molecular electronics 

and spintronics. The field of molecular electronics deals with the study of molecular materials for the 

application of electronic devices. Spintronics on the other hand aims at exploiting the spin of the electron to 

transport and store information, whereas conventional electronics only use the charge. 

A novel effect in these two fields, called organic magneto resistance (OMAR), has recently been discovered in 

room temperature non-magnetic molecular materials. It is generally observed in OLED devices, where an 

organic semiconducting material is sandwiched between two metal electrodes. The current through these 

devices can change up to ∼10% when an external magnetic field in the order of 10 mT is applied. All 

contemporary models explaining the OMAR effect are based on the interactions of the spin of the charge 

carriers, which can be electrons or holes. However, there is no consensus on the exact origin. Therefore, 

extensive research is required to verify the correct underlying model. 

The spin interactions that play an important role in these organic semiconductors manifest themselves as pairs 

of particles. This includes Coulomb bound electron-hole pairs, but also interactions of particles with the same 

charge, such as electron-electron pairs, which are called bipolarons. Furthermore, an electron-hole pair can 

convert into a particle called an exciton. These excitons can interact with free unpaired charges again. Most 

OMAR models distinguish between these three different spin interactions; bipolarons, electron-hole pairs and 

exciton-charge interactions. 

In our research the versatility of organic materials has been used test and verify these models. This is done in a 

unique way by following the path from an OLED to a blended organic photovoltaic. The presence of different 

molecules in an OLED can influence the charge carriers significantly and under the right circumstances a phase 

separated network develops, thereby creating separate paths for the electrons and holes to follow and reducing 

their interaction with each other. The effect of the changes in spin interactions and morphology on OMAR could 

point to the correct underlying model. 


In this work, a blend of poly(phenylene-vinylene) (MDMO-PPV), acting as electron donor, with fullerene 

(PCBM) molecules, acting as electron acceptor, has been studied. This blend is a well known and extensively 

studied organic photovoltaic. The morphology and charge transport through such a blend are schematically 

depicted in Fig. 1a. Within a few weight percent PCBM, virtually all excitons can find a PCBM site within their 

diffusion length at which rapid charge transfer can take place. At around 20 wt.-% PCBM electrons will be able 

to hop through the fullerenes, thereby significantly altering the transport properties of the blends. Finally, phase 

separation sets in at around 67 wt.-% PCBM, which reduces the electron-hole interactions, but further enhances 

the transport properties. 

We have studied the magnetic field effects on the current as a function of applied magnetic field, voltage and 

fullerene concentration. Typical results for the magnetoconductance, defined as the relative change in current 

when applying a magnetic field, are shown in Fig. 1b-d. Changing the bias voltage and PCBM content causes 

the line shapes to change dramatically.


Fig 1. (a) Schematic representation of the morphology and charge transport in a PPV – PCBM blend. (b)–(d) Examples of 

magnetoconductance (MC) traces as a function of magnetic field (B) using different bias voltages and PCBM concentrations. 

Two different magnetic field effects have been identified, which behave different with applied voltage and 

fullerene concentration. Furthermore, drift-diffusion simulations have been performed to gain insight on the 

behavior of the devices as a function of voltage and fullerene concentration. We are able to relate the changes in 

the MC to the morphology of the blend and we are trying to unravel the organic magnetoresistance by 

comparing the proposed models with our observed data and simulations. 

So far we have been able to conclude that exciton-charge interactions are the dominant mechanism behind the 

magnetic field effect in PPV devices with little PCBM content. In blended devices with more than 20 wt.-% 

PCBM these effects are fully quenched. In these devices the electron-hole pairs have been identified as the 

dominant spin interactions causing the magnetic field effects. When phase separation sets in, the bipolaron 

mechanism becomes dominant, as can be observed in the magnetoconductance traces. 

By tuning the spin interactions in a blend of organic materials we have thus concluded that different 

mechanisms are responsible for OMAR. However, which mechanism is dominant is based on the exact material 

choice and operating conditions. Currently, we are aiming to combine the different models in one unified 

picture.


6.3 Ultrafast Spin Dynamics 

6.3.1 Theory of femtosecond laser-induced magnetization dynamics 

A.J. Schellekens, T. Roth b , G. Malinowski, F. Dalla Longa, K.C. Kuiper, D. Steiauf a , M. Fähnle a , M. Cinchetti b , 

and M. Aeschlimann b and B. Koopmans 

a 

Max-Planck-Institut für Metallforschung Stuttgart, Germany 

b 

Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, Germany 


All-optical techniques exploiting femtosecond laser pulses have opened the way towards the exploration of the 

ultimate limits of magnetization dynamics, providing means to manipulate magnetic systems at down to 

femtosecond time scales. Apart from addressing fundamental issues in the field of (nano)magnetism the 

approach is also considered to be of extreme relevance for future progress in (high data rate) magnetic 

recording and spintronic applications. In 1996, Beaurepaire and coworkers found that magnetic order in 

ferromagnetic transition metals can be quenched within a few hundred femtoseconds after laser heating. In 

contrast, earlier work by Vaterlaus et al. on gadolinium reported a much slower response of 100 ± 80 ps, i.e. a 

factor of thousand slower! The apparent incompatibility of the two results, combined with the large uncertainty 

in the earlier measurements on gadolinium, has fuelled intense scientific discussion about its origin, and even 

whether results for gadolinium could be trusted at all. 


Recently, in a joint effort with researchers from the University of Kaiserslautern and the MPI Stuttgart we 

succeeded in providing a coherent explanation for the contrasting results for the different materials. We 

introduced a theoretical framework based on Elliott-Yafet type of spin-flip scattering that successfully explains 

all phenomena and timescales on equal footing. Calculations were based on a simple model Hamiltonian 

describing (spinless) free electrons, representing phonons within the Einstein or Debye model, and treating spin 

excitations using a mean-field Weiss model. Thus we derived a microscopic version of the three-temperature 

model (M3TM, Fig. 1c & 1d), describing the magnetization dynamics by a simple differential equation. Fig. 1a & 

1b show calculated profiles of the electron (red) and lattice (blue) temperature, as well as the magnetization 

(green) after pulsed laser heating for Ni and Gd, resp., using spin scattering probabilities of ~ 10% in both cases. 

Despite these similar values, the calculated transients match well with experimentally observed 

demagnetization time scales of ~ 100 fs and ~ 50 ps, resp. The values of the spin-flip probability were shown to 

agree well with ab initio calculations of the spin-mixing in these materials. More recently, we performed detailed 

temperature- and laser-fluence-dependent experiments on Ni and Co, and refined our M3TM. We find that in 

all aspects the model's predictions agree well with experiment, including the appearance of a ‘two-step’ 

demagnetization –such as prominently visible for Gd (Fig. 1b)– when performing measurement on Ni at 

elevated temperature.


These results can be understood by further inspection of the theory. It is found that the demagnetization rate is 

governed by the ratio of the Curie temperature to the atomic magnetic moment. Comparing Ni and Co, this 

ratio is almost identical, explaining the similar dynamics for the two transition metals. However, Gadolinium 

has twice a lower Curie temperature than Ni, while its atomic moment is a factor of 13 larger (Fig. 1f & 1g). 

Thus, the demagnetization process gets so slow, that the demagnetization is not yet completed once the electron 

and lattice systems have reached mutual thermal equilibrium. This explains both the much longer time scale, as 

well as the occurrence of a two-step process. More generally, we constructed a generic view on laser-induced 

demagnetization, introducing a phase diagram separating two classes of dynamics. After having made this 

significant step forward in our understanding of laser-induced ultrafast demagnetization, next challenges will 

be the implementation of more realistic spin excitation spectra, and applying the models to more complicated 

magnetic materials (including ferrimagnetics), and new laser-based scenarios (including switching by circularly 

polarized light). 

