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.