EPSRC DTP PhD studentship: X-ray detection of spin current propagation in multi-layered spintronic materials

University of Exeter - College of Engineering, Mathematics and Physical Sciences

Main supervisor: Prof. R. J. Hicken (University of Exeter)
Co-supervisor: Dr A. Shytov (University of Exeter)

Project Description:

The structure of a multilayered magnetic thin film can be tailored so as to facilitate the generation and propagation of spin currents. While transport of spin-polarized electrons underlies the Giant Magnetoresistance effect (2007 Nobel Prize), injection of spin current into a ferromagnet can result in spin transfer torque that enables “active” spintronic function in memory and logic devices. Such effects are ordinarily confined to nanoscale devices due to the large current densities required. However spin currents may alternatively be generated by precessional spin pumping in an unpatterned source magnetic layer and detected either through the torque exerted on a sink layer [1], or by polarising paramagnetic impurities in an adjacent nonmagnetic layer [2]. Both effects are detected by phase resolved x-ray ferromagnetic resonance (XFMR), which we pioneered at the Diamond Light Source (DLS) and the Advanced Light Source. Here XFMR measurements will be performed on spin-valve structures in which source and sink are separated by either a non-magnetic or an antiferromagnetic layer. Spin transport through individual layers and interfaces will be understood in terms of microscopic theory [3], while the efficient conduction of spin current through an anti-ferromagnet will be confirmed. Samples will be provided by UC Berkeley, while off-line characterisation will be performed by vector network analyser FMR (VNAFMR) and time resolved scanning Kerr microscopy (TRSKM) in Exeter.

The student will receive training from a group with internationally leading expertise in high frequency measurement, and will develop skills in time resolved microwave, optical and measurement techniques that are highly marketable as information technology moves towards increased miniaturisation and higher bandwidths. Following initial training the student will be expected to lead the initial characterisation of samples received from Berkeley.

Synchrotron beamtime will be sought through the usual competitive proposal mechanisms where Prof Hicken has an excellent track record, so that we can expect on average one beamtime in each 6 month allocation period. During beamtimes the student will join a cohesive team comprised of beamline staff and collaborators from UC Berkeley, Exeter, York, and the Magnetic Spectroscopy group at DLS, providing access to a broad range of expertise in synchrotron use. Theoretical input on spin transport will be provided by Dr Shytov.

[1] “Phase-Resolved X-ray Ferromagnetic Resonance Measurements of Spin Pumping in Spin Valve Structures”, M. K. Marcham, L. R. Shelford, S. A. Cavill, P. S. Keatley, W. Yu, P. Shafer, A. Neudert, J. R. Childress, J. A. Katine, E. Arenholz, N. D. Telling, G. van der Laan, and R. J. Hicken Phys. Rev. B 87, 180403(R) (2013).

[2] “Direct detection of pure ac spin current by x-ray pumpprobe measurements”, J. Li, L. R. Shelford, P. Shafer, A. Tan, J. X. Deng, P. S. Keatley, C. Hwang, E. Arenholz, G. van der Laan, R. J. Hicken, and Z. Q. Qiu, Phys. Rev. Lett. 117, 076602 (2016).

[3] “Non-local charge transport mediated by spin diffusion in the spin Hall effect regime”, D. A. Abanin, A. V. Shytov, L. S. Levitov, B. I Halperin, Physical Review B 79, 035304 (2009).

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South West England