Chiral Non-Collinear Spin Textures in Nominally Centrosymmetric Magnets

One of the major topics in spintronics today is to study magnetic materials in which neighboring magnetic moments are neither parallel nor anti-parallel but are rather non-collinear and can thereby form complex, topological, spin textures. Perhaps the most extensively studied of these is the skyrmion; this is a tiny magnetic nano-object with a chiral magnetic boundary and which is typically circular in shape. A prerequisite for the formation of such chiral nano-objects is that the parent material has a structure that is non-centrosymmetric. What has surprised the spintronics community recently is the observation of skyrmions in several materials that are centrosymmetric. Here we show evidence for skyrmions in a member of the family of van der Waals materials, CrTe₂, that is nominally centrosymmetric. The symmetry of the structure of this 2D compound is lowered in a very special manner by self-intercalation of Cr atoms within the so-called van der Waals gaps between tri-layers of Te-Cr-Te from which the structure is formed. Furthermore, we show from detailed, high-resolution, x-ray structure analysis that the self-intercalated Cr atoms form an ordered 3D superstructure that hasn’t previously been observed in any known van der Waals compound. The superstructure gives rise to two inequivalent van der Waals gaps, both partially filled by Cr atoms but in different ways. As a consequence, neighboring Te-Cr-Te tri-layers experience an asymmetric environment involving relaxations of the atomic positions along the vertical <i>c</i>-axis, thereby reducing the crystal symmetry from Pm to P3m1, the latter being non-centrosymmetric. The latter has a <i>C</i>_3<i>v</i> point group that is compatible with the appearance of Néel-type skyrmions, which were directly imaged using state-of-the-art Lorentz transmission electron microscopy over a wide temperature range.