Spin dynamics of magnetic nanoparticles: Beyond Brown’s theory

An investigation of thermally induced spin dynamics of magnetic nanoparticles is presented. We use an atomistic model for the magnetic interactions within an effective, classical spin Hamiltonian constructed on the basis of first-principles calculations for $L{1}_{0}$ FePt. Using Langevin dynamics we investigate how the internal degrees of freedom affect the switching behavior at elevated temperatures. We find significant deviations from a single-spin model, arising from the temperature dependence of the intrinsic properties, from longitudinal magnetization fluctuations, and from both thermal and athermal finite-size effects. These findings underline the importance of atomistic simulations for the understanding of fast magnetization dynamics.