Very many congratulations to the EPSRC CDT in Metamaterials PGR Tim Spicer, who has recently submitted his PhD thesis on “Excitation of picosecond magnetization dynamics by spin transfer torque“. Tim was supervised by Rob Hicken and Volodymyr Kruglyak. Tim is currently working on 3 more papers to publish his results in addition to his article in the Journal of Applied Physics (2016) on Time-resolved scanning Kerr microscopy of flux beam formation in hard disk write heads. He has presented his research through posters and talks at various conferences over the past 4 years, such as the annual Magnetism conferences (2015 – 2018), the Joint European Magnetic Symposia (JEMS), and the 6th IEEE International Conference on Microwave Magnetics (ICMM2018).
We wish Tim good luck for his viva and look forward to welcoming him back as part of our growing research community to inspire future generations in subsequent years.
“The last four years in the CDT have given have taught me invaluable skills and provided me with fantastic opportunities. One of the reoccurring highlights for me was the traveling to attend and present at conferences, despite attending quite a few I only wish I could have been to more! Through these did I not only get to see the latest research but meet with other researchers (at all levels) and discuss our respective work.
This is perhaps most epitomised by the three weeks I spent in Gothenburg University, in a very snowy February of 2017. Our collaborators very graciously lent me the use of their computing cluster (dubbed Rangnarok) for simulating magnetization dynamics, while leaving me the chance to explore the beautiful city on the weekends.
I was fortunate enough to share this journey with both very talented and very amiable scientists, both inside and outside of magnetics. Sharing music, discussing film & TV, and pub trips certainly made the PhD a more enjoyable experience, I can’t imagine doing it without any of them!
My advice to future students would be this: write everything down and try to keep a level head! Research at its best is about dealing with unexpected results for which there is no standard or established procedure, these times can be taxing but sometimes the smallest detail can tie everything together. Also having a good distraction for when you’re not in the office definitly helps!
From here I’m looking for a career in London, either within industrial R&D or analytics and software development.” (Tim Spicer, August 2018)
Thesis Abstract:
This thesis presents the results from investigations of ultrafast magnetization dynamics driven by pure spin currents. Spin orbit coupling in heavy metal layers – such as Tungsten, Tantalum or Platinum – allows for the generation of pure spin currents, whereby spin up and spin down electrons move in opposite directions. Hence, a flow of angular momentum can be controlled through the manipulation of charge current through a heavy metal layer. When a spin current is injected into a ferromagnet, a torque is exerted on its magnetization, with potential to induce a wide variety of ultrafast dynamics. The experimental investigation of these phenomena employed a variety of high frequency electrical techniques and time-resolved scanning Kerr microscopy (TRSKM) methods. In addition, various simulative and analytical approaches were used to gain insight into the underlying mechanisms.
Spin Hall nano-oscillators (SHNOs) have recently been shown to support a tuneable GHz spin wave `bullet’ under injection of direct current (DC), making it an exciting candidate for microwave communication applications. While the device is designed to focus DC current to within a finite active region, a new optically-based FMR technique demonstrates that the spin torques present under injection of radio frequency (RF) current are not subject to the same confinement. The competition between self-inductance and focusing within the device geometry results in a modified distribution of spin current. Further TRSKM measurements suggest that the bullet mode exhibits reduced localization within the modified torque landscape.
Devices that exploit spin currents for magnetization reversal have received interest from academia and industry for their potential use as memory elements. The perpendicular magnetic anisotropy present in Ta/CoFeB/MgO leads to lower write currents and higher thermal stability. However, ultrafast processes have not been previously observed in such devices. TRSKM measurements of Hall bar devices were compared with a macrospin model to reveal the dynamics that can be driven by a damping-like torque and Oersted field. Elements built from the same stack structure exhibited dynamics highly dependent on the amplitude and orientation of the static magnetic field, as well as motion of the domain structure for. These could also be understood in terms of the effect of an Oersted field and damping-like torque acting on the static magnetization. Measurements with a bi-polar electrical pulse demonstrated that meta-stable switching can be achieved in micron-scale elements.
