Exploration of the sub-nanosecond magnetisation dynamics of partially built hard disk drive write-head transducers and other topical magnetic and spintronic materials and devices
Valkass, Robert Alexander James
Thesis or dissertation
University of Exeter
Reason for embargo
Work from, and related to, this thesis is being prepared for publication.
In this thesis both the static and dynamic magnetic behaviour of complex three-dimensional nanoscale commercial hard disk drive write heads and thin film structures of interest to emerging spintronic devices have been investigated using a plurality of experimental techniques. The magneto-optical Kerr effect (MOKE) provides the basis for an optical microscopy technique sensitive to the magnetisation of a sample, detectable as a change in polarisation of light reflected from the sample surface. With a modelocked laser light source, synchronised electrical pulse generator and lock-in amplifier (LIA), a stroboscopic technique has been used to observe the magnetisation dynamics of hard disk drive write heads at 600 nm spatial resolution and 10 ps time resolution in response to a driving electrical pulse. The equilibrium magnetic state of these devices has been directly imaged by x-ray photo-emission electron microscopy (XPEEM), as well the stability of the equilibrium state in response to the application of an external bias field. Direct images of the equilibrium state obtained by XPEEM were found to agree with inferences made from MOKE images. Time-resolved scanning Kerr microscopy (TRSKM) images of magnetisation dynamics showed that flux does not form in ‘beams’ as commonly believed, but instead nucleates in separate sites across the writer. Static and time-resolved x-ray techniques have also been used to investigate a number of thin films of interest to spintronics. Spin pumping and spin transfer torque in Co2MnGe / Ag / Ni81Fe19 spin valves were explored using time-resolved x-ray ferromagnetic resonance (XFMR) carried out at Diamond Light Source (DLS), a as well as static x-ray magnetic circular dichroism (XMCD) for sample characterisation. This has provided element-specific measurements of the spin state in the source and sink layers of the spin valve, revealing a clear sign of spin transfer torque, while also investigating the role of sink layer thickness in spin pumping and damping. Ferrimagnetic yttrium iron garnet (Y3Fe2(FeO4)3) (YIG), a material of great interest in spintronics, has been studied by static and dynamic XMCD in comparison with ferromagnetic Co. While static and dynamic spectra for Co were identical, those for YIG differed markedly. While this may hint at a phase difference between the precession of Fe moments on different lattice sites, the true source of this difference has not been identified. Comparisons between vector network analyser ferromagnetic resonance (VNA-FMR) and XFMR measurements further suggest the presence of long-range inhomogeneities in the YIG. The spin dynamics of an antiferromagnet being driven by a ferromagnet have also been investigated using XMCD and x-ray magnetic linear dichroism (XMLD). A CoO / Fe / Ni81Fe19 trilayer wherein the thickness of the CoO layer varies across the sample has been thoroughly characterised by static XMCD and XMLD, providing information necessary to fully interpret time-resolved MOKE measurements on these samples. Measurements have shown that even small amounts of ordered CoO significantly modify the resonant field and linewidth of the adjacent ferromagnetic layers. Phase-resolved measurements of CoO spins have shown these spins to precess in phase with those of the adjacent Fe. The viability of dynamic XMLD measurements has also been confirmed. Finally, potential directions for future work in each project are discussed.
R. A. J. Valkass et al, Applied Physics Letters 106, 232404 (2015)
R. A. J. Valkass et al, Journal of Applied Physics 119, 233903 (2016)
Hicken, Robert J
PhD in Physics