Output 

Explaining the paradoxical diversity of ultrafast laser-induced demagnetization 

B. Koopmans, G. Malinowski, F. Dalla Longa, D. Steiauf, M. Fähnle, T. Roth, M. Cinchetti and M. Aeschlimann 

Nature Materials 9, 259 (2010) 

Temperature dependence of laser-induced demagnetization in Ni: A key for identifying the underlying 

mechanism 

T. Roth, A. J. Schellekens, S. Alebrand, O. Schmitt, D. Steil, M. Cinchetti, B. Koopmans and M. Aeschlimann. 

(Submitted) 

Fig. 1 Calculated dynamics of laser-induced 

demagnetization for Ni and Gd. (a), Ultrafast 

demagnetization m(t) (green), as well as Te(t) 

(red) and Tp(t) (blue) profiles, simulating 

experimental results for Ni. (b), Similar for 

the two-step process, as observed for Gd. (c), 

Schematic representation of the threetemperature 

model for Ni, as a 

representative for the 3d transition metals. 

Energy equilibration is indicated by twosided 

arrows; angular momentum flow is 

controlled by interaction with the lattice 

(dashed arrow). (d), Similar for Gd, with the 

extra 4f system. (e), Elliott–Yafet spin-flip 

scattering on emission of a phonon, taking 

over angular momentum. (f), Spin-flip 

scattering in the 3d4sp band of Ni. The 

orange shading represents the number of 

uncompensated spins. (g), Similar diagram 

for Gd; scattering is occurring only in the 

5d6sp band with small magnetic moment, 

whereas localized 4f states predominantly 

contribute to the magnetic moment.


6.3.2 Magnetism and dynamics of Pt / Co / for domain wall devices 

A.J. Schellekens, F. Elich and B. Koopmans 


Magnetization dynamics in perpendicularly magnetized materials have received much interest from the 

scientific community, as they are important candidates for new magnetic storage media and spintronic devices. 

The typically large perpendicular anisotropy results in narrow and simple Bloch domain walls, which facilitates 

current driven domain wall motion as well as high density data storage. A problem of many materials with a 

perpendicular anisotropy is the hindered domain wall motion due to pinning. However, recently large domain 

wall velocities (> 100 m/s) have been measured in a new type of perpendicular magnetized material, namely 

trilayers, making them extremely interesting for future spintronic devices. 

Magnetization dynamics are governed by the Landau-Lifschitz-Gilbert equation. An important parameter in 

this equation is the Gilbert damping parameter α, as it determines the magnetization relaxation rate and thereby 

also the domain wall velocity, the current to trigger domain wall propagation and the timescales of switching a 

magnetic memory element. The goal of this project is to determine α for 

trilayers, as it is a crucial 

parameter for the spin dynamics and could elucidate the origin of the large domain wall velocities in these 

materials. 


The samples used in this project are Pt / Co / Al cross wedges (Fig. 1), where on the same sample both the cobalt 

and aluminum thickness is varied. The sample is oxidized by plasma oxidation resulting in an Al2O3 top layer. 

To see for which Co and Al thicknesses the sample is perpendicularly magnetized, the magnetization 

component perpendicular to the sample surface is measured by means of the magneto-optical Kerr effect 

(MOKE). The results are depicted in Fig. 2. Three distinct regions are visible in this contour plot, namely (i) a 

region where the aluminum is under-oxidized and the magnetization lies in plane, (ii) a region where the 

sample is optimally oxidized and (iii) the magnetization is out of plane, and a region where not only the 

aluminum but also the cobalt is oxidized, resulting in a quenched magnetization. 

Fig. 1 Pt / Co / Al wedge structure, where both the Al as 

the Co thickness are varied. 

Fig. 2 Out-of-plane normalized remanent magnetization of 

a 

cross wedge.


The Gilbert damping parameter is determined by measuring laser-induced magnetization dynamics exploiting 

the time-resolved magneto-optical Kerr effect (TR-MOKE). An external magnetic field is applied at an angle of ~ 

10˚ with the sample surface. Typical TR-MOKE traces are depicted in Fig. 3. After ~ 10 ps a damped precession 

of the magnetization occurs due to a change in anisotropy during laser pulse excitation. From the LLG-equation 

it is derived that , where is the precession frequency and is the damping time, hence can be 

determined by fitting this damped precessional motion. 

In Fig. 4 the fitted values for are plotted as a function of the applied magnetic field for different thicknesses of 

the aluminum layer. A magnetic field dependence of is observed, which becomes more pronounced for small 

aluminum thicknesses. This magnetic field dependence has in literature been suggested to be caused by 

dephasing of the precession by a spread in the local anisotropies. The data in Fig. 4 is fitted by a macro-spin 

model including such a spread. What can be observed is that for a large Al thickness, so a moderate magnetic 

field dependence, the experimental data and fits correspond well. However, for a thinner Al top layer with a 

stronger field dependence, the data cannot be fitted by the simple model. This raises the question whether the 

magnetic field dependence of the damping parameter is really caused by a dispersion in the anisotropy or that a 

different mechanism is at play. 

A conclusion that can be drawn from the experimental data is that seems to converge to values between 0.1 

and 0.2 on increasing the applied field, which is comparable to the values found for Pt / Co / Pt and 

Pt / CoFeB / Pt films. Further experiments and calculations are required to pinpoint the origin of the magnetic 

field dependence of the Gilbert damping parameter in these ferromagnetic thin films. 

2.0 

1.8 nm Co 

0.8 

1.8 nm Co 

1.5 

0.6 

d al 

= 6.6 Å 

d al 

= 6.1 Å 

M z 

(arb. units) 

1.0 

0.5 

d al 

= 6.1 Å 

d al 

= 6.6 Å 

d al 

= 7.1 Å 

d al 

= 7.6 Å 

 

0.4 

0.2 

d al 

= 7.1 Å 

d al 

= 7.6 Å 

d al 

= 8.1 Å 

0.0 

d al 

= 8.1Å 

0 50 100 150 200 

Delay (ps) 

0.0 

0.25 0.50 0.75 1.00 

B applied 

(T) 

Fig. 3 Measurements and fits of the precessional dynamics 

of Pt / Co / Al2O3 as a function of Al thickness for an 

applied field of 1 Tesla. 

Fig. 4 Obtained values for the Gilbert damping parameter 

as a function of the applied field. The lines are fits assuming 

a dispersion in the perpendicular anisotropy.


6.3.3 Experiments and simulations on femtosecond laser-induced magnetization dynamics 

K.C. Kuiper, A.J. Schellekens, T. Roth a , M. Cinchetti a , M. Aeschlimann a and B. Koopmans 

a 

Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, Germany 


For more than 10 years, it is known that the magnetization of a ferromagnetic transition metal can be quenched 

within a few hundred femtoseconds (fs) by an intense fs-laser pulse. Since the measurements of Beaurepaire in 

1996, several models have been introduced, which try to unravel the demagnetizing processes. One of these 

models is called the 3 temperature model (3TM), which divides the ferromagnet into 3 subsystems, being the 

electron-, lattice- and spin system. All these subsystems have their individual temperature. The quenching of 

the magnetization is assumed to be caused by the increase of the spin temperature. 

Although the 3TM can successfully reproduce experimental observations, its main shortcoming is that only the 

energy flow among the subsystems is modeled, whereas the conservation of angular momentum is not 

considered. This problem was recently solved by the introduction of a microscopic extension of the Three 

Temperature Model (M3TM), in which the transfer of angular momentum between the electron-, lattice- and 

spin system is mediated by Elliott-Yafet type of spin scattering. 


M3TM simulations on nickel films were carried out to demonstrate that utmost care has to be taken in deriving 

quantitative information from experiments when using high laser fluences. More specifically, modeling the 

dynamics including a finite penetration of the laser light and heat diffusion, it is shown that extrinsic 

parameters, such as the thickness of the ferromagnetic layer or the sample structure, can cause a change in the 

observed demagnetization time by up to a factor of three. 

