Macrospin in external magnetic field: Entropy production and fluctuation theorems

Swarnali Bandopadhyay, Debasish Chaudhuri, A. M. Jayannavar

Published 2014-12-21Version 1

We consider stochastic rotational dynamics of a single magnetic domain in an external magnetic field, at constant temperature. Starting from the appropriate Langevin equation of motion, we calculate entropy production along stochastic trajectories to obtain fluctuation theorems, by presenting several possibilities of choosing conjugate trajectories. One of these choices gives entropy production in the reservoir that is consistent with prediction from Fokker-Planck equation. We further show the relation between heat absorbed and entropy production in the reservoir, using stochastic energy balance. For a time-independent field, the magnetization obeys Boltzmann distribution, however, also supports an azimuthal current making the dynamics inherently non-equilibrium. We use numerical simulations to obtain distribution functions for entropy production along trajectories which show good agreement with the detailed fluctuation theorem.