Macrospin in external magnetic field: Entropy production and fluctuation theorems

TL;DR

This paper investigates the stochastic rotational dynamics of a macrospin under an external magnetic field, deriving entropy production and fluctuation theorems, and analyzing the system's out-of-equilibrium behavior through simulations.

Contribution

It derives fluctuation theorems for macrospin dynamics with multiplicative noise and clarifies the nature of excess entropy production due to spin torque.

Findings

Distribution functions for entropy production match fluctuation theorem predictions

System exhibits excess entropy production due to spin torque

Redefinition of heat and work removes entropy anomaly

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.