EPSRC CDT in Metamaterials (PhD studentship): Detection of magnetic meta-atom dynamics by x-ray ferromagnetic resonance

University of Exeter - Departments of Physics and Engineering

Joint supervisors: Prof Robert J Hicken, Prof Gino Hrkac

Industry partner: Seagate Technology

The interaction of microwave radiation with a magnetic multilayer is determined by the thickness and composition of its constituent layers. Since the layer thicknesses generally lie within the deep nanoscale range, and are much smaller than the microwave wavelength, the multilayer can be though of as a one-dimensional electromagnetic metamaterial. Each layer can be thought of as a meta-atom and full understanding of the dynamic behaviour of the metamaterial requires that the dynamic properties of each meta-atom be known and understood.  X-ray ferromagnetic resonance (XFMR) has a unique ability to determine the dynamical behaviour of meta-atoms of different composition because the photon energy of radiation generated by a synchrotron can be tuned to core level transitions that are element specific.

In this project the unique capabilities of XFMR will be used to study two highly topical problems.  Firstly, XFMR will be used to explore the spin wave excitations supported by magnetic exchange springs, which were first probed by ultrafast optical techniques in Exeter [1], in alignment with an EPSRC grant [2] that began in June 2017.  Secondly, the propagation of spin currents through antiferromagnetic layers will be studied.  Multilayer stacks will be studied in which two types of ferromagnetic layer with different magnetic resonance conditions are separated by antiferromagnetic layers.  When one ferromagnetic layer is at resonance, its precessing magnetization drives a spin current into the adjoining antiferromagnet through which it will propagate to the other ferromagnetic layer, inducing a dynamical response.  The technique is already well established having been demonstrated for the case of copper spacer layers [3], [4], as Exeter pioneered the development of phase-resolved XFMR at the Diamond and Advanced Light Sources.

Ultrafast and high frequency phenomena are of ever increasing importance in the development of information technology.  There are opportunities to apply underlying principles to new types of materials and devices, and to draw upon knowledge gained in other areas of science and technology. Therefore it is essential to remain aware of developments within neighbouring fields.  Membership of the CDT cohort and community will foster this awareness, while the skills training available within the CDT will provide a solid foundation for future employment.

[1] “Ultrafast optical parametric pumping of magnetization reorientation and precessional dynamics in DyFe2/YFe2 exchange springs”, L. R. Shelford, Y. Liu, U. Al-Jarah, P. A. J. de Groot, R. C. C. Ward, and R. J. Hicken, Phys. Rev. Lett. 113, 067601 (2014).
[2] “Picosecond Dynamics of Magnetic Exchange Springs”, EPSRC EP/P008550/1.
[3] “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).
[4] “Direct detection of pure ac spin current by x-ray pump‐probe 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).

This studentship is part of the Centre of Doctoral Training in Metamaterials. Please see all fully funded opportunities.

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