Multipolar interactions and complex phases in ferromagnetic ultrathin films

TL;DR

This paper introduces a model for ultrathin magnetic films that incorporates dipolar and quadrupolar interactions, predicting complex phases including a nematic phase with orientational order of domain walls, and discusses experimental signatures of these phases.

Contribution

It presents a novel model emphasizing quadrupolar degrees of freedom to explain complex magnetic phases and predicts an isotropic-nematic transition of the Kosterlitz-Thouless type in ultrathin films.

Findings

Prediction of a nematic phase with orientational order of domain walls

Identification of the isotropic-nematic transition as Kosterlitz-Thouless type

Experimental observables include magnetic structure factor and susceptibility

Abstract

We present a model to describe complex phases observed at mesoscopic scales in ultrathin magnetic films. The model is based on the interaction between dipolar as well as quadrupolar magnetic moments. In the special case of strong perpendicular crystal anisotropy, we show that quadrupolar degrees of freedom associated with orientation of domain walls are essential in order to correctly describe the observed phenomenology of domain formation. A nematic phase characterized by orientational order of domain walls (stripe-like) but without translational order is predicted. This isotropic-nematic transition belongs to the Kosterlitz-Thouless type in the thermodynamic limit. However, we found that in actual experimental scales, the fluctuations of the nematic order parameter are regularized by the sample size, and orientational long range order, as predicted by mean field, should be observable. The transition may be completely characterized experimentally from measurements of the magnetic structure factor as well as from the non-linear in-plane magnetic susceptibility.