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Evaluation of spin-orbit torques in thin film magnetic materials and their use in driving topological structures


Project Description

Joint supervisors: Prof R J Hicken, Dr P S Keatley

External supervisor: Prof K L Livesey (University of Colorado, UCCS)

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Statement of Research

Spin orbit torques (SOTs) result from the spin Hall and Rashba effects within multilayered thin film magnetic materials. An injected charge current generates a spin current that can transfer angular momentum to local magnetic moments, thereby exerting a torque upon them. hin the Spintronics, Magnonics and Magnetic Materials theme. SOTs have the advantage that charge current may be passed within the plane of the film, rather than through a delicate tunnel barrier, so that larger current densities, sufficient to change the magnetic state of the sample, may be used. It is expected that SOTs will be exploited in future generations of magnetic random access memory (MRAM) device, and within new data storage and processing paradigms that harness topological structures such as domain walls vortices and skyrmions.

The PhD project is aligned with 2 EPSRC grants [1,2] that support experienced researchers who will be happy to share their expertise. The first grant uses time resolved scanning Kerr microscopy to detect the magnetization dynamics detected by spin orbit torques within thin film magnetic materials formed into Hall bars and spin transfer oscillators. The second uses time resolved optical measurements to study magnetization dynamics in a wide range of magnetic structures and devices. The motion of topological structures within such materials is a major theme within the EXTREMAG project.

Existing measurement methods rely on magnetoresistive response to characterise spin-orbit torques. However this approach does not sense the magnetization directly, provides a null response in certain geometries of interest, and lacks spatial resolution. Direct optical detection of the magnetization overcomes all of these difficulties and, together with the detection of the resulting dynamics in candidate MRAM and spin transfer oscillator devices, is the goal of the first EPSRC grant [1]. The proposed PhD project is intended to provide the theory and modelling needed to extract the values of the torques reliably and understand how they induce dynamic response within devices where the magnetization undergoes quasi-uniform precession. This theory will then be extended to more complicated topological objects such as skyrmions. Specifically, the means by which skyrmions move within thin films will be explored, with particular focus upon whether the internal dynamics of skyrmions affect their translational motion and interaction with defects and pinning sites, such as those found at the edges of nanowires that are used to guide their motion.

[1] “Optical detection of magnetisation dynamics induced by spin-orbit torques”, EP/P008550/1.
[2] “EXTREMAG: an Exeter-based Time Resolved Magnetism Facility”, EP/R008809/1.

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The succesful candidate will have the opportunity to join the additional training programme of the Centre for Doctoral Training in Metamaterials (XM²).

XM² now has over 80 post graduate researchers. Our aim is to undertake world-leading research, while training scientists and engineers with the relevant research skills and knowledge, and professional attributes for industry and academia.

Please visit http://www.exeter.ac.uk/metamaterials for more information about the current CDT and an indication of what to expect.

This studentship will be based on the Streatham Campus in Exeter.

Funding Notes

This exciting 3.5 year project is for self-funded students only. You need to be able to cover at least fees (UK/EU approx. £4,320 per year; International approx. £23,700 per year) and your living expenses (minimum of £15,000 per year is recommended).

How good is research at University of Exeter in Physics?

FTE Category A staff submitted: 40.20

Research output data provided by the Research Excellence Framework (REF)

Click here to see the results for all UK universities

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