Ferronematics: integrality of the limiting interface in the strong coupling regime

Abstract

We consider a vectorial energy functional proposed in the physical literature as a simplified model for thin films of ferronematics -- composite materials formed by dispersing magnetic nanoparticles into a nematic liquid crystals host. The model features two order parameters: a reduced $\Q$-tensor field $\Q$, describing the nematic liquid crystal component, and a vector field $\M$, accounting for the average polarisation vector field generated by the included particles. The energy contains a coupling term promoting alignment between $\Q$ and $\M$. It has been shown in~\cite{CDS1,CDS2} that, as a small parameter $\eps$ tends to zero, the energy of critical pairs $(\Q_\eps,\,\M_\eps)$ concentrates on \emph{distinct} singular sets: a finite set of points for the $\Q_\eps$-component and the support of a $\H^1$-rectifiable varifold for the $\M_\eps$-component, with first variation supported on the singular set for the $\Q_\eps$-component. We show in this paper that, if the coupling constant is above a certain (explicit) threshold, then, up to rescaling its density by a material constant, such a limiting varifold has integer multiplicity.