Category Archives: Papers

Strain Induced Vortex Core Switching in Planar Magnetostrictive Nanostructures

As part of a collaboration with Diamond Light Source, The University of Nottingham and the University of York this open 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.

Laser Induced Magnetization Reversal for Detection in Optical Interconnects

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.

Schematic view of focusing of the laser beam on the detecting MTJ.
Schematic view of focusing of the laser beam on the detecting MTJ.

 

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).

Temperature Dependent Ferromagnetic Resonance in FePt

Ferromagnetic resonance (FMR) is a technique for measuring the magnetic properties of materials such as, damping, gyromagnetic ratio and anisotropy. The underlying theory was outlined as long ago as the 1950’s by Charles Kittel and has since been extensively studied both experimentally and theoretically. The temperature dependence of ferromagnetic resonance curves and the properties derived from them can often be tricky to predict. By using the Landau-Lifshitz-Bloch (LLB) equation that describes the time-dependence of an ensemble of magnetic moments in a spatially averaged way, we have derived in a recently published article a new equation for the power absorbed during ferromagnetic resonance.

This paper predicts a number of temperature dependent magnetic properties using input functions into the LLB that have been parameterised from ab-initio calculations through atomistic spin dynamics simulations. This provides a link directly between electronic structure calculations to macroscopic observables.

As well as studying the properties analytically we have also extended the model to incorporate the effects of exchange between the macrospins, demagnetising fields and stochastic thermal fluctuations. By utilising GPU acceleration large magnetic structures can be simulated for the long times required to get good enough averages to simulate ferromagnetic resonance. Our results of simulating FMR in thin films have shown that there is a strong variation in the damping when the film thickness is varied. The thinner films show the largest damping at high temperatures due to the dominance of the demagnetising fields. This has a knock on effect in terms of the dynamic properties such as the reversal times, an important property in magnetic storages devices utilising heat assisted magnetic recording.

The GPU model that we have developed is capable of calculating a wide range of scenarios for large magnetic systems for long time-scales. This paves the way for new theoretical studies that can be compared to experimental measurements.