Simulations of magnetic reversal properties in granular recording media

With increasing demand for high density magnetic recording devices a paradigm shift is required to overcome the super-paramagnetic limit. By using a high anisotropy material, such as L1_0 FePt, and heat assisted magnetic recording the areal density can be taken well beyond 1 Tbit/in^2. For FePt the grains on which the data is stored can be reduced to 3nm in size before thermal noise becomes an issue. Therefore, the understanding of how magnetic grains of only a few nanometres in size behave and switch is highly important. Here an atomistic model of magnetic materials, based on the Heisenberg exchange interaction and localised classical atomic moments, is used to investigate the magnetic reversal properties of granular recording media. A detailed model for FePt, parametrised from ab-initio, is presented and this is used to investigate the finite size effects in nanometre grains. Using this model, the finite size effects on the linear reversal regime is investigated. Comparing a model using the Landau-Lifshitz-Bloch macrospin equation to the atomistic results show that multi-scale modelling may be valid down to the 3 nm length scale. The inter-granular exchange interaction between neighbouring grains is investigated using a model invoking the presence of magnetic impurities in the grain inter-layer. Using constrained Monte-Carlo methods the effective exchange is calculated for different impurity density, grain separation and temperature. Following this, helicity dependent all optical switching in granular FePt is investigated. The phase space for switching through the Inverse Faraday effect is explored. Using the Master equation, a model for thermal switching with magnetic circular dichroism is explored which qualitatively explains the induced magnetisation observed experimentally. Finally, the coupling of the spin and lattice system is modelled by combining spin and molecular dynamics. The coupling to the lattice excites magnons which appear to decay implying that some damping processes are occurring.