Adiabatic demagnetization of antiferromagnetic systems: A computer simulation of a planar-rotor model.

Abstract

We report a computer simulation of the adiabatic demagnetization of a two-dimensional antiferromagnetic (AFM) model system, based on the molecular-dynamics method, and motivated by recent studies in the nuclear magnetism of copper. As opposed to Monte Carlo computations, we are able to calculate directly the isentropes B(T) with perfect control of adiabaticity, having macroscopic reversibility available as a signature. We then find a distinct minimum temperature at a nonzero external field in the vicinity of the phase-transition line separating the antiferromagnetically ordered phase from the high-temperature region. This is compatible with both earlier Monte Carlo calculations of Lindgard, Vierti\"o, and Mouritsen [Phys. Rev. B 38, 6798 (1988)] and a mean-field-approximation analysis also presented in this paper. We propose that the shape of the isentropes is a feature of simple AFM systems with next-neighbor interactions.