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.

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.

Ultrafast Magnetism Conference

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.

Image from www.cntraveller.com

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.

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.

Spinwaves Symposium 2013, St. Petersburg, Russia

Last week saw the 2013 spinwaves symposium held in St. Petersburg and hosted by the Ioffe Physical-Technical Institute of the Russian Academy of Sciences. A combination of invited and contributed talks made for a very interesting meeting demonstrating the latest advances in magnetism. I presented a talk looking at spin-spin correlations during ultrafast demagnetisation processes.

Spin Waves 2013

St. Petersburg is a very impressive city and gave a good impression on my first visit to the country. I look forward to returning to Russia, perhaps for spinwaves 2015 (hopefully next time I won’t leave my car keys in my hotel room!).

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.

Demonstration of Working HAMR Drive

At the 2012 TMRC conference there was a very impressive talk from Rausch et al.where they gave an overview of HAMR performance and integration challenges in moving from a spin stand demonstration to a fully operational drive. In the grand finale they showed that the presentation was actually entirely being stored on an operational HAMR drive! A very impressive demonstration.