Towards multiscale modeling of magnetic materials: Simulations of FePt

The established methods for the numerical evaluation of magnetic material properties exist only in certain limits, including first-principles methods, spin models, and micromagnetics. In the present paper, we introduce a multiscale modeling approach, bridging the gaps between the three approaches above. The goal is to describe thermodynamic equilibrium and nonequilibrium properties of magnetic materials on length scales up to micrometers, starting from first principles. In the first step, we model, as an example, bulk FePt in the ordered L1(0) phase by using an effective, classical spin Hamiltonian that was constructed earlier on the basis of first-principles methods. The next step is to simulate this spin model by using the stochastic Landau-Lifshitz-Gilbert equation. The temperature dependent micromagnetic parameters, which are evaluated with these atomistic simulations, are consequently used to develop a many macrospin micromagnetic approach, based on the Landau-Lifshitz-Bloch equation. As an example, we calculate the magnetization dynamics following a pico-second heat pulse resembling pump-probe experiments.