Research Profile - Department of Materials Science and Metallurgy ...

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James Loudon Royal Society University Research Fellow MA MSci University of Cambridge PhD University of Cambridge +44 (0) 1223 334564 jcl25@cam.ac.uk www-hrem.msm.cam.ac.uk/people/loudon Imaging Flux Vortices in Superconductors I work in the High-Resolution Electron Microscopy Group researching the structure and dynamics of flux vortices in superconductors using real-time imaging. Previously, I studied the unconventional phases of mixed-valent manganites at Cambridge and Cornell Universities. Superconductors and flux vortices Superconductors have two remarkable properties: they have no electrical resistance whatsoever and expel magnetic flux from their interiors. Applying an increasing magnetic field to an ideal (type-I) superconductor, no flux enters until a critical field strength at which the material suddenly ceases to be superconducting. In contrast, in type-II superconductors, the superconducting state is not wholly destroyed at once but magnetic flux penetrates the material along narrow channels of non-superconducting material called flux vortices. Each vortex contains one quantum of flux: the smallest amount of magnetic flux allowed by the laws of quantum mechanics. To emphasize their quantum nature, they are often called ‘fluxons’. The behaviour of fluxons is crucial to the performance of almost every superconducting device, including superconducting power lines, SQUIDs for very sensitive measurements of magnetic fields, superconducting magnets in medical MRI scanners, and future technologies such as superconductor-based quantum computers. To improve performance, it is necessary to observe fluxons in action to determine their response to pinning sites and other constraints and stimuli. Imaging flux vortices We use transmission electron microscopy to observe individual vortices in real time. This technique has a higher resolution than the alternatives and can measure the magnetic fields quantitatively. Worldwide, it had been successfully employed by only two collaborating groups using specially adapted microscopes. However, we have recently imaged vortices with a modern commercial electron microscope (see figure), opening up a world of possibilities. We shall investigate the structure of individual vortices, observe and quantify their dynamics and investigate their response to pinning sites and constrained geometries. JC Loudon, LF Kourkoutis, JS Ahn, CL Zhang, SW Cheong & DA Muller, “Valence changes and structural distortions in ‘charge ordered’ manganites quantified by atomic-scale scanning transmission electron microscopy” Phys. Rev. Lett. 99, 237205 (2007). JC Loudon & PA Midgley, “Comparison of the ferromagnetic phase transitions in La 0.7 Ca 0.3 MnO 3 and single crystal nickel by micromagnetic imaging” Philos. Mag., 86, 2941–2956 (2006). JC Loudon & PA Midgley, “Micromagnetic imaging to determine the nature of the ferromagnetic phase transition in La 0.7 Ca 0.3 MnO 3 ” Phys. Rev. Lett. 96, 027214 (2006). JC Loudon & PA Midgley, “Real-space imaging of coexisting chargeordered and monoclinic phases in La 1‐x Ca x MnO 3 (x = 0.67 and 0.71)” Phys. Rev. B 71, 220408 (2005). JC Loudon, S Cox, AJ Williams, JP Attfield, PB Littlewood, PA Midgley & ND Mathur, “Weak charge-lattice coupling requires reinterpretation of stripes of charge order in La 1–x Ca x MnO 3 ” Phys. Rev. Lett. 94, 097202 (2005). Flux vortices in superconducting Bi 2 Sr 2 CaCu 2 O 8+ō : A pseudo-hexagonal array of flux vortices in superconducting Bi 2 Sr 2 CaCu 2 O 8+ō at 20 K in a field of 30 G. Individual vortices are seen as black-white features 28 Research Profile
Neil Mathur Reader in Materials Physics MA University of Cambridge PhD University of Cambridge +44 (0) 1223 762980 ndm12@cam.ac.uk www.msm.cam.ac.uk/dmg/ Thin-Film Device Physics I use thin-film devices to study the physics of materials that display notable electrical and magnetic properties. Thin films present large areas for interfacing and imaging, and thin-film devices collect information from precisely controlled materials and materials combinations. My scientific goals are academic but reflect long-term technological challenges. Multiferroic and magnetoelectric materials It is possible to address magnetic materials via an electrical signal, or vice versa. The magnetoelectric coupling that permits this cross-talk suggests applications in sensors, actuators and data storage. Meanwhile, the related challenge of finding multiferroic materials that are both ferromagnetic and ferroelectric remains a hot topic. Manganites Complex magnetic and electronic textures arise naturally in these crystalline perovskite oxides of manganese. By controlling these textures it is possible to explore the links with macroscopic measurements. Studies of this type provoke dreams of some future nanotechnology based on self-organized devices. Spintronics Electrons can carry magnetic information over long distances in non-magnetic materials, provided they travel sufficiently fast. Carbon nanotubes are fast electronic conductors, and therefore constitute plausible building blocks for e.g. proof-of-principle spin transistors, or quantum computers based on magnetic qubits. Electrocaloric materials These materials permit the interconversion of heat and electricity. The ability to heat and cool a material via a voltage-driven phase change provides the basis for an electrically driven solid-state heat pump. The reverse effect permits electrical power generation from waste or other heat. Thin films and large electric fields offer an exciting avenue for exploration. C Israel, ND Mathur & JF Scott, “A one-cent room-temperature magnetoelectric sensor” Nature Mater. 7, 93-94 (2008). LE Hueso, JM Pruneda, V Ferrari, G Burnell, JP Valdés-Herrera, BD Simons, PB Littlewood, E Artacho, A Fert & ND Mathur, “Transformation of spin information into large electrical signals using carbon nanotubes” Nature 445, 410–413 (2007). W Eerenstein, ND Mathur & JF Scott, “Multiferroic and magnetoelectric materials” Nature 442, 759–765 (2006). AS Mischenko, Q Zhang, JF Scott, RW Whatmore & ND Mathur, “Giant electrocaloric effect in thin-film PbZr 0.95 Ti 0.05 O 3 ” Science 311, 1270–1271 (2006). Size and cost comparison for a one-cent multilayer capacitor that inadvertently functions as a room-temperature magnetic-field sensor that requires no electrical power [A one-cent roomtemperature magnetoelectric sensor. C Israel, ND Mathur & JF Scott, Nature Materials 7, 93–94 (2008)] Research Profile 29
Page 1 and 2: Department of Materials Science and
Page 3 and 4: Materials Science at the University
Page 5 and 6: Magnetic domains in a thin cobalt f
Page 7 and 8: Zoe Barber Senior Lecturer MA Unive
Page 9 and 10: Harry Bhadeshia FREng FRS FNAE Tata
Page 11 and 12: Mark Blamire Professor of Device Ma
Page 13 and 14: Tim Burstein Professor of Materials
Page 15 and 16: Tony Cheetham FRS Goldsmiths’ Pro
Page 17 and 18: Bill Clyne FREng Professor of Mecha
Page 19 and 20: Caterina Ducati Royal Society Unive
Page 21 and 22: James Elliott Lecturer MA Universit
Page 23 and 24: Bartek Glowacki Reader in Applied S
Page 25 and 26: Colin Humphreys CBE FREng Emeritus
Page 27 and 28: Krzysztof Koziol Royal Society Univ
Page 29: John Little Senior Lecturer MA Univ
Page 33 and 34: Michelle Moram Oppenheimer Research
Page 35 and 36: Cathie Rae Lecturer MA University o
Page 37 and 38: Karl Sandeman Royal Society Univers
Page 39 and 40: Rob Wallach Senior Lecturer MA Univ
Page 41 and 42: Alan Windle FRS Professor of Materi
Page 43 and 44: Dr Rosie Ward Academic Secretary +4

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