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Spontaneous skyrmion ground states in magnetic metals

Abstract

Since the 1950s, Heisenberg and others have addressed the problem of how to explain the appearance of countable particles in continuous fields1. Stable localized field configurations were searched for an ingredient for a general field theory of elementary particles, but the majority of nonlinear field models were unable to predict them. As an exception, Skyrme succeeded in describing nuclear particles as localized states, so-called ‘skyrmions’2. Skyrmions are a characteristic of nonlinear continuum models ranging from microscopic to cosmological scales3,4,5,6. Skyrmionic states have been found under non-equilibrium conditions, or when stabilized by external fields or the proliferation of topological defects. Examples are Turing patterns in classical liquids7, spin textures in quantum Hall magnets8, or the blue phases in liquid crystals9. However, it has generally been assumed that skyrmions cannot form spontaneous ground states, such as 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 based on a few material-specific parameters that can be determined experimentally. Our model has a condition not considered before: we allow for softened amplitude variations of the magnetization, characteristic of, for instance, metallic magnets. Our model implies that spontaneous skyrmion lattice ground states may exist 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.

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Figure 1: Three chiral modulated structures for noncentrosymmetric ferromagnets and comparison of their energy density.
Figure 2: Phase diagram of a chiral ferromagnet in terms of temperature versus longitudinal stiffness parameter ( T, η).
Figure 3: Details of skyrmion solutions for η = 0.4 and b = 0.05 at various temperatures.
Figure 4: Structure of a two-dimensional skyrmion lattice, derived as a minimum energy solution for the model's equation (1) with Dzyaloshinskii–Moriya interactions: equation (4).

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Acknowledgements

The permanent address of A.N.B. is the Donetsk Institute for Physics and Technology, Donetsk, Ukraine. We wish to thank B. Binz, H. Eschrig, I. Fischer, A. Möbius, A. Rosch, H. von Löhneysen, M. Vojta and W. Zwerger for support and discussions. We are particularly grateful to P. Böni for discussions on his studies of longitudinal magnetic fluctuations in EuS, Ni and MnSi. A.N.B. thanks the DFG-Graduiertenkolleg GRK 284 ‘Kollektive Pänomene im Festkörper’ for financial support. C.P. acknowledges support in the form of a Helmholtz-Hochschul-Nachwuchsgruppe at the Universität Karlsruhe in the initial part of this project.

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Rößler, U., Bogdanov, A. & Pfleiderer, C. Spontaneous skyrmion ground states in magnetic metals. Nature 442, 797–801 (2006). https://doi.org/10.1038/nature05056

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