Coexistence of dipolar frustration and criticality in ferromagnets

Abstract

In real magnets the tendency toward ferromagnetism - promoted by exchange coupling - is usually frustrated by dipolar interaction. As a result, the uniformly ordered phase is replaced by modulated (multidomain) phases, characterized by different order parameters rather than the global magnetization. The transitions occurring within those modulated phases and toward 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. We corroborate this proposal with analytic and numerical calculations on the 2D Ising model frustrated by dipolar interaction (isotropic part). Our analysis is directly applicable to thin magnetic films with out-of-plane anisotropy.