Applications of Spin-Polarized Electrons in Condensed ... - CASA

Applications of Spin-Polarized Electrons in
Condensed Matter Physics
Dan Pierce
Electron Physics Group, NIST
http://physics.nist.gov/epg
Supported in part by the Office of Naval Research

Principle of GaAs Polarized e - Source
Optical excitation of spin polarized electrons
For positive helicity σ + light, the theoretical polarization of the
electrons photoexcited by bandgap radiation is
N −N
N + N
P= ↑ ↓
↑ ↓
=
1-3
1+3
= -50%
Polarization is easily and rapidly reversed by switching helicity of light
Pierce and Meier, PR B 13,5484-5500 (1976)

Negative Electron Affinity (NEA) GaAs
Surface treatment gives world’s best photoemitter
Source characteristics:
P ~ 30% for bulk wafer
I = 20 µA/mW
continuous or pulsed operation
low initial energy and energy spread
high figure of merit = P 2 I

Charlie’s Hippie Days

Spin Polarized Electron Gun
Spin polarized e - scattering apparatus
Measure
I −I
A= I + I
↑↑ ↑↓
↑↑ ↑↓
∝M
Pierce, Celotta, Wang, Unertl, Galejs, Kuyatt, Mielczarek, RSI 51, 478-499 (1980)

Polarized e - Gun for SLAC Parity Violation Experiment
Beam polarization, 35% to 45% Beam current, 1 mA cw to 15 A peak

Complete Polarized e - Injection System
for SLAC Parity Violation Experiment

Technical Support in the Early Days

Development of Measurement Techniques
Using Spin-Polarized Electrons
Spin-polarized electron gun (NEA GaAs)
Spin polarized electron scattering
Spin polarized low energy electron diffraction (SPLEED)
Spin polarized inverse photoemission spectroscopy (SPIPES)
Spin polarized electron energy loss spectroscopy (SPEELS)
Spin polarized low energy electron microscopy (SPLEEM)
Electron spin polarization analyzer
Spin polarized photoemission
Spin polarized Auger electron spectroscopy
Spin polarized secondary electron emission
Scanning Electron Microscopy with Polarization Analysis (SEMPA)

Surface Sensitivity
Surface sensitive due to short electron mean free paths
Inelastic mean free path especially short in transition metals with many d holes
Spin dependent attenuation length in ferromagnets at low energy
Pappas et al, PRL 66, 504 (1991)

Spin Polarized Electron Gun
Spin polarized e - scattering apparatus
Measure
I −I
A= I + I
↑↑ ↑↓
↑↑ ↑↓
∝M
Pierce, Celotta, Wang, Unertl, Galejs, Kuyatt, Mielczarek, RSI 51, 478-499 (1980)

Spin Polarized Low Energy Electron
Diffraction (SPLEED)
First surface magnetization measurement with SPLEED
IAA , IAB
A=A(E,θ,H,T)
θ
A(H)
Test for true magnetic effect by measuring field and temperature dependence
A(T)
Celotta, Pierce, Wang, Bader, Felcher, PRL 43, 728 (1979)

Spin-Polarized Electron Scattering
Measure low temperature surface magnetization
Ni 40 Fe 40 B 20
Thermal excitation of spin waves
M( T)
=- 1 BT32
/ + .... Theory:
M(
0)
B
B surface
bulk
B s /B b =3
Results explained by extending theory to include
weaker exchange coupling of surface to bulk, J S⊥ 2

Spin-Polarized Electron Scattering
Determine surface critical exponent
A lot of theory but few experiments for
magnetic critical behavior at surfaces
A ex ∝ t β , where t=[1-(T/T C )]. Find β≅0.78 for both
Ni(110) and Ni(100) surfaces, independent of E and θ
Neutron measurements give β=0.38 for bulk
Alvarado, Campagna, Hopster, PRL 48, 51 (1982)

Spin Polarized Inverse Photoemission Spectroscopy
A= P
Spin dependent bandstructure of unfilled states
Energy, angle (k), and spin-resolved inverse photoemission
Photon detector sensitivity
Measure
hw = 97 . ± 035 . eV
F
G
HG
1
0 cosa
I
J
KJ
A B
A B
n -n
n + n
Large minority spin peak just above
Fermi level from the d-holes
responsible for ferromagnetism in Ni
Unguris, Seiler, Celotta, Pierce, Johnson, Smith, PRL 49, 1047 (1982)

Running with Friends
Bay to Breakers

Stopping to read while others catch up

Tough

Happy on top
of the mountain

A Sinclair Grand Canyon
Expedition

You’re not tryin’ if
your not bleedin’!
Determined

Principled
Two things I can’t
tolerate:
Incompetence and
Cold Coffee

Married in 1983!

