Separation of domain walls with nonlocal interaction and their renormalised energy by $\Gamma$-convergence in thin ferromagnetic films

TL;DR

This paper studies the asymptotic behavior of N9el walls in thin ferromagnetic films using -convergence, revealing how nonlocal interactions influence their energy and interactions, with implications for micromagnetic models.

Contribution

It extends previous work by removing the separation assumption and develops a -convergence framework for the effective energy of N9el walls, including models with anisotropy.

Findings

Renormalised energy determines N9el wall interactions.

Pattern reversal compared to Ginzburg-Landau vortices.

Anisotropy alters the renormalised energy in unexpected ways.

Abstract

We analyse two variants of a nonconvex variational model from micromagnetics with a nonlocal energy functional, depending on a small parameter $\epsilon > 0$. The model gives rise to transition layers, called N\'eel walls, and we study their behaviour in the limit $\epsilon \to 0$. The analysis has some similarity to the theory of Ginzburg-Landau vortices. In particular, it gives rise to a renormalised energy that determines the interaction (attraction or repulsion) between N\'eel walls to leading order. But while Ginzburg-Landau vortices show attraction for degrees of the same sign and repulsion for degrees of opposite signs, the pattern is reversed in this model. In a previous paper, we determined the renormalised energy for one of the models studied here under the assumption that the N\'eel walls stay separated from each other. In this paper, we present a deeper analysis that in particular removes this assumption. The theory gives rise to an effective variational problem for the positions of the walls, encapsulated in a $\Gamma$-convergence result. In the second part of the paper, we turn our attention to another, more physical model, including an anisotropy term. We show that it permits a similar theory, but the anisotropy changes the renormalised energy in unexpected ways and requires different methods to find it.