Ising metamagnets in thin film geometry: equilibrium properties

TL;DR

This paper investigates the equilibrium properties of thin Ising metamagnets using Monte Carlo methods, revealing a complex phase diagram with an additional surface-aligned phase and a critical end point.

Contribution

It provides the first detailed analysis of the phase diagram of thin Ising metamagnets, including surface effects and phase transitions, using Wang/Landau and importance sampling techniques.

Findings

Identification of an intermediate phase with surface layer alignment

Discovery of a discontinuous phase transition ending at a critical point

Phase diagram complexity differs from bulk systems with additional phases

Abstract

Artificial antiferromagnets and synthetic metamagnets have attracted much attention recently due to their potential for many different applications. Under some simplifying assumptions these systems can be modeled by thin Ising metamagnetic films. In this paper we study, using both the Wang/Landau scheme and importance sampling Monte Carlo simulations, the equilibrium properties of these films. On the one hand we discuss the microcanonical density of states and its prominent features. On the other we analyze canonically various global and layer quantities. We obtain the phase diagram of thin Ising metamagnets as a function of temperature and external magnetic field. Whereas the phase diagram of the bulk system only exhibits one phase transition between the antiferromagnetic and paramagnetic phases, the phase diagram of thin Ising metamagnets includes an additional intermediate phase where one of the surface layers has aligned itself with the direction of the applied magnetic field. This additional phase transition is discontinuous and ends in a critical end point. Consequently, it is possible to gradually go from the antiferromagnetic phase to the intermediate phase without passing through a phase transition.