Category Archives: Papers

Interaction effects in granular media

Controlling the relaxation of magnetisation in magnetic nano-structures is key to optimising magnetic storage devices. Present day magnetic storage devices have what is known as a granular structure where the magnetic orientation of a section of grains (see the schematic) store the binary information (1’s and 0’s). At the nano-scale these grains can interact which affects how the magnetisation reacts to an external stimulus and therefore how the magnetisation is controlled.

Schematic of an optical pump probe simulation on granular media.
Schematic of an optical pump probe setup granular media. The grains (represented as individual magnetic moments) have a given configuration (giving an initial magnetisation M’) and upon laser excitation relax to a new magnetic state. This can be used to probe the relaxation time-scales of the material.

In collaboration with experimental partners at Seagate Technology, in the Netherlands, as well as with, theoretical collaborators in the UK, our recently published article in Physical Review B we have shown that the effects of the exchange interaction between grains has a strong effect on the relaxation processes and time-scale of the dynamics. Experimentally a sample series with different intergrain exchange was measured using a pump-probe technique (optical ferromagnetic resonance) and showed that the damping decreased significantly with increasing interaction strength, confirmed by both (semi)-analytic and computational models, providing new insights into technologically relevant magnetic materials.

Without funding this work would not have been possible so the authors are gratefully to; the Marie Curie Incoming BeIPD-COFUND fellowship program at the University of Liège; the Advanced Storage Technology Consortium; and the European Commission under contract number 281043 (FEMTOSPIN). Thanks to Jamie Verwey for the schematic diagram.

BeIPD COFUND

Ultrafast and Distinct Spin Dynamics in Magnetic Alloys

Controlling magnetic order on ultrashort timescales is crucial for engineering the next-generation magnetic devices that combine ultrafast data processing with ultrahigh-density data storage. An appealing scenario in this context is the use of femtosecond (fs) laser pulses as an ultrafast, external stimulus to fully set the orientation and the magnetization magnitude of a spin ensemble. Achieving such control on ultrashort timescales, e.g., comparable to the excitation event itself, remains however a challenge due to the lack of understanding the dynamical behavior of the key parameters governing magnetism; the elemental magnetic moments and the exchange interaction.

Screen Shot 2015-08-21 at 11.13.08In a new article published in the journal SPIN, we investigate the fs laser-induced spin dynamics in a variety of multi-component alloys and reveal a dissimilar dynamics of the constituent magnetic moments on ultrashort timescales. Moreover, we show that such distinct dynamics is a general phenomenon that can be exploited to engineer new magnetic media with tailor-made, optimized dynamic properties. Using phenomenological considerations, atomistic modeling and time-resolved X-ray magnetic circular dichroism (XMCD), we demonstrate demagnetization of the constituent sub-lattices on significantly different timescales that depend on their magnetic moments and the sign of the exchange interaction. The results can be used as a “recipe” for manipulation and control of magnetization dynamics in a large class of magnetic materials.

Cover for issue 3 of volume 5 of the SPIN journal
Cover for issue 3 of volume 5 of the SPIN journal

This work was lead by Ilie Radu (TU Berlin) and carried out in collaboration with a number of experimental and theoretical partners across Europe and Japan. The article is made publicly available through the journal’s open access format and was selected as the front cover highlight of the issue (see image above) and was in the top five most downloaded articles in 2015 in the journal SPIN. The work would not have been possible without the support of the European Community’s Seventh Framework Program (FP7/2007–2013) Grants No. NMP3-SL-2008-214469 (UltraMagnetron), No. 214810 (FANTOMAS) and No. 281043 (FEMTOSPIN) and ERC Grant No. 257280 (Femtomagnetism) as well as Grant No. 226716 and ERC-2013- AdG339813-EXCHANGE, the German Federal Ministry of Education and Research (BMBF) Grant No. 05K10PG2 (FEMTOSPEX), the Foundation for Fundamental Research on Matter (FOM) and the Netherlands Organization for Scientic Research (NWO) is gratefully acknowledged.

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.

Microscopic Explanation of Thermally Induced Magnetization Switching

Microscopic explanation of thermally induced magnetisation switching

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.

Thanks to Joe Barker for creating this blog post.

New Paper on Ultrafast Switching in Ferrimagnets

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.

New Paper on Relaxation Processes in Rare-Earth doped Permalloy

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.

Femtosecond Heating as a Sufficient Stimulus for Magnetization Reversal

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.