Spontaneous Skyrmion Ground States in Magnetic Metals

TL;DR

This paper demonstrates theoretically that skyrmion textures can spontaneously form as ground states in magnetic materials with chiral interactions, challenging previous assumptions and broadening potential applications.

Contribution

It introduces a phenomenological continuum model allowing for spontaneous skyrmion ground states without external fields or defects, considering softened amplitude variations.

Findings

Spontaneous skyrmion ground states are theoretically possible in magnetic systems.

The model applies to materials with chiral interactions, especially at surfaces and in thin films.

Skyrmion lattices may be stable without external stabilization mechanisms.

Abstract

Since the 1950s Heisenberg and others have attempted to explain the appearance of countable particles in quantum field theory in terms of stable localized field configurations. As an exception Skyrme's model succeeded to describe nuclear particles as localized states, so-called 'skyrmions', within a non-linear field theory. Skyrmions are a characteristic of non-linear continuum models ranging from microscopic to cosmological scales. Skyrmionic states have been found under non-equilibrium conditions, or when stabilised by external fields or the proliferation of topological defects. Examples are Turing patterns in classical liquids, spin textures in quantum Hall magnets, or the blue phases in liquid crystals, respectively. However, it is believed that skyrmions cannot form spontaneous ground states like ferromagnetic or antiferromagnetic order in magnetic materials. Here, we show theoretically that this assumption is wrong and that skyrmion textures may form spontaneously in condensed matter systems with chiral interactions without the assistance of external fields or the proliferation of defects. We show this within a phenomenological continuum model, that is based on a few material-specific parameters that may be determined from experiment. As a new condition not considered before, we allow for softened amplitude variations of the magnetisation - a key property of, for instance, metallic magnets. Our model implies that spontaneous skyrmion lattice ground states may exist quite generally in a large number of materials, notably at surfaces and in thin films as well as in bulk compounds, where a lack of space inversion symmetry leads to chiral interactions.