As part of a collaboration with Diamond Light Source, The University of Nottingham and the University of York thisopen access article at Physical Review Letters demonstrates the possibility of low energy reversal of magnetic vortex core. The work, lead by Dr Stuart Cavill (The University of York) shows that by applying a time-varying strain to a ferroelectric layer that induces a strain in a magnetostrictive magnetic layer (Galfenol), vortex core dynamics are stimulated. The flux closure state is topologically symmetric and cannot be moved by simply applying a time-varying strain, therefore the symmetry must be broken. We achieved this by applying a gradient to the strain which moves one domain more than another in the vortex alternately. If the strain gradient is large enough the precession of the vortex core can be driven to force the vortex to reverse. Below is a short movie demonstrating the process.
The work was published on the 7th of August 2015 in Physical Review Letters as under the open access under a creative commons license. This was made available through the York open access fund. The work would have not been possible without the funding of the European Framework 7 project (FemtoSpin), the EPSRC, Diamond Light Source and industrial funding from Seagate Technology.
The use of optical interconnects has become a front runner to replace more traditional (usually Cu based) electrical interconnects in many modern devices. One of the major drawbacks of optical interconnects is overcoming the need for photodetectors and (power hungry) amplifiers at the receiver. Such detection is in most cases performed by CMOS circuits or direct band gap semiconductors. As part of a collaboration lead by engineers at Purdue University, IN, USA a new use of ultrafast heat induced switching, originally published in Nature Communications, has been proposed as a means of using optical signals directly with standard CMOS circuits.
The data is transmitted using femtosecond laser pulses that induce magnetisation reversal in a magnetic tunnel junction (MTJ) in the receiver. The proposed scheme offers almost a 40% energy improvement over current technology and speeds of up to 5 GBits/sec for a single link. The preprint of the article can be found on arXiv (or downloaded from this link).
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.
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.
Marie Curie COFUND postdoctoral research fellow at the University of Liège, Belgium