## Computational micromagnets and nanomagnetism research projects:

### (PhD/MSc) Spin/conduction electron/photon interactions

Prof RW ChantrellInterest in this topic is currently very high, prompted by a number of 'pump-probe' experiments in which a magnetic material is subjected to a laser pulse of 100fs duration and the magnetic properties following the pulse measured using the Magneto-optic Kerr effect with a low power laser. These measurements are giving new insight into the sub-ps dynamics of the magnetisation. One of the major challenges is to understand the mechanisms underlying the transfer of energy from the laser to the magnetic spin system via the conduction electrons. The project involves the development of theoretical approaches to the problem, in particular the derivation of spin Hamiltonian representations, which could be used in large-scale atomistic simulations. The practical interest in the project arises from the fact that the spin/electron/photon interaction is fundamental to the technique of 'Heat Assisted Magnetic Recording', in which the magnetic recording medium is rapidly heated by a laser during the writing process. HAMR is at a very early stage and is being developed with a view to long-term applications. The project will involve collaboration with the Seagate research center in Pittsburgh via an experimental group carrying out Pump-probe measurements and with a condensed matter theorist (Dr. A Rebei) who has already made an initial study of the problem.

### (PhD/MSc) Atomistic calculations of magnetisation dynamics

Prof RW ChantrellThe term atomistic calculations refers to the use of classical spin Hamiltonians to carry out simulations of systems with large numbers of atoms (greater than can be achieved with ab-initio calculations. Atomistic simulations are especially important in magnetic materials, which are generally nanostructured in that they contain grains of less than 10nm in size. In such systems finite size effects are of great importance, and atomistic calculations are vital to the understanding of their equilibrium and non-equilibrium properties. The project will focus on the understanding of interface properties, in particular the simulation of mixing at the interface between a magnetic and non-magnetic material and its effect on the magnetic and electron transport properties.

### (PhD/MSc) Order/disorder phenomena in L1_{0} alloys

Prof RW Chantrell
L1_{0} alloys such as FePt are extremely important because of their large magnetic anisotropy. However, thin films and nanostructured FePt tend to form in the low anisotropy fcc phase in which the Fe and Pt atoms are distributed randomly among the lattice sites. The fcc phase must be annealed in order to achieve a transformation to the L1_{0} phase. The project concerns the development of a model of the phase transformation, and will involve ab-initio calculations to determine the parameters for a Molecular Dynamic simulation of the phase transformation. Of special interest is the dynamics of the transformation and the effects of finite size and surface segregation potentials on the ordering temperature.

### (PhD/MSc) Magnetic domain wall motion driven by spin currents

Prof RW ChantrellMagnetic materials build the basis for state-of-the-art data storage. Magnetic devices based especially on domain walls have been suggested for future data storage and logic. Rather than using conventional magnetic fields to move domain walls, recently, current-induced domain wall motion has received much attention since it opens up a route for simple device fabrication, as no field-generating parts are necessary. Apart from possible applications, the interplay between spin currents and domain walls in magnetic nanostructures is of fundamental interest, since the basic physical mechanisms involved are not completely understood. Nevertheless, the controlled current-induced motion of single domain walls in magnetic nanostructures has been achieved experimentally.

The usual Landau-Lifshitz-Gilbert equation of motion for a magnetic system does not account for the spin transfer torque effect, which leads to current-induced domain wall motion, and so it has to be extended. Theoretically, the phenomenon of current-induced domain wall motion has been known for a long time but the underlying theory of the interaction between current and magnetisation is still controversial. It is the goal of this project to perform numerical calculations of equations of motion which give rise to current-induced domain wall motion. Rather than the conventional micromagnetic finite difference or finite element methods, an atomistic Heisenberg spin model is used. Quantities of interest are the domain wall velocity, domain wall pinning effects, and the influence of heating processes which cannot be avoided in experimental situations due to the very high current densities.

### (PhD/MSc) Spin dynamics in the picosecond regime: the response to a Laser pulse

Prof RW ChantrellLaser-induced spin dynamics in the pico- and sub-picosecond regime opens new perspectives for applications in magnetic storage devices, spintronics, and quantum computation. Writing information on time-scales below conventional magnetisation reversal schemes appears to be within reach. Of fundamental interest in this context is the understanding of magnetisation processes, reflecting the fundamental physical mechanisms involved in the coupling between spin, charge and lattice. Experimentally, at the forefront of these investigations is the use of pump-probe techniques. These involve the use of a high-energy femtosecond laser to cause local rapid heating of a ferromagnetic material with a consequent change of magnetisation. Typically the response of the magnetisation is then measured using the Magneto-Optical Kerr Effect.

In this project we investigate the dynamic response of a magnetic system to pulsed laser heating by solving the stochastic Landau-Lifshitz-Gilbert equation of motion for the spin system numerically. The goals are the determination of typical time scales for the rapid decrease of the magnetisation as well as its recovery, the estimation of lattice, electron, and spin temperature, and the investigation of the possibility of magnetisation reversal processes in the picosecond regime. The figure shows the calculated magnetisation distribution in a Ni film shortly after the laser pulse.