Linearly polarised light has the effect of generating heat in the magnetic material. To incorporate the laser heating into the model we require a well defined temperature, which will vary with time. The model we are using allows time variations of the temperature via the stochastic process. We use a two-temperature model that defines a temperature associated with delocalized electrons and the phonons of the system[3]:

Ultrafast and Distinct Spin Dynamics in Magnetic

Alloys and Heterostructures

I. Radu1, T. A. Ostler2* et al.

1Helmholtz Zentrum, Berlin, Germany2The University of York, York, UK1. Introduction

2. Atomistic Model

Our model solves a set of coupled Landau-Lifshitz-Gilbert equations for an (3D) ensemble of spin:

The Hamiltonian of the system contains only Heisenberg (nearest neighbour) exchange and anisotropy:

The effective field in the LLG equation is then written, ,

with, , being the (stochastic) thermal term. The thermal term has the properties:

3. Disordered Moment Model

Using our model of atomistic spin dynamics we modelled a ferrimagnetic (GdFeCo[1,2]) and a ferromagnetic material (NiFe) with two distinct species

4. Incorporating Laser Heating

5. Results

We predict that the demagnetization time scales with the atomic magnetic moment based on our model:

Supported experimentally for a range of materials.

6. Conclusions

References:[1] – Radu et al. Nature 472, 205-208 (2011).[2] – Ostler et al. Phys. Rev. B 84, 024407 (2011).[3] – Chen et al. Int. Journ. Heat and Mass Trans. 49, 307-316 (2006).

Although the next generation of ultrahigh density magnetic data storage will rely on tuned material properties, such as, Curie temperature or magnetic anisotropy, realized on alloys consisting of several elements, theoretical treatment of their magnetization dynamics has so far assumed equilibrium between their sublattices. Such a picture fails to describe processes that occur on the sub-picosecond timescale, such as femtosecond laser excitation.

Using a model of atomistic spin dynamics and X-ray magnetic circular dichroism (XMCD) experiments, we show magnetization processes on the sub-picosecond timescale vary depending on the magnetic moments and the exchange interaction that couples them.

Numerical Simulations

Experimental Measurements

Both simulations and experiments show different demagnetization times for different species for both ferromagnetically and antiferromagnetically coupled alloys.