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.

Magnetism 2015 – Leeds

Screen Shot 2015-03-31 at 16.08.00The 2015 meeting of the UK magnetism community closes today after two days of parallel sessions as well as posters on all things magnetism. Highlights include a plenary session by Professor Stuart Parkin and the Wohlfarth lecture given by Professor Laura Heyderman. Each sessions also enjoyed an invited talk including the IEEE Distinguished Lecturer Professor Russel Cowburn. The wide range of talks from theoretical and experimental groups is an excellent showcase of the magnetism community in the UK. As well as learning new physics the event also involves a great dinner and a great chance to network. My talk on ultrafast thermally induced magnetisation switching can be found here.

IOP Postgraduate Techniques Workshop

IOP2014Logo

This years IOP magnetism techniques workshop for postgraduate students closes today after a packed two day program. The workshop is mainly based around various experimental techniques but with a smidgen of theory thrown in for good measure. The workshop takes place each year close to christmas and is a good opportunity to meet new students and academics involved in magnetism and to see the wide range of skills and techniques the UK has. The social program usually includes a christmas dinner and festive beer or two on the first night. Further information can be found at this link. My slides can be found here.

Cubic Spline Interpolation Library in C++

This afternoon I was looking around for a cubic interpolation routine to do some data analysis. The numerical recipes one works fine but I also found a nice library from the website of Tino Kluge which works (so far anyway) very nicely indeed. It is as simple as including a header file, defining a set of (sorted) \(x_i\) and their corresponding \(y_i\) (for example using the vector class from the STL), declaring an instance of that class and calling the classes routine “set_points”. Below is the example provided on the authors website.


#include <cstdio>
#include <cstdlib>
#include <vector>
#include "spline.h"

int main(int argc, char** argv) {

std::vector<double> X(5), Y(5);
X[0]=0.1; X[1]=0.4; X[2]=1.2; X[3]=1.8; X[4]=2.0;
Y[0]=0.1; Y[1]=0.7; Y[2]=0.6; Y[3]=1.1; Y[4]=0.9;

tk::spline s;
s.set_points(X,Y); // currently it is required that X is already sorted

double x=1.5;

printf("spline at %f is %f\n", x, s(x));

return EXIT_SUCCESS;

The header is available from the authors website at the following link along with a some example programs and the explanation of the mathematics (see here). Many thanks to the author for providing it under the GNU GPLv2 licence.

Math symbols are rendered by MathJax which requires JavaScript.

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.

IOP Postgraduate Magnetic Techniques Workshop

 

The 2013 “Postgraduate Magnetic Techniques Workshop” organised by the Insitute of Physics is taking place today. It is a workshop to provide introductory training in key experimental and theoretical techniques used in magnetic research, aimed at new postgraduate students and research fellows new to magnetic research. My talk will be on the atomistic spin dynamics model and how it can be used to describe magnetization dynamics in the femtosecond regime. I will focus on how the model is constructed and how it is being used to compare to experimental observations. The talk will be available on the conference presentations page.

Installing Rasmol on OS X (10.8.5)

I am used to using rasmol for quick viewing of atomic positions. For example to check that my system generation creates an fcc structure with two layers, one with Nickel and one Iron:

Screen Shot 2013-12-03 at 18.35.58

To avoid compiling the binary from source there are a number of pre-compliled binaries for different platforms available here. My aim was to install a working version to /usr/local/ on the mac filesystem. I went for a filetype that had a name like:

RasMol_2_7_5_i386_OSX_21Jul09.tar.gz

This should run on a mac with an intel chip (I hope). Download this and untar it. Once it has been untarred upen up a terminal and move into the newly unpacked directory. In the terminal you can do this by downloading the file to, for example, /Users/username/Downloads, where you put your username in <username>. Then change to that directory and untar the file.

This made a directory called RasMol_2_7_5_i386_OSX_21Jul09, change to that directory.

install to /usr/local/ by running the rasmol_install.sh file with the path prefix as /usr/local

It will ask you if this is where you want to put the bin and lib directories. Hit ‘y’ twice and enter. From a terminal you should be able to call rasmol now as /usr/local/bin should be in the executable path.