The influence of the sample thickness on the quenching of the magnetization is shown in Fig. 1. From this 

figure, it is clear that for a thin isolated film, the quenching rapidly increases for increasing fluence and the film 

is completely demagnetized abruptly. However for a relative thick film, the quenching seems to saturate for 

large laser fluences. This saturating behavior was observed in many earlier experiments. 

In Fig. 2, the effect of the sample structure on the demagnetization time is shown. In this case, two different 

nickel films were simulated. Both samples consisted of a nickel film with variable thickness. However, one film 

was assumed to be on thermally isolated substrate, whereas the other was on a conductive substrate. The 

overall higher demagnetization times of the isolated structure can be explained by slower heat dissipation, 

leading to overall higher ambient temperature and thereby slower magnetization dynamics. 

An important parameter governing the magnetization dynamics within the M3TM is the Elliott-Yafet spin-flip 

probability . In search for materials with deviating , and to explore the influence of multi-layer structures 

with many interfaces, we performed experiments on especially engineered Co/Pt composite multilayers over a 

wide fluence range (cooperation with the University of Kaiserslautern). The demagnetization traces can be well 

fitted in a global fitting routine to the M3TM model as shown in Fig. 3. In these fits, only the laser fluence is 

allowed to vary among the separate curves. The extracted demagnetization times (Fig. 4) are smaller than those 

of normal cobalt (typically around 250 fs) and the spin-flip parameter is 2 times as large. This is probably caused 

by the increased spin-orbit coupling at the cobalt-platinum interface. The individual data points in Fig. 4 

represent the demagnetization times as determined by fitting the individual demagnetization traces with the 

ordinary 3TM.


Output 

Nonlocal Ultrafast Demagnetization Dynamics in the High Fluence Limit 

K.C. Kuiper, G. Malinowski, F. Dalla Longa, and B. Koopmans 

Journal of Applied Physics 

(Accepted) 

M max 

/M 0 

1.0 

0.8 

0.6 

0.4 

0.2 

d = 5 nm 

d = 30 nm 

0.0 

0.0 0.5 1.0 1.5 2.0 2.5 3.0 

0 5 10 15 20 25 

T pump 

/T thickness (nm) 

C 

M 

* (fs) 

240 

220 

200 

180 

160 

isolated 

conductive 

Fig. 1 The maximum quenching of the magnetization 

( ) as a function of laser fluence . 

Open symbols: optically thin Ni film compared to the laser 

penetration depth, i.e. 15 nm. Closed symbols: optically 

thick Ni film 

Fig. 2 The demagnetization time ( ) as a function of the 

sample thickness on a thermally isolated and conductive 

substrate 

M/M 0 

1.0 

0.8 

0.6 

0.4 

0.2 

M 

* (fs) 

120 

100 

80 

60 

40 

data 3TM 

M3TM fit 

0.0 0.2 0.4 1 2 3 4 

delay (ps) 

0 20 40 60 80 

M max 

/M 0 

(%) 

Fig. 3 Demagnetization traces of a Co/Pt-multilayer for 

several laser fluences. The full lines represent the results of a 

global M3TM fitting routine. 

Fig. 4 The demagnetization times as determined by the 

M3TM compared to the fits of the individual traces of 

Figure 3 using the ordinary 3TM.


6.3.4 Towards ultrafast studies of current induced domain wall motion 

R. Paesen, E. Murè, K.C. Kuiper and Bert Koopmans. 


With the advent of spintronics, in 1996, the realization of a storage device based on current induced DW motion 

became an attractive challenge for the scientific community. Recently, shift register devices, based on the 

nucleation and displacement of several domains in the same structure, have been proposed and studied. For the 

development of such complex systems a better understanding and characterization of the domain wall 

dynamics is needed. This project is aimed at probing the DW dynamics on a picoseconds timescale. This is 

made possible thanks to the combination of electric and optical experimental schemes, based on the use of an 

ultrafast laser set-up (laser pulse duration 70 fs). Thanks to the pulsed excitation of DW dynamics, we expect to 

be able to exit the creep motion regime and study the DW motion in flow regime, deducing a clearer picture of 

the spin torque induced dynamics. 


The results achieved in the past year mainly concern the generation, propagation and detection of ultrafast 

current pulses. A SEM image of the test device used for it is shows in Fig. 1. The pulse generation is based on an 

Auston switch device: a pulsed laser is shone on a GaAs substrate, connected to two biased gold pads. The 

incident photons excite the electrons from the valence to the conduction band and make the GaAs 

instantaneously conductive. This allows us to convert the femtosecond laser pulse into an electric pulse, suitable 

for DW motion experiments. The Auston switch is connected to a gold wire with magnetic island patterned on 

it. Under the effect of an applied voltage the current pulse propagates through the gold wire, inducing an 

Oersted field around it. The study of the time resolved magneto optic Kerr effect (TRMOKE) on the Co islands 

permits information on the field pulse to be inferred. In Fig. 2 is shown the Kerr signal measured on a Co island 

for different values of the bias voltage applied to the waveguide. From the LLG analysis of those data we 

deduce a current pulse with typical rise time 

and a decay time 

The next step will be to nucleate a domain wall in a magnetic wire and use the ultrashort current pulses to 

excite its dynamics. Our magnetic wire consists of a Pt(4nm) / Co(0.5nm) / Pt(2nm) stack, characterized by a 

strong perpendicular magnetic anisotropy. A Ga focused ion beam is used to irradiate and damage a region of 

the wire, in such a way to create an anisotropy gradient and therefore a pinning site for DWs. The nucleation of 

a DW is done by applying a static magnetic field, whose value is comprised between the coercive field of the 

irradiated and non irradiated regions. The DW is then pushed by the current produced in an Auston switch. 

The use of extremely short pulses allows us to increase the current amplitude without risking a damage of the 

magnetic wire. Typically, a current pulse with a peak current density of about 

would induce a 

heating of about 30K (equivalent to the effect of a continuous current density of 

). A 1D model 

calculation shows that such a high current density ensures a DW displacement of the order of 100 nm, in the 

span of time of our measurement. The high temporal resolution and signal to noise ratio of the experiment is 

based on the use of a stroboscopic technique. Therefore the DW has to be resettled to its initial position after 

each measurement shot. This can be done by applying a second current pulse of opposite polarity of by using 

an offset magnetic field. In Fig. 3 is sketched the expected response of the DW position ( ) and canting angle 

( ) to a train of current pulses having the typical , and repetition rate achievable with our 

TRMOKE set-up. In Fig. 4 is shown a possible scheme of the final measurement set-up, suitable for both an 

optical detection (Kerr signal from the Pt / Co / Pt wire) and an electrical detection. Indeed we suggest the 

possibility to use a second Auston switch to perform a ‚pulsed‛ detection of the extraordinary Hall effect (EHE) 

at a Hall Cross patterned in the magnetic structure.


Output 

Toward ultra fast spin transfer detection 

R. Paesen 

Master project 

Fig. 1 Test sample for ultrafast current pulse generation and 

detection. The pump beam is shone on the GaAs region and 

produces a current pulse which propagates through the gold 

wire and induces a magnetic field on the Co islands. The 

probe beam is used to detect the Kerr signal from the Co. 

Fig. 2 Kerr signal measured on a Co island. The external 

field and the laser fluence are maintained constant and the 

voltage across the waveguide is varied, varying the intensity 

of the Oersted field on the Co. The inset shows the signal 

dependence on the voltage amplitude (e.g. on the amplitude 

of the current pulse). 

Fig. 3 Qualitative response of DW position and canting 

angle to a train of high density current pulses (extracted 

from 1D model calculations). 

Fig. 4 Schematic sample design for ultrafast CIDM 

experiment. This measurement scheme would permit us to 

do both the optical and electrical detection (double Auston 

switches scheme).