How ‘bout them legs

Spin Polarized Secondary Emission
High spin polarization where have high secondary intensity
Fe 81.5 B 14.5 Si 4
Valence electron polarization
n −n
n
P=
↑ ↓
n + n
=
n
=
↑ ↓
B
V
M =-m ( n -n
)
B
# Bohr magnetons
# Valence electrons
A B
n B n V P
Fe 2.2 8 0.28
Co 1.7 9 0.19
Ni 0.055 10 0.055
Unguris, Pierce, Galejs, Celotta, PRL 49, 72 (1982)

Scanning Electron Microscopy
with Polarization Analysis (SEMPA)
High resolution magnetization imaging
Incident Electrons From
Scanning Electron Microscope
Magnetic
Specimen
Spin-Polarization Analyzer
Spin-Polarized
Secondary Electrons
Magnetization
M= -µ B (N ↑ -N ↓)
P = N ↑ -N ↓
N ↑ + N ↓

NIST SEMPA Apparatus
Auger
Electron
Energy
Analyzer
Ion Gun
Secondary &
Backscatter
Electron
Detectors
Electron
Source
Sample
RHEED Screen
In-Plane
Metal
Evaporators
Polarization
Analyzers
Out-of-Plane
Acquisition time (min)
1000
100
10
1
0.1
Old LaB 6
New Field Emission
10 keV
20 keV
0.01
1 10 100 1000
Beam diameter (nm)

4 cm
Low Energy Diffuse Scattering Analyzer
Input Optics
Anode
MCP
G2
G1
E2
Drift Tube
E1
Au & C Targets
150 eV e -
Y
A
D B
C
End View
Au evaporator
X

SEMPA Example – Iron Crystal
Measure the topography and two magnetization components
(simultaneously but separately):
Intensity
Then derive the
magnetization
direction and
magnitude:
M x
Direction
θ = tan -1 (M x/M y)
M y
Magnitude
⏐M⏐ = (M x 2 + My 2 ) 1/2

Out-of-plane M
M contour
In-plane M
Cobalt Vector Magnetization

SEMPA – MFM Comparison
Hard disk test sample with Au patterned overlay
SEMPA
Intensity M AFM MFM
350 nm

Interlayer Exchange Coupling
Cr wedge schematic
Cr Wedge
∼ 500 µm
Fe Whisker
M
Fe Film
∼ 1-2 nm
0 - 20 nm
Unguris, Celotta, Pierce, PRL 67, 140 (1991)

Fe/Cr/Fe Oscillatory Exchange Coupling
M
P(Fe)
P(Cr)
0.2
0.0
-0.2
0.04
0.0
-0.04
Precise measurement of coupling periods
RHEED
0 10 20 30 40 50 60 70 80
Cr Thickness (layers)
Interlayer Measured period
(layers)
Cr
0.144 nm/layer
Ag
0.204 nm/layer
Au
0.204 nm/layer
2.105 ±0.005
12 ±1
2.37 ±0.07
5.73 ±0.05
2.48 ±0.05
8.6 ±0.3

Leadership

Vision
We’ll get there if
you can keep up
with me

Between a rock and a hard place

Magnetic Depth Profiling Co/Cu Multilayers
Magnetoresistance and domain structure
Current through magnetic multilayer depends on magnetization alignment,
but real domain structures can be complex:
Cu
Co
Ion milled crater
(2 keV Ar + )
i
M
Top Co layer
2 nd Co layer
Top layer magnetization predominantly
antiparallel to 2 nd layer
Borchers, Dura, Unguris, Tulchinsky, Kelley, Majkrzak, Hsu, Loloee, Pratt, Bass, PRL 82, 2796 (1999)

Interlayer Domain Correlations
Quantify layer-by-layer angle distributions
Angle images:
1st layer 2nd layer
P(∆φ)
Correlation distribution:
FM AFM
1.0
0.8
0.6
0.4
0.2
0.0
0 90 180
∆φ (°)

Magnetic materials
Air Side Wheel Side
Transformer core ferromagnetic glass
(Allied Signal)

Cr
Magnetic Nanostructures
Fe “nanowires”
Fe
Fe evaporated at grazing incidence on Cr lines
fabricated by laser focused atom deposition
M
213 nm
Tulchinsky, Kelley, McClelland, Gupta, Celotta, JVST A16, (1998)

Ready to retire???

Acknowledgements
Spin-polarized electron work at NBS/NIST
Many contributors,
especially Bob Celotta and John Unguris
Pictures of Charlie
Joel Falk
Jim Murray
Roger Route