Modulated phases in external fields: when is reentrant behavior to be expected?

TL;DR

This paper presents a new coarse-grained model for two-dimensional ferromagnetic systems with competing interactions and external fields, revealing conditions under which reentrant phase behavior occurs or is absent.

Contribution

It introduces an improved mean field model that accurately predicts phase diagrams and explains reentrant behavior in ultrathin ferromagnetic films with competing interactions.

Findings

Reentrant behavior occurs in certain regimes of interaction strength.

Reentrance is linked to suppression of domain wall degrees of freedom.

Phase diagram topology varies with the ratio of competing interactions.

Abstract

We introduce a new coarse grain model capable of describing the phase behavior of two dimensional ferromagnetic systems with competing exchange and dipolar interactions, as well as an external magnetic field. An improved expression for the mean field entropic contribution allows to compute the phase diagram in the whole temperature versus external field plane. We find that the topology of the phase diagram may be qualitatively different depending on the ratio between the strength of the competing interactions. In the regime relevant for ultrathin ferromagnetic films with perpendicular anisotropy we confirm the presence of inverse symmetry breaking from a modulated phase to a homogenous one as the temperature is lowered at constant magnetic field, as reported in experiments. For other values of the competing interactions we show that reentrance may be absent. Comparing thermodynamic quantities in both cases, as well as the evolution of magnetization profiles in the modulated phases, we conclude that the reentrant behavior is a consequence of the suppression of domain wall degrees of freedom at low temperatures at constant fields.