6.3.5 Resolving the genuine laser-induced ultrafast dynamics of exchange interaction in 

ferromagnet/antiferromagnet bilayers 

F. Dalla Longa, J.T. Kohlhepp and B. Koopmans 


The so-called exchange bias effect which describes the magnetic exchange coupling between a ferromagnet (FM) 

and an antiferromagnet (AFM) is an issue of practical interest because of its importance for the realization and 

the functionality of present and future magneto and spintronics devices, since it creates the possibility to 

manipulate the switching behavior of magnetic layers. As a result of the exchange interaction at the interface the 

FM displays a unidirectional anisotropy and its hysteresis loop is shifted along the field axis by a quantity HEB 

called exchange bias field. Although this effect was already discovered more than 50 years ago and also 

extensively exploited in magneto-electronic devices since about 20 years, a comprehensive microscopic 

understanding of the mechanisms involved is still far from being reached. A particularly exciting new challenge 

in the field is to control the spin dynamics by modifying the exchange interaction between FM/AFM bilayers. 

The aim of this project is gaining fundamental understanding of the phenomenon of exchange coupling and in 

particular the physics of the dynamics of the exchange interaction on the sub-picosecond time scale in carefully 

engineered samples. The experiments are carried out by driving exchange coupled bilayers out of equilibrium 

through laser excitation and following the dynamics of the ferromagnetic spins thereof by means of timeresolved 

magneto-optics. 


We chose for our study polycrystalline bilayers consisting of ferromagnetic Co and antiferromagnetic IrMn (see 

Fig. 1). The experimental geometry is sketched in Fig. 1: the sample lies in the x-y plane, the exchange bias field 

acts along the negative x direction, and an external field is applied in the sample plane along the y 

axis. The effective field acting on the FM magnetization is given by the vectorial sum of and 

When the laser hits the sample the exchange interaction is quenched, leading to a change in the orientation and 

magnitude of the (from point 1 to point 2) and therefore a torque acts on the magnetization and a 

precession according to the Landau-Lifshitz-Gilbert (LLG) equation is triggered. Precessional transients were 

measured with TRMOKE for a range of applied fields; one example is shown in Fig. 2. 

The precession is determined at each time by the value and orientation of the effective field acting on the 

magnetization at that particular time, and, in turn, these depend on the value of The idea was to retrieve 

the 

by carefully analyzing the precessional transients. To do this we wrote the LLG 

equation in the approximation of a small perturbation and invert it, expressing as a function of 

(the measured data), its derivative and its integral. The calculation was performed for all the measured 

transients, and the resulting field pulses have been averaged and smoothed by adjacent averaging over a period 

of 7 ps, yielding the plot shown in Fig. 3 (dots). The data could be fitted (dashed line) revealing that after laser 

excitation the exchange bias field is quenched to a minimum with a characteristic decay time 

, and its recovery can be described by two exponentials with time scales and 

The fitting function was then deconvoluted to get rid of the effect of the adjacent 

averaging, yielding the genuine temporal evolution of the exchange bias field (solid line). 

We conjecture that the quenching of the interface exchange interaction is caused by laser-induced disordering of 

the spins at the FM/AFM interface. A loss of spin ordering in the AFM which has a significant lower ordering 

temperature then the FM could trigger the fast quenching of . Therefore, we anticipate that our method 

could prove useful not only for investigating the dynamics of exchange interaction but also to indirectly probe 

the loss and recovery of magnetic ordering in AFM’s in the femtosecond regime.


Output 

Resolving the genuine laser-induced ultrafast dynamics of exchange interaction in 


F. Dalla Longa, J.T. Kohlhepp, W.J.M. de Jonge, and B. Koopmans 

Physical Review B 81, 094435 (2010) 

Fig. 1 Left: sketch of the experimental 

configuration showing the quenching of 

by an intense laser pulse and the 

consequent change of the equilibrium 

direction of the magnetization from A to B. 

Right: sample structure and normalized 

magnetization loops parallel and 

perpendicular to the exchange bias 

direction. 

hot e 

thermalized e-p-s 

Fig. 2 Precessional transient obtained at , 

the changing slope during the first 5 ps is highlighted by the 

dashed line. 

Fig. 3 Average of the reconstructed pulses on the short and 

long time scales (symbols); the solid line describes the 

exchange bias field pulse after deconvolution.


6.3.6 Magnetization dynamics in racetrack memory 

B. Bergman a , R. Moriya b , L. Thomas b , M. Hayashi b , X. Jiang b , and S.S.P. Parkin b and B. Koopmans 

a 

carrying out Ph.D. work at IBM 

b 

IBM Almaden Research Center, San Jose, California, US 


Several interesting concepts have been proposed recently for memory and logic devices based on the 

manipulation of magnetic domain walls (DWs). This has stimulated research into DW dynamics, particularly 

those resulting from interactions with current passing through the DW via the phenomenon of spin momentum 

transfer (SMT). This interaction can result in the motion of domain walls along magnetic nanowires and is a 

basic concept of the Magnetic Racetrack Memory. A better understanding of DW dynamics is of vital 

importance for the further development of such devices. 

In this project we probe the DW magnetization dynamics through an optical pump-probe technique, in which 

the magneto-optical Kerr effect (MOKE) is used to probe changes in a nanowire’s magnetization M when it is 

‘pumped’ with short current pulses. 

Results 

At IBM’s Almaden research laboratory a pump-probe Kerr magnetooptical scanning microscope has been 

developed. In order to control DW injection, motion and reset, magnetic fields have to be applied locally on the 

nanowire. For this a special Damascene CMOS chip has been fabricated at the 200 mm wafer facility at IBM 

Microelectronics Research Laboratory (MRL). Probing of the local magnetization is done with a focused pulsed 

laser spot of 400 nm diameter where the polarization rotation caused by the Kerr effect is measured after 

reflection. In order to achieve optimal focusing a perpendicular incident laser beam is focused with a high 

numerical aperture objective. Synchronized ‘pumping’ in this scheme is achieved by successively: 1. injecting a 

DW; 2. propagate the DW down the nanowire with either current through or an applied field pulse over the 

nanowire; and 3. resetting the whole nanowire to its original magnetization by applying a large field together 

with the injection of an opposite magnetic domain. With this setup field and current induced DW motion is 

studied in permalloy nanowires ranging in width from 200 to 700 nm and thickness of 20 nm. 

For control of DWs in Racetrack memory it is important to understand the different mechanism for driving a 

DW already in motion (dynamic) and driving a DW that is currently at rest (static). The propagation field, the 

minimum field below which no DW motion takes place, is measured for both dynamic DWs and static DWs 

(Fig. 1). It is found that Static DWs require a much higher field than DWs already in motion. A model is build 

where this effect is related to the wire roughness, successfully describing the existence of a propagation field, 

the difference between both propagation fields and a specific effect related to the method of injection. 

One important property affecting DW velocity and possibly also the critical current is Gilbert damping. Gilbert 

damping in permalloy can be tuned by doping the nanowires with osmium. This is used to prepare a sample 

series with increasing Gilbert damping. Measurement of both magnetic field and current-induced DW velocity 

revealed a profile well known that includes the Walker breakdown (a maximum field where further increasing 

field strength does not further increase the DW velocity). From these profiles the dependence of the Walker 

breakdown, DW mobility and maximum DW velocity, as well as the spin torque efficiency (β/α) on Gilbert 

damping has been determined.


