Stability of in-plane and out-of-plane chiral skyrmions in epitaxial MnSi(111)/Si(111) thin films: surface twists versus easy-plane anisotropy

TL;DR

This paper revises theoretical phase diagrams for thin film cubic helimagnets, showing that surface twists and easy-plane anisotropy stabilize both in-plane and out-of-plane skyrmions, with implications for experimental studies.

Contribution

It demonstrates that surface twists enable out-of-plane skyrmion stability and that easy-plane anisotropy extends in-plane skyrmion stability in epitaxial MnSi(111)/Si(111) films.

Findings

Surface twists stabilize out-of-plane skyrmions.

Easy-plane anisotropy increases in-plane skyrmion stability.

A stable oblique spiral state emerges due to anisotropy and surface twists.

Abstract

The revisited theoretical phase diagrams for thin films of cubic helimagnets with the easy-plane anisotropy are shown to have different topology as previously reported [Phys. Rev. B 85, 094429 (2012)]. For both in-plane and out-of-plane directions of an applied magnetic field, the phase diagrams exhibit extensive areas of stable skyrmions, which overlap for a wide range of anisotropy parameters. Although the existence of the out-of-plane skyrmions was contradicted within the previous theoretical models, we prove that additional surface twists lead to their stability, while the moderate easy-plane anisotropy increases the stability range of in-plane skyrmions. Moreover, the interplay between the anisotropy and the surface twists gives rise to a stable spiral state canted with respect to the surfaces. Being absent in bulk helimagnets, this oblique spiral occupies vast areas at the phase diagrams in thin-film nanosystems and serves as a connecting-link between cones and helicoids. Our theory gives clear directions for renewed experimental studies of in-plane and out-of-plane skyrmions in epitaxial MnSi(111)/Si(111) thin films.