Long-periodic spin textures in vector magnetic fields as seen by resonant elastic x-ray scattering

Victor Ukleev

Resonant elastic small-angle soft x-ray scattering (REXS) is the unique tool that allows to study long-periodic spin textures in noncentrosymmetric magnets with unprecedented reciprocal-space resolution. Moreover, the unique sample environment allows one to investigate hitherto unexplored pathways of spin texture transformations and extract small parameters that are inaccessible with other real or reciprocal-space methods. Here we will focus on two recent case studies of an antiskyrmion host Mn1.4PtSn and of a chiral skyrmion host Cu2OSeO3 using REXS.

Multiple intriguing phenomena have recently been discovered in tetragonal Heusler compounds, where D2d symmetry sets a unique interplay between Dzyaloshinskii-Moriya (DM) and magnetic dipolar interactions. In the prototype D2d compound Mn1.4PtSn, this has allowed the stabilization of exotic spin textures, such as first-reported antiskyrmions or elliptic Bloch-type skyrmions [1,2,3]. Although less attention has so far been given to the low-field spiral state, this remains extremely interesting as a simplest phase scenario on which to investigate the complex hierarchy of magnetic interactions in this materials family. Here, via REXS experiments on high-quality single crystals of Mn1.4PtSn at low temperatures, we evidence how the underlying D2d symmetry of the DM interaction in this material is reflected in its magnetic texture. Our studies reveal the existence of a novel and complex metastable phase, which possibly has a mixed character of both the Néel-type cycloid and the Bloch-type helix, that forms at low temperature in zero fields upon the in-plane field training. This hybrid spin-spiral structure has a remarkable tunability, allowing to tilt its orientation beyond high-symmetry crystallographic directions and control its spiral period [4].

On the other hand, in a chiral cubic skyrmion host Cu2OSeO3 the ground-state helical magnetic structures of cubic chiral systems is well explained by the Bak-Jensen model that considers the interplay between Heisenberg exchange interaction, DMI, anisotropic exchange interaction (AEI), and cubic anisotropy [5]. The weak cubic anisotropy determines the spin-wave gap and some additional peculiarities of the helix axis orientation under a magnetic field, while the exchange, DMI and AEI are responsible for the helical spiral and its orientation relative to crystal axes [6]. The latter interaction is often neglected due to its weak impact on experimental observations. However, the cubic and exchange anisotropies are important for defining the propagation direction of the helix, and ultimately the orientation of any field-induced skyrmion lattice in these materials. Moreover, their role which has been neglected for decades, has been recently found to be manifested in exotic low-temperature states of Cu2OSeO3: tilted conical spiral and disordered skyrmion phase [7]. We used an elegant REXS method in vector magnetic fields [8] to quantify the AEI in Cu2OSeO3 found that the AEI is strongly pronounced at low temperatures < 35 K resulting in the conical spiral pitch variation of 10% for particular magnetic field orientations relatively to crystal axes.

These results broaden the richness of the exotic magnetic phase diagrams of noncentrosymmetric magnets and extend their tunability, thus, enhancing a relevant playground for further fundamental explorations and potential applications in energy-saving technologies.

 

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  8. Ukleev, et. al., Phys. Rev. Res. 3, 013094 (2021).