Output 

An investigation of the static and dynamic domain wall propagation fields in permalloy nanowires using 

pump-probe Kerr microscopy 

Bastiaan Bergman, Rai Moriya, Masamitsu Hayashi, Luc Thomas, Bert Koopmans and Stuart S.P. Parkin 

(Submitted) 

M X 

/ M S 

Probability 

1 

0 

-1 

1 

0 

a 

b 

random 

Dynamic 

Static 

2.3 m 

2.7 m 

3.3 m 

4.3 m 

5.2 m 

0 Oe 

2 Oe 

4 Oe 

Fig. 1 Example of experiments and simulations 

distinguishing static and dynamic domain wall 

motion. (a) Experimental results of 

propagation field for dynamic DWs (open 

symbols) and static DWs (closed symbols) for 

five different starting positions. Incomplete 

switching (from –MS to +MS) should be seen as 

a limited probability for DW propagation. (b) 

Propagation probability for static DWs 

obtained from a 1D model using a wash board 

energy landscape. For DWs at a random initial 

position (half open symbols) and for DWs 

positioned using a bias field (closed symbols). 

For details see PhD thesis. 

0 5 10 15 

H (Oe)

60 FNA Annual Report 2010 6.Results

FNA Annual Report 2010 7.Output 61 

7 Output 

7.1 Publications 

Explaining the paradoxical diversity of ultrafast laser-induced demagnetization 

B. Koopmans, G. Malinowski, F. Dalla Longa, D. Steiauf, M. Faehnle, T. Roth, M. Cinchetti, and M. 

Nature Mat. 9, 259 (2010) 

Reduced DW pinning in ultrathin Pt-CoB-Pt with perpendicular magnetic anisotropy 

R. Lavrijsen, G. Malinowski, J.H. Franken, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans 

Appl. Phys. Lett. 96, 022501 (2010) 

Current-induced domain wall motion in Co/Pt nanowires: Separating spin torque and Oersted-field effects 

J. Heinen, O. Boulle, K. Rousseau, G. Malinowski, M. Klaeui, H.J.M. Swagten, B. Koopmans, C. Ulysse, G. Faini 

Appl. Phys. Lett. 96, (2010) 

Controlled domain-wall injection in perpendicularly magnetized strips 

R. Lavrijsen, J.H. Franken, J.T. Kohlhepp, H.J.M. Swagten en B. Koopmans 



W. Wagemans, P. Janssen, E.H.M. van der Heijden, M. Kemerink, B. Koopmans 


Fe:O:C: grown by focused-electron-beam-induced deposition; magnetic and electric properties 

R. Lavrijsen, R. Cordoba, F.J. Schoenaker, T.H. Ellis, B. Barcones, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans, 

J.M. De Teresa, C. Magen, M.R. Ibarra, P. Trompenaars, J.J.L. Mulders 

Nanotech 22, 025302 (2010) 

Resolving the genuine laser-induced ultrafast dynamics of exchange interaction in 


F. Dalla Longa, J.T. Kohlhepp, W.J.M. de Jonge, and B. Koopmans 

Phys. Rev. B 81, 094435 (2010) 

Spin transport and magnetoresistance in organic semiconductors 

W. Wagemans and B. Koopmans 

Phys. Stat. Sol. (b), Published on-line 3 November 2010 

Point-defect interactions in electron-irradiated titanomagnetites - as analyzed by magnetic after-effect 

spectroscopy on annealing within 80 K < T < 1200 K 

F. Walz, V.A.M. Brabers, H. Kronmueller 

J. Phys. Cond. Mat. 22, 046007 (2010) 

Separating photocurrent and injected current contributions to the organic magnetoresistance 

W. Wagemans; W.J. Engelen, F.L. Bloom, B. Koopmans 

Synth. Met. 160, 266 (2010) 

Spin relaxation and magnetoresistance in disordered organic semiconductors 

P.A. Bobbert, T.D. Nguyen, W. Wagemans, F.W.A. van Oost, B. Koopmans, M. Wohlgenannt 

Synth. Met. 160, 223 (2010)

62 FNA Annual Report 2010 7.Output 

7.2 Presentations 

Femtosecond laser-induced demagnitzation from a thermodynamic perspective 

B. Koopmans 

Magnetism & Magnetic Materials; Washington, DC, USA (18 jan - 22 jan 2010) 

Sub-surface nanoclusters and near-surface quantum wells in anisotropic metals (Invited) 

O. Kurnosikov 

Invited talk at Georg-August-Universitaet Gottingen, Germany (29 jan 2010) 

Laser-induced femtosecond magnetization dynamics - Reconciling a paradoxical history 

B. Koopmans 

Kolloquium Forschungszentrum Juelich, Deutschland (29 jan 2010) 

Spin in organics, a new route to spintronics (Invited) 

B. Koopmans 

Fruhjahrstagung 2010; Molecular Spintronics; Regensburg, Germany (21 jan - 29 jan 2010) 

Molecular Spintronics - Currrent status and Challenges (Focused Session) 

Spin electronica - geleiding en toepassing op de nanometer schaal 

H.J.M. Swagten 

Annual DocNdag; Eindhoven, Netherlands, the (17 mrt 2010) 

Spinning dynamics! - New trends in spintronics (Invited) 

B. Koopmans 

Fysica 2010; Utrecht, Netherlands, the (23 apr 2010) 

Focussessie Nanofysica 

Spinnende elektronica - Spannende technologie 

B. Koopmans 

Voordracht voor alumnivereniging Veni, Trafalgar Pub, Eindhoven (2 mei 2010) 

Recent progress on domain-wall physics in perpendicular systems (Invited) 


Trends in Spintronics, Korean-Dutch Workshop; Eindhoven, Netherlands, the (23 jun - 27 jun 2010) 

Influencing the exchange interactions at ferromagnet/antiferromagnet interfaces (Invited) 

J.T. Kohlhepp 


Spin in organics, a new route to spintronics (Invited) 

B. Koopmans 


Dutch-Korean Workshop Trends in Spintronics 

Tunable domain wall injection in perpendicularly magnetized strips 

J.H. Franken, M. Hoeijmakers, R. Lavrijsen, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans 

Joint European Events Office; Krakow, Poland (22 aug - 28 aug 2010) 

Spin-spin interactions in organic magnetoresistance 

W. Wagemans, A.J. Schellekens, M. Kemper, F.L. Bloom, B. Koopmans 

SPINOS III - Conference on Spins in Organic Semiconductors; Amsterdam, Netherlands, the (30 aug - 3 sep 

2010) 


P. Janssen, W. Wagemans, E.H.M. van der Heijden, M. Kemerink, B. Koopmans 



Investigating OMAR device models experimentally 

F.L. Bloom, H. Moons, J.M. Veerhoek, E. Goovaerts, B. Koopmans 


2010)


Subsurface nanoclusters in metallic substrate: localized quantum wells, electron interference and inner 

electron 

O. Kurnosikov, C.O. Avci, H.J.M. Swagten, B. Koopmans 

European Conference Surface Science; Groningen, Netherlands, the (29 aug - 3 okt 2010) 

Femtosecond laser-induced magnetization dynamics (invited) 

B. Koopmans 

Workshop 2010 Strongly correlated transition metal compounds III; Bergisch Gladbach, Germany (8 sep - 10 sep 


Domain walls in perpendicularly magnetized stripes violating spin-transfer torque? (Invited) 

R. Lavrijsen, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans 

International symposium on metallic multilayers; Berkeley, California, U.S. (13 sep - 17 sep 2010) 

De diverse gezichten van nanotechnologie (Popular scientific) 

B. Koopmans 

Workshop met docenten middelbare school; Venice, Italy (27 sep - 27 sep 2010) 

Femtosecond magnetization dynamics - From demagnitization to spin transfer (Invited) 

G. Malinowski, B. Koopmans 

Workshop Laser-induced magnetization in nanostructures; Stoos, Switzerland (6 okt - 8 okt 2010) 

Controlling domain wall motion in perpendicularly magnetized materials 

J.H. Franken 

Joint FOM Spin/NanoNed Spintronics Meeting; Nijmegen, Netherlands, the (14 okt - 15 okt 2010) 

Domain wall motion in perpendicularly magnetized strips. Violating spin transfer torque? 

