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

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

Time resolved scanning Kerr microscopy of flux beam formation in hard disk write heads

This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.The underlying research dataset supporting this publication is available under a Creative Commons Attribution-ShareAlike 4.0 International License (https://creativecommons. org/licenses/by-sa/4.0/) and can be publicly accessed in Open Research Exeter via the following persistent identifier: http://hdl.handle.net/10871/21108.To meet growing data storage needs, the density of data stored on hard disk drives must increase. In pursuit of this aim the magnetodynamics of the hard disk write head must be characterized and understood, particularly the process of “flux beaming”. In this study, seven different configurations of perpendicular magnetic recording (PMR) write heads were imaged using time-resolved scanning Kerr microscopy, revealing their detailed dynamic magnetic state during the write process. It was found that the precise position and number of driving coils can significantly alter the formation of flux beams during the write process. These results are applicable to the design and understanding of current PMR and next-generation heat-assisted magnetic recording (HAMR) devices, as well as being relevant to other magnetic devices.The authors gratefully acknowledge financial support from the Seagate Plan

Effect of sink layer thickness on damping in CoMnGe (5 nm) / Ag (6 nm) / NiFe (x nm) spin valves

Poster presented at Magnetism 4 – 5 April 2016, Sheffield.In spin valve structures the damping of a ferromagnetic layer driven at resonance can be modified by the transfer of spin angular momentum into a ‘sink’ ferromagnetic layer. This effect, known as spin pumping, is interface dominated and expected to increase with increasing sink layer thickness up to a saturation absorption depth, previously reported to be 1.2 nm regardless of the sink layer’s composition [1]. Using vector network analyser ferromagnetic resonance (VNA-FMR), we have studied the variation in damping as a function of sink layer thickness in a series of CoMnGe (5 nm) / Ag (6 nm) / NiFe (x nm) spin valves. These measurements show only small variations in the CoMnGe Gilbert damping parameter for x ≤ 1.8 nm, although damping is observed to increase at x = 0.3 and 0.6 nm. Element-resolved x-ray detected ferromagnetic resonance (XFMR) [2] measurements confirm spin transfer torque due to spin pumping as the origin of the damping for x = 1.5 and 1.8 nm, with both thicknesses having the same effective spin mixing conductance, supporting the findings of Ghosh et al [1]. For thicker sink layers the source and sink FMR fields are seen to coincide, hampering the identification of spin pumping. [1] A Ghosh, et al. Physical Review Letters 109, 127202 (2012) [2] M Marcham, et al. Physical Review B 87, 180403 (2013)We thank the Advanced Light Source for access to beamlines 4.0.2 and 6.3.1 (ALS-06433, ALS-07116). The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.We thank Diamond Light Source for access to beamlines I06 and I10 (SI8782, SI11585, SI13063) that contributed to the results presented here.This work was supported by the Engineering and Physical Sciences Research Council [grant number EP/J018767/1]

Time-resolved scanning Kerr microscopy of flux beam formation in hard disk write heads (dataset)

Data behind the published paper "Time-resolved scanning Kerr microscopy of flux beam formation in hard disk write heads"Seagate Technology PL