Research

Model/Code Development

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A large part of my research involves the development and use of computational models of magnetic materials. I have a number of codes that I actively develop. I have my own atomistic spin dynamics code that I developed throughout my PhD and am developing a new GPU based code. For simulations of larger, more extended systems, I have developed a micromagnetic-like code based on the Landau-Lifshitz-Bloch equation of motion. As well as these large codes I have also written a number of smaller ones, such as simple mean field models or data analysis. None of my codes are currently publicly available, however, I am open to collaboration to work on new problems in magnetism. I am always open to discussion via email.

Related Publications:

  • Crystallographically Amorphous Ferrimagnetic Alloys: Comparing a Localized Atomistic Spin Model with Experiments. Physical Review B84, 024407 (2011). (read more)
  • Strain Induced Vortex Core Dynamics in Planar Magnetostrictive Nanostructures. Physical Review Letters, 115, 067202 (2015). (read more)

Theory and Experiment

A large part of my research involves collaboration with experimental partners, which can take the form of verifying theoretical predictions, using model calculations to gain key insight into measurements or making a direct comparison or calibration. This is thanks, in part, due to the fact that the theoretical approaches I use are able to describe real space and time dynamics of magnetic systems. Below is a list of articles where theoretical calculations have been directly compared with experimental measurements.

  • Substrate Induced Strain Field in FeRh Epilayers Grown on Single Crystal MgO (001) Substrates. (Open Access) Scientific Reports, 7, 44397 (2017). (read more).
  • Modelling and Experiments of the Effects of Interfaces on the Phase Transition in FeRh. Physical Review B95, 064415 (2017). (read more).
  • Effects of Interactions on the Relaxation Processes in Magnetic Nanostructures. Physical Review B94,  134431 (2016). (read more). Read the blog post here.
  • Ultrafast and Distinct Spin Dynamics in Magnetic Alloys. SPIN5, 1550004 (2015). (read more)
  • Ultrafast Heating as a Sufficient Stimulus for Magnetization Reversal. Nature Communications, 3, 666 (2012). (read more)
  • Transient Ferromagnetic-like State Mediating Ultrafast Reversal of Antiferromagnetically Coupled Spins. Nature, 472, 205-208 (2011). (read more)
  • Crystallographically Amorphous Ferrimagnetic Alloys: Comparing a Localized Atomistic Spin Model with Experiments. Physical Review B84, 024407 (2011). (read more)

Modelling of Ultrafast Magnetization Dynamics in Multi-Component Alloys

 

Many models of multi-component materials assume that the behaviour of the individual species in such magnetic alloys is the same. However, on the femtosecond timescale this is not necessarily true. One aspect of my research is using numerical models of these multi-component materials and looking at their behaviour on the sub-picosecond timescale, particularly after excitation with a femtosecond laser pulse.Two example materials of particular interest include, NiFe and GdFeCo.

Related Publications:

  • Conditions for thermally-induced all-optical switching in ferrimagnetic alloys: modeling of  TbCo.Physical Review B, 96, 014409 (2017). (read more)
  • Optimal electron, phonon and magnetic characteristics for ultra-low energy thermally induced magnetization switching. Applied Physics Letters, 107, 192402 (2015). (read more)
  • The Landau-Lifshitz Equation in Atomistic Models. Low Temperature Physics/Fizika Nizkikh Temperatur, 41, N 9, pages 908-916 (Sept 2015). (read more Russian/read more English). arXiv version available here.
  • Ultrafast and Distinct Spin Dynamics in Magnetic Alloys. SPIN5, 1550004 (2015). (read more)
  • Computer Simulations of Ultrafast Magnetization Reversal. Ph.D Thesis (2013). Download here or access it at the White Rose Ethesis Online Server via this link. (read more).
  • Classical Spin Model of the Relaxation Dynamics of Rare-Earth Doped Permalloy. Physical Review B86, 174418 (2012). (read more)
  • (Conference Proceedings) Ultrafast Magnetism as Seen by X-rays. Ultrafast Phenomena and Nanophotonics Xvi8260, 82601M-82601M-9 (2012). (read more)
  • Crystallographically Amorphous Ferrimagnetic Alloys: Comparing a Localized Atomistic Spin Model with Experiments. Physical Review B84, 024407 (2011). (read more)

Ultrafast Magnetisation Switching

 

Magnetisation switching and the more general area of magnetisation dynamics are important areas of research. Control of magnetisation dynamics and switching has potential technological applications, as well as being of fundamental interest from a scientific point of view. There are a number of ways of switching magnetisation, one way that we are currently exploring is using femtosecond laser systems to excite the system.