R. Lavrijsen 


EBID of magnetic nanostructures 

T.H. Ellis 


Magnetism and dynamics of Pt/Co/ for domain wall devices 

A.J. Schellekens 


Ultrafast spin-transfer torque in nanomagnets 

B. Koopmans 


Tunable domain wall injection and pinning in perpendicularly magnetized strips 

J.H. Franken, M. Hoeijmakers, R. Lavrijsen, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans 

Conf. Magnetism & Magnetic materials; Atlanta, Georgia, U.S. (14 nov - 18 nov 2010) 

Domain wall in perpendicularly magnetized stripes violating spin-transfer torque? 

R. Lavrijsen, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans 



P. Jansssen, W. Wagemans, E.H.M. van der Heijden, M. Kemerink, B. Koopmans 


Nonlocal ultrafast magnitization dynamics in the high fluence limit 

K.C. Kuiper, G. Malinowski, A.J. Schellekens, B. Koopmans, T. Roth, S. Alebrand, D. Steil, M. Cinchetti, M. Aeschlimann 


Nano-engineering for magnetic domain wall devices (Invited) 

J.T. Kohlhepp 

MicroNano conference; Twente, Netherlands (17 nov - 18 nov 2010) 

Spin in organics, a new route to spintronics (Colloquium) 

B. Koopmans 

Kolloquium Universiteit Duisburg-Essen Germany (1 dec 2010)


Laser induced magnetization dynamics in exchange coupled FM/AFM Co/Ir/Mn bilayers (Invited) 

J.T. Kohlhepp 

Internation conference of AUMS, 2010; Jeju Island, Korea (5 dec - 8 dec 2010) 

Local formation of a Heusler type structure in CoFeAl current perpendicular to the plane GMR spin valves 

J.T. Kohlhepp 

Internation conference of AUMS, 2010; Jeju Island, Korea (5 dec - 8 dec 2010) 

Tunable domain-wall injection and propagation in ferromagnetic nanowires (Invited) 


Internation conference of AUMS, 2010; Jeju Island, Korea (5 dec - 8 dec 2010)


7.3 Chapters 

Magnetoresistance and spin transport in organic semiconductor devices 

M. Wohlgenannt, P. A. Bobbert, B. Koopmans, and F. L. Bloom 

In: Organic Spintronics 

Ed. by: Z.V. Vardeny (Taylor & Francis, 2010), pp. 67-136 

Magnetic tunnel junctions 

H.J.M. Swagten, P.V. Paluskar 

In: Encyclopedia of Material Science & Technology 

Ed. by: (Elsevier Ltd., ISBN: 978-0-0804-3152-9, p. 1-7, 2010), pp. 1-7 

7.4 Guest Lectures 

Towards nanomagnetic probing in SEM-STM 

Serhiy Vasnev (Radboud University, Nijmegen) 

FNA seminar, 6 apr 2010 

Influence of local laser heating on domain wall propagation 

Philipp Möhrke (University of Konstanz) 

FNA seminar, 16 apr 2010 

Electron scattering and Kondo resonances at Co subsurface impurities in copper 

M. Wenderoth (Uni. Goettingen, Germany) 

FNA seminar. 17 mei 2010


7.5 Posters 

Laser-induced magnetization dynamics in Co/Pt based multilayers 

K. Kuiper, G. Malinowski, R. Lavrijsen, B. Koopmans 

FOM -dagen jan. 2010; Veldhoven, Netherlands, the (19 jan - 20 jan 2010) 

Magnetic nanowires by EBID 

R. Lavrijsen, F. Schoenaker, T. Ellis, B. Barcones, H. Swagten, B. Koopmans, J. de Teresa, R. Cordoba, H. Mulders, P. 


Organic magnetoresistance: unravelling the origin of sign changes 

P. Janssen, W. Wagemans, F.L. Bloom, B. Koopmans 


Multiple quantum wells states induced by subsurface nano-reflectors 

C.O. Avci, O. Kurnosikov, H.J.M. Swagten, B. Koopmans 

Dutch SPM symposium; Eindhoven, Netherlands, the (5 feb 2010) 

Organic magnetoresistance: unravelling the origin of sign changes 

P. Janssen, W. Wagemans, F.L. Bloom, B. Koopmans 

cNM Research Day 2010; Eindhoven, Netherlands, the (30 jun 2010) 

Magnetic effects in FeNi structures codeposited with atom nanolithography 

T. Meijer, J. Beardmore, E. Vredenbregt, M. Hoeijmakers, J.H. Franken, B. Koopmans, T. van Leeuwen 

NNV-AMO; Lunteren, Netherlands (12 aug 2010) 

Modeling spin interactions in disordered organic semiconductors 

A.J. Schellekens, M.J.M. van Schijndel, W. Wagemans, B. Koopmans 



Magnetic properties of Fe nanowires grown by focused-electron-beam-induced deposition 

R. Lavrijsen, R. Cordoba, F. Schoenaker, T. Ellis, B. Barcones Campo, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans, J.M. 

de Teresa, C. Magen, M.R. Ibarra, P. Trompenaars, J.J.L. Mulders 

International symposium on metallic multilayers; Berkeley, California, U.S. (13 sep - 17 sep 2010) 

Tunable domain wall injection PMA stripes by focused He and Ga beams 

J.H. Franken, M. Hoeijmakers, R. Lavrijsen, J.T. Kohlhepp, E. van Veldhoven, D.J. Maas, H.J.M. Swagten, B. Koopmans 

International symposium on metallic multilayers; Berkeley, California, U.S. (13 sep - 17 sep 2010)


7.6 PhD Theses 

Plastic Spintronics; spin transport and intrinsic magnetoresistance in organic semiconductors 

W. Wagemans 

June 2010 

Organic Magnetoresistance; An investigation of microscopic and device properties 


November 2010 

7.7 Master Theses 

Exploring spin interactions in organic semiconductors 

A.J. Schellekens 

February 2010 

Domain wall motion in perpendicularly magnetized ultrathin Pt/CoFeB/Pt films 

J.H. Franken 


Exploring the fabrication of ferromagnetic nanostructures by electron beam induced deposition 

F.J. Schoenaker 


Near surface quantum wells induced by buried nanoparticles in Copper 

C.O. Avci 

Augustus 2010 

Novel experimental and modeling approaches to organic spin-valves (Intern) 

M.J.M. van Schijndel 

October 2010 

Toward ultra fast spin transfer detection 

R. Paesen 

October 2010 

Nano-stencil fabrication process for spin-torque devices 

P.E.D. Soto Rodriquez 

November 2010 

Racing domain walls - A micromagnetic study 

G.C.F.L. Kruis 

December 2010


7.8 Internship Reports 

The Gilbert damping constant in Ga irradiated Pt/CoFeB/Pt 

M. Herps 

January 2010 

Innovative materials and characterization techniques for organic spin valve devices 

M. Hoeijmakers 

ISMN-CNR Bologna, Italy 

March 2010 

Resistive behavior of Fe-rich amorphous 

M.M. Haverhals 

University of Uppsala, Sweden 

Augustus 2010 

Gilbert damping of perpendicularly magnetized Pt/Co/ 

F.H.A. Elich 

October 2010


7.9 Publicity 

(See also pages 72-76 for a selection of items shown below) 

Bronzen schaatsmedaille voor professor Koopmans 

N.a.v. behalen van derde plaats in het gewestelijk kampioenschap marathonschaatsen voor Brabant, Limburg 

en Zeeland in de categorie Masters 

Cursor 16 (21 jan 2010) 

En ik vind… BKO op herhaling 

Blog Henk Swagten 

Cursor, (4 feb 2010) 

VICI-hoogleraar Henk Swagten, interview 

Reprint in first trial edition of N! 

