Macrospin in external magnetic field: Entropy production and fluctuation theorems

Abstract

We consider stochastic rotational dynamics of a macrospin at a constant temperature, in presence of an external magnetic field. Starting from the appropriate Langevin equation which contains multiplicative noise, we calculate entropy production (EP) along stochastic trajectories, and obtain fluctuation theorems. The system remains inherently out of equilibrium due to a spin torque supporting azimuthal current, leading to an excess EP apart from the EP due to heat dissipation. The anomaly may be removed using a redefinition of dissipated heat and stochastic work done. Using numerical simulations, we obtain distribution functions for entropy production along stochastic trajectories to find good agreement with the detailed fluctuation theorem.