On the coexistence of dipolar frustration and criticality in ferromagnets

TL;DR

This paper explores how dipolar interactions in ferromagnets lead to complex phase behavior, including modulated phases and avoided criticality, and proposes a generalized scaling hypothesis to explain experimental observations.

Contribution

It introduces a generalized scaling hypothesis for ferromagnets that accounts for dipolar interactions acting as a relevant field, explaining the coexistence of criticality and frustration.

Findings

Standard critical behavior is observed under small applied fields.

Dipolar interactions act as a relevant field in the renormalization group sense.

The proposed scaling hypothesis explains experimental power laws near the avoided critical point.

Abstract

In real magnets the tendency towards ferromagnetism, promoted by exchange coupling, is usually frustrated by dipolar interaction. As a result, the uniformly ordered phase is replaced by modulated (multi-domain) phases, characterized by different order parameters rather than the global magnetization. The transitions occurring within those modulated phases and towards the disordered phase are generally not of second-order type. Nevertheless, strong experimental evidence indicates that a standard critical behavior is recovered when comparatively small fields are applied that stabilize the uniform phase. The resulting power laws are observed with respect to a putative critical point that falls in the portion of the phase diagram occupied by modulated phases, in line with an avoided-criticality scenario. Here we propose a generalization of the scaling hypothesis for ferromagnets, which explains this observation assuming that the dipolar interaction acts as a relevant field, in the sense of renormalization group.