N! periodic of SVTN "J.D. van der Waals", STOOR and VENI, (spring 2010) 

Nieuws op onderzoeksgebied; Snellere harde schijven door laserlichtflitsen 

Interview met B. Koopmans 

Matrix (spring 2010) 

Spins in organische materialen 

Interview met A.J. Schellekens 

N! periodic of SVTN "J.D. van der Waals", STOOR and VENI, (autumn 2010) 

OMAR: over plastic en koelkast magneetjes 

Interview met Wiebe Wagemans 

Cursor, (17 jun 2010) 

OMAR: over plastic en magneetvelden 

Interview met Wiebe Wagemans 

MATRIX, (autumn 2010) 

Vraag het vier vaders 

Paul Janssen (board member VENI), interview met Bram van Gessel, Maikel Goosen, Jurgen Schoonus and Henk Swagten 

N! periodic of SVTN "J.D. van der Waals", STOOR and VENI, (summer 2010) 

Hoe?zo! Radio (RTN radio) 

Interview met Bert Koopmans 

Hilversum, (30 sep 2010) 

Nominatie voor Huijbregtsenprijs ‘Wetenschap & Maatschappi’ 

B. Koopmans 

Ridderzaal, Den Haag, (1 nov 2010)


70 

En ik vind... 

BKO op herhaling 

Prof. Fred Steutel, emeritus hoogleraar Wiskunde, vraagt zich in de rubriek ‘En ik vind


71 

Bronzen schaatsmedaille voor professor Koopmans 

21 januari 2010 - Prof.dr. Bert Koopmans van de faculteit Technische Natuurkunde is tijdens het 

gewestelijk kampioenschap marathonschaatsen voor Brabant, Limburg en Zeeland derde 

geworden in de categorie Masters. Op een natuurijsbaan in Wijk en Aalburg moest hij woensdag 

13 januari slechts twee andere concurrenten voor laten gaan. 

Koopmans bereikte de finish na 45 ronden, omgerekend ruim twintig kilometer. Over de piste niets 

dan lof. ‚Geweldig ijs. Geen scheurtje te zien. Ze hadden de baan goed geveegd en de bochten lagen 

er prachtig bij. Ach, goed of slecht, natuurijs is altijd een genoegen.‛ 

Het gebeurt niet iedere dag dat een TU/e-hoogleraar een medaille haalt. En zeker niet tijdens een 

kampioenschap op natuurijs, dat door de zachte winters nog maar sporadisch mogelijk is. Maar dat 

Koopmans in de prijzen rijdt, is voor insiders niet echt een verrassing. Hij mag dan wel hoogleraar 

Fysica van Nanostructuren zijn, maar is tevens een begenadigd schaatser. Geboren in Norg, in de 

strenge winter van 1963. Het legendarische schaatsjaar waarin Reinier Paping de Elfstedentocht 

won. 

In zijn jeugdjaren zat hij in de schaatsselectie van Jong Oranje en op zijn 23ste kreeg hij een plaats 

binnen de Nederlandse kernploeg. Baantjes draaien met Hein Vergeer en Leo Visser, met een 

voorliefde voor de lange afstand. Op NK’s veroverde hij een reeks medailles en in 1987 was hij 

reserve voor het EK en WK. ‚De mate van getraindheid is tegenwoordig wat minder‛, zegt hij. ‚Ik 

oefen nog maar twee keer een uurtje per week op de ijsbaan in Eindhoven.‛ (FvO)/.


72 

Reprint


73 

OMAR: over plastic en koelkast magneetjes 

17 juni 2010 - Stroomgeleidende plastics vinden razendsnel 

toepassing in displays en lichtgevende folies (op basis van 

zogeheten OLED’s) en zonnecellen. Maar daarmee zijn de 

grenzen van deze plastics, die in potentie snel, goedkoop en 

milieuvriendelijk kunnen worden geproduceerd, nog niet 

bereikt. Dr.ir. Wiebe Wagemans onderzocht in de groep 

Physics of Nanostructures van prof.dr. Bert Koopmans de 

effecten van magneetvelden op geleidende plastics. 

Wiebe Wagemans bij de gloveboxopstelling. 

Foto: Bart van Overbeeke 

Wiebe Wagemans (29) vertelt enthousiast over zijn promotieonderzoek: ‚Zo rond de tijd dat ik bij Koopmans afstudeerde, werd duidelijk 

dat de weerstand voor stroomgeleiding van deze plastics verandert als je ze blootstelt aan een magneetveld.‛ Een intrigerend fenomeen, 

vindt hij. ‚De weerstand kan wel tot wel tientallen procenten veranderen bij magneetvelden kleiner dan een koelkastmagneet. En dat ook 

nog eens gewoon bij kamertemperatuur. Van vrijwel alle materialen verandert de elektrische weerstand onder invloed van magneetvelden, 

maar dat is vaak pas bij heel sterke magneetvelden of extreem lage temperaturen.‛ 

Dat dit magnetische effect, OMAR gedoopt (van ‘organic magnetoresistance’), optreedt onder zulke milde omstandigheden maakt het 

volgens Wagemans zeer bruikbaar voor toepassing in magneetsensoren. Het effect lijkt bovendien te bestaan bij alle geleidende plastics. 

‚OMAR is zelfs gewoon zichtbaar in de bestaande OLED’s. Het lijkt erop dat je magnetische pennen kunt ontwikkelen waarmee je op 

OLED’s kunt schrijven, daarvoor hoef je de OLED niet eens van een extra laag te voorzien.‛ Je hebt ook geen heel bijzonder materiaal nodig, 

zegt Wagemans. Vrijwel elk plastic dat stroom geleidt, vertoont het OMAR-effect. ‚Het maakt niet uit of je plastics maakt uit lange 

polymeren, of dat je organische materialen maakt uit kleine moleculen. Alleen voor de sterkte van het effect maakt het wat uit of je bij wijze 

van spreken spaghetti of macaroni gebruikt.‛ 

Magnetoresistance is een term die ook bekend is uit de reguliere elektronica. Zo werken afleeskoppen van moderne harddisks op basis van 

giant magnetoresistance (GMR), dat de ontdekkers in 2007 nog de Nobelprijs opleverde. Toch werkt OMAR heel anders dan zijn 

tegenhanger in metalen. Dat hangt samen met de manier waarop lading wordt getransporteerd in plastics, zegt Wagemans. ‚De elektronen 

zijn er minder vrij dan in metalen. Ze hoppen echt van een plek naar de volgende, waarna ze weer een tijdje stilzitten. Ze gaan 101 stappen 

naar voren en 100 terug. Daarbij bewegen ze zich als het ware door een soort berglandschap, waar ze over bergpaadjes moeten wandelen. 

Hierdoor komen de elektronen elkaar ook regelmatig tegen. Stel nu dat er een elektron in een kuil is beland en een ander elektron er langs 

wil, dan lukt dat alleen als de twee elektronen tegengestelde spin hebben.‛ 

Spin is een eigenschap van elektronen die kan worden opgevat als een inwendig magneetje dat ontstaat doordat het elektron snel om zijn as 

‘spint’. De richting van deze elektronspin bepaalt of het ‘vrije’ elektron het elektron in de kuil gemakkelijk kan passeren. ‚Twee elektronen 

die elkaar ontmoeten, hebben in de helft van de gevallen tegengestelde spin. Dan hebben ze geen last van elkaar. Zolang de spins echter in 

dezelfde richting staan, zitten ze vast.‛ Gelukkig gaan de spins draaien als ze worden blootgesteld aan kleine lokale magneetvelden, 

opgewekt door naburige waterstofatomen. Dan is het een kwestie van wachten totdat de spins een verschillende richting krijgen en het vrije 

elektron kan doorstromen. En die wachttijd bepaalt de elektrische weerstand van het materiaal. 