Related Publications:

  • Optimal electron, phonon and magnetic characteristics for ultra-low energy thermally induced magnetization switching. Applied Physics Letters, 107, 192402 (2015). (read more)
  • Laser Induced Magnetization Reversal for Detection in Optical Interconnects. IEEE Electron Device Letters, 35, 1317-1319 (2014). (read more).
  • Ultrafast Thermally Induced Magnetic Switching in Synthetic Ferrimagnets, Applied Physics Letters104, 082410 (2014). (read more).
  • Two magnon bound state causes ultrafast thermally induced magnetisation switching. Nature Scientific Reports, 3, 3262 (2013). (read more).
  • Ultrafast Dynamical Path for the Switching of a Ferrimagnet After Femtosecond Heating. Physical Review B87, 224417 (2013). (read more).
  • Ultrafast Heating as a Sufficient Stimulus for Magnetization Reversal. Nature Communications, 3, 666 (2012). (read more)
  • Transient Ferromagnetic-like State Mediating Ultrafast Reversal of Antiferromagnetically Coupled Spins. Nature, 472, 205-208 (2011). (read more)

Relaxation Processes in Magnetic Nanostructure

 

Understanding and quantifying the relaxation processes in magnetic materials across a range of time-scales is an interesting and technologically relevant for optimising the dynamics of magnetic materials. I use a range of methods in this area to reveal such properties, outlined below.

Ferromagnetic resonance (FMR) – is one such technique technique used to determine specific properties of a magnetic material. It works by applying static and oscillating magnetic fields to a system simultaneously and measuring the power loss. From the resonance peak a number of properties, such as, the damping, resonance frequency and gyromagnetic ration can be determined from the experiment. A complimentary methodology is the use of so-called optical FMR which uses a combination of applied fields (to determine the equilibrium position of the magnetisation) and short laser pulses to induce a temporary change in the internal fields of the magnetic via a relaxation process (precession). This can be used to determine the relaxation processes in high anisotropy materials, such as, FePt which requires large fields to saturate.

Spinwaves – are collective excitations of magnetic moments relevant across a broad range of time and length-scales in magnetic materials. Understanding their dispersion can provide important insights into observed dynamic effects.

Related Publications:

  • Temperature Dependent Ferromagnetic Resonance via the Landau-Lifshitz-Bloch Equation: Application to FePt. Physical Review B, 90 094402 (2014). (read more).
  • Two magnon bound state causes ultrafast thermally induced magnetisation switching. Nature Scientific Reports, 3, 3262 (2013). (read more).
  • Effects of Interactions on the Relaxation Processes in Magnetic Nanostructures. Physical Review B94,  134431 (2016). (read more). Read the blog post here.

 

Strain Induced Magnetization Dynamics in Composite Multiferroics

 

Magnetic vortex cores, often found in planar magnetic structures, arise from the complex interactions between the magnetostatic and exchange energy. In recent years the magnetization dynamics of the core have also been studied in great detail as the gyrotropic mode has applications not only in spin torque driven magnetic microwave oscillators, but also provides a means to flip the direction of the core such as magnetic random access memory. The excitation of the core gyrotropic mode can be achieved using various stimuli such as RF or pulsed magnetic fields, spin-polarized currents or the topological inverse Faraday effect. By constructing large scale micromagnetic models of planar magnetostrictive structures under the influence of a time-varying ferroelectric layer we are investigating a new means of stimulating magnetization dynamics in the magnetic layer.

Related Publications:

  • Strain Induced Vortex Core Dynamics in Planar Magnetostrictive Nanostructures. Physical Review Letters, 115, 067202 (2015). (read more)