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.
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.
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.