Wagemans: ‚Omdat de magneetvelden van de omliggende waterstofatomen in willekeurige richting wijzen, komen de spins op een 

gegeven moment altijd in tegengestelde richting te staan. Behalve wanneer je een sterker extern magneetveld aanlegt; dan gaan alle 

elektronen daar op dezelfde manier omheen draaien en verandert de relatieve oriëntatie niet.‛ Met andere woorden: met een extern 

magneetveld blijven de elektronen in een impasse zitten, met een grotere elektrische weerstand als gevolg. 

Wagemans stelde een eenvoudig model op waarin slechts het elektron in de kuil en de wachtende voorbijganger figureren. ‚Heel simpel, 

maar het bevat alle essentiële ingrediënten.‛ Daarnaast bracht hij veel tijd in het lab door, waar hij een gloveboxopstelling -een luchtdichte 

ruimte waar je van buitenaf met handschoenen bij kunt- bouwde voor de organische samples die hij maakte met hulp van de groep van 

prof.dr.ir. René Janssen, expert op organische zonnecellen en andere geleidende plastics. ‚Die samples zijn heel gevoelig voor water en 

zuurstof. Daarom heb ik de experimenten in de glovebox onder beschermende stikstofatmosfeer uitgevoerd.‛ 

In de glovebox positioneerde Wagemans de samples tussen twee forse elektromagneten, waarmee hij de sterkte en frequentie van het 

magneetveld kon variëren. Hij ontwikkelde een extra gevoelige techniek om de invloed van de oriëntatie en frequentie van het externe 

magneetveld op de weerstand van het sample te kunnen meten. Hierdoor ontdekte Wagemans dat de spins niet alleen afhankelijk zijn de 

magneetvelden uit de omgeving, maar dat de elektronspins ook met elkaar ‘praten’, zoals hij het noemt. 

Wagemans onderzocht ook of je geleidende plastics kunt gebruiken in GMR-sensoren. ‚In principe werkt dit zelfs beter met organisch 

materiaal dan met koper, omdat de spin van de elektronen beter behouden blijft in organische materialen. Dat blijkt te kloppen, maar je 

hebt nog wel last van de wisselwerking tussen de elektronspins onderling.‛ 

Toch concludeert Wagemans dat het zeker mogelijk is om afleeskoppen en andere magnetische elementen van organische materialen te 

maken. ‚Je moet dat niet zien als concurrentie voor de huidige metalen GMR-systemen, maar het is voor de organische elektronica wel een 

belangrijke stap vooruit als je sensoren en ook geheugenelementen van plastic kunt maken.‛ (TJ)/.

74 FNA Annual Report 2010 7.Output 75 

74

FNA Annual Report 2010 8.Social Events 75 

8 Social Events 

Publication parties 

Within FNA it is common that great academic success 

is followed up by even greater parties, where the 

group comes together with food, beer and sometime 

even champagne. This year’s scientific output ensured 

that many afternoons ended with a toast. 

Garden party 

Also this year the whole group moved to the far east 

of Nuenen to enjoy the hospitability of Bert during the 

yearly garden party. Unless the blistering cold, most 

group members stayed outside until the bitter end, 

while enjoying the great food, wine, beer and other 

drinks. 

Spin ‘m d’r in – Championship 

The final season of the indoor soccer 

championship was one not to forget. The 

talented squad of Spin ‘m d’r in won all 

of their matches with relative ease and 

therefore ranked 1 st in their competition! 

Unfortunately team coach Koen forgot to 

thank his team with delicious ‘vlaai’, but 

star players Jeroen and Sjors saved the 

day so that FNA could celebrate their 

unbeaten championship. 

TU/e Experience 

Since we are always eager to share our dedication 

to fundamental physics with a broad audience, 

FNA highly contributed to the successful TU/e 

Experience (formerly known as the 

‘publieksdag’). With for example a challenging 

Nanoquiz, a tour through the decorated ‘kopzaal’ 

and a demonstration of the magnetic coil gun, 

FNA managed to give many people some more 

insight in the work that we do. 

Table tennis tournament 

When someone enters the e-wing of our n-laag building, there is one object in particular that will immediately 

draw your attention. One of the two student rooms is equipped with a professional table tennis setup to prevent 

the hard working students to suffer from RSI. The combination of the setup and a group of ambitious FNA 

physicists of course resulted in a challenging tournament. This year Koen and Jef made it towards the final, but 

after a true epic battle, Koen turned out to be the deserved winner of the tournament!

76 FNA Annual Report 2010 8.Social Events 

Group outing 

By Sjors Schellekens 

Every year there is one day in particular where physics is not the most debated topic between the group 

members. This day is of course the legendary annual FNA group outing. This year the bus with all the members 

set course for Oirschot, where some typically Dutch activities were prepared waiting. After arriving at a 

beautiful small farm, and drinking a fast coffee to make sure that everybody was awake, the group was split in 

two. The first group started ‚Skiking‛, which can best be described as cross-country skiing on wheels. After a 

hesitating start everybody got the hang of it, and a wonderful tour through the countryside was made. 

Soaked in sweat the group finally skiked back to the farm, where the other group was playing the typically 

Dutch game ‚Ganzebord‛. However, this version of Ganzebord was not played on a board but on a small 

pasture. The players had to get to the end of the game by rolling an enormous dice and answering difficult 

questions about the Dutch countryside. As if this was not hard enough, players had to cooperate in small 

groups of two, were the legs of the players were tied together. After a tough game of Ganzebord everybody was 

hungry, indicating that it was time for typically Dutch farmer’s lunch. 

When all group members were stuffed, both groups switched activities. The day at the farm ended by another 

typically Dutch game, namely ‚klootschieten‛. The idea of the game is simple; each team has to throw a ball 

along a route over the countryside. The team that requires the smallest amount of throws to get to the finish 

wins. However, in practice the game turned out to be far more difficult than expected. Sharp turns, shallow 

waters and large trees were all over the route, making the journey to the finish far from trivial. Luckily all teams 

made it to the end without losing the precious kloot. 

They day ended back in Eindhoven, were Can organized a dinner at a delightful Turkish restaurant. The food 

was terrific, the Turkish beer was surprisingly good, and the atmosphere was even better. A perfect way to end 

a successful group outing, let’s hope next year’s outing will be as good as this one! 

Sinterklaas 

In december 2010, Sinterklaas and his 

Black Pete (Mathijs and Matthijs) came 

all the way from Boekel to Eindhoven to 

visit the group of FNA, where they 

surprised the members with spicenut, 

marzipan, chocolate, drinks and other 

traditional snacks. On top of that, each 

member was given a small gift and a 

short but brilliant poem, mostly 

containing a wholehearted hint for the 

future.

FNA Annual Report 2010 8.Social Events 77 

Oudejaarsbraspartij 

If you ask one of the FNA members about the best way to celebrate Christmas and New Year’s Eve in one night, 

they will definitely tell you about their yearly ‘Oudejaarsbraspartij’. At this evening, all members came together 

in the decorated presentation room and contributed to the dinner with starters, main courses and deserts. This 

year, the menu (which was coordinated by Sonja, girlfriend of Matthijs Cox) consisted for example among other 

things of Freshly Fried Turkish Feta Rolls, Traditional Italian Tiramisu and Extremely Heavy Meat Loaf. Despite 

the surprising absence of ‘Oliebollen’ on the menu, we all can 

say that it just was delicious! The evening was ended up in 

style by the traditional ‘piekschieten’ (tree-peak-shooting) and 

the election of the Blunderbokaal election. 

Blunderbokaal 

The person who is responsible for the greatest work 

related blunder within FNA, is every year rewarded with 

the famous Blunderbokaal. This year, the competition 

was fierce, but at the end it was clear that the nomination 

of Sjors Schellekens was most voted for. In spite of his not 

so strong pledge for innocence, he won the trophy for his 

attempt to ruin both of the laser setups and thereby 

bringing himself (and others) in danger. Sjors did not 

want to comment on his blunder, but claimed that he 

received the ‘Blunderbokaal’ on a personal note

FNA Annual Report 2010 - Technische Universiteit Eindhoven

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