Computer Simulation and Mean Field Calculation of Phase Diagram and Adiabatic Demagnetization Paths for an Antiferromagnet

A Monte Carlo simulation of a two-dimensional nuclear antiferromagnet is presented and compared with mean field calculations. The model includes nearest neighbor dipolar and exchange interactions and a single ion term. The phase diagram as a function of field and temperature is calculated, and the constant entropy lines, which are the paths of adiabatic nuclear cooling, are included. The fluctuations reduce the transition temperatures strongly, whereas the isentrops are altered much less relative to those calculated by the mean field theory. The dynamics of the ordering process after "quenches" is studied under constant field, temperature, and entropy conditions.