# The Sine-Gordon regime of the Landau-Lifshitz equation with a strong easy-plane anisotropy

## TL;DR

This paper rigorously proves that solutions to the Landau-Lifshitz equation for biaxial ferromagnets with strong easy-plane anisotropy converge to solutions of the Sine-Gordon equation, confirming the regime's dynamics.

## Contribution

It provides a rigorous justification and sharp convergence results for the Landau-Lifshitz equation approaching the Sine-Gordon regime under strong easy-plane anisotropy.

## Key findings

- Convergence of Landau-Lifshitz solutions to Sine-Gordon solutions established.
- High order Sobolev space analysis for well-posedness and convergence.
- Derivation of the free wave regime of the Landau-Lifshitz equation.

## Abstract

It is well-known that the dynamics of biaxial ferromagnets with a strong easy-plane anisotropy is essentially governed by the Sine-Gordon equation. In this paper, we provide a rigorous justification to this observation. More precisely, we show the convergence of the solutions to the Landau-Lifshitz equation for biaxial ferromagnets towards the solutions to the Sine-Gordon equation in the regime of a strong easy-plane anisotropy. Moreover, we establish the sharpness of our convergence result. This result holds for solutions to the Landau-Lifshitz equation in high order Sobolev spaces. We first provide an alternative proof for local well-posedness in this setting by introducing high order energy quantities with better symmetrization properties. We then derive the convergence from the consistency of the Landau-Lifshitz equation with the Sine-Gordon equation by using well-tailored energy estimates. As a by-product, we also obtain a further derivation of the free wave regime of the Landau-Lifshitz equation.