Since the discovery of a purely thermally induced magnetisation switching (TIMS) in GdFeCo, there has been much effort to identify the cause of this unexpected phenomenon. While several works have studied the macroscopic relaxation behaviour (Mentink et al., Phys. Rev. Lett. 108, 057202 (2012). Atxitia et al., Phys. Rev. B 87, 224417 (2013)), there has been little headway made in finding the material origins of the switching. In our new work “Two-magnon bound state causes ultrafast thermally induced magnetisation switching” published in the open access journal Scientific Reports we have found, through simulation and described with a combination of theoretical approaches, that the switching is caused by angular momentum transfer from a two magnon bound state which exists in this class of ferrimagnetic materials. Specifically, within GdFeCo we have shown that the amorphous properties of the material affect the switching behaviour because the antiferromagnetic interactions which couple the rare-earth and transition metal species have a large effect only at the interfaces of Gd clusters within the FeCo background. Our work provides a new insight into the switching which is induced by femtosecond laser pulses and gives new directions for experimentalists to focus their research.
The first Ultrafast Magnetism Conference (UMC) was held last week in Strasbourg, France. A week focused on magnetism on the sub-picosecond timescale. Talks from both experimental and theoretical groups included talks on laser induced magnetization dynamics, THz stimulation and ultrafast magneto-acoustics.
This meeting was a huge success with over 150 participants from all over the world and a large number of invited talks. The format was a single session talks and posters so that all participants could see everything on offer. The single session format promoted a comfortable environment in which the audience could ask questions and discuss things amongst themselves.
I gave a contribution based on the thermally induced switching phenomena in GdFeCo, recently accepted to Nature Scientific Reports (the preprint can be found on arXiv). This work by Joe Barker shows that the thermally induced switching is caused by the excitation of a two magnon bound state. This bound state is possible when sufficient energy is provided to excite two spinwave bands. This lays out a criteria for ultrafast switching with heat to occur, which Joe tested using the atomistic spin model.
The second Ultrafast Magnetism Conference will take place in 2015 in Nijmegen, where I am sure the event will be even larger and the talks will be equally as interesting.
Yesterday saw a long awaited paper into the mechanism behind heat induced switching in ferrimagnetic materials. Using the newly developed Landau-Lifshitz-Bloch equation for a ferrimagnet we linearize the equations of motion in the conditions seen in heat induced switching, arriving at a set of dynamical equations. These dynamical equations show that the reversal path occurs via a transfer of angular momentum from the linear motion to the transverse motion. We support these analytics by making comparisons with atomistic spin dynamics.
Last week saw the 2013 spinwaves symposium held in St. Petersburg and hosted by the Ioffe Physical-Technical Institute of the Russian Academy of Sciences. A combination of invited and contributed talks made for a very interesting meeting demonstrating the latest advances in magnetism. I presented a talk looking at spin-spin correlations during ultrafast demagnetisation processes.
St. Petersburg is a very impressive city and gave a good impression on my first visit to the country. I look forward to returning to Russia, perhaps for spinwaves 2015 (hopefully next time I won’t leave my car keys in my hotel room!).
On the 19th November this month Matt Ellis, a colleague at York, finally had his paper on Rare-Earth doped Permalloy published in Physical Review B. This latest paper uses a localized Heisenberg model, combined with the Landau-Lifshitz-Gilbert equation of motion for atomic magnetic moments, to study the effects of doping of different rare-earth metals on the magnetization dynamics of Permalloy. The model allows one to study the effect of different energy transfer channels to and from the spin system.
This systematic study looks at the effects of doping on properties such as, the longitudinal relaxation after femtosecond heating, and the transverse relaxation time after exciting the system away from it’s anisotropy axis.
At the 2012 TMRC conference there was a very impressive talk from Rausch et al.where they gave an overview of HAMR performance and integration challenges in moving from a spin stand demonstration to a fully operational drive. In the grand finale they showed that the presentation was actually entirely being stored on an operational HAMR drive! A very impressive demonstration.
In February this year, as part of a large collaborative project, my colleagues and I published a paper in Nature Communications showing that heat alone can stimulate deterministic magnetization reversal in GdFeCo. This project was stimulated by the results of Stanciu et al. who showed that all-optical control of magnetization was possible using femtosecond laser pulses of different chiralities. The aim of this work was initially to use my model of GdFeCo to provide insight into the processes occurring on the femtosecond timescale with atomic resolution. As it turned out we discovered that, as part of a systematic study, the switching seen in GdFeCo was possible without using circularly polarized light. This means that all of the laser light is absorbed as heat and we showed, using our model, that it was this heat that was driving the reversal. Until this point, it was believed that heat could only assist in magnetization reversal by driving down the energy barrier associated with switching.
In the paper we showed that using heat alone it was possible to induce reversal in micro-structures of GdFeCo experimentally. This was an important step forward in realizing this mechanism for applications as the switching had only previously been seen in large thin films. Though the micro-structures were nowhere near the size of confined magnetic structures seen inbit-pattern media, this was an important proof of principle of the concept of heat driven switching.
Work is now underway to explain and provide insights into the mechanism behind the switching. The hope is that by being able to explain more thoroughly what is going on, we can find new materials that exhibit this behavior.
Senior Lecturer of Physics at Sheffield Hallam University