Chiral surface spin textures in Cu$_2$OSeO$_3$ unveiled by soft x-ray scattering in specular reflection geometry

TL;DR

This paper introduces a novel soft x-ray scattering method in specular reflection geometry to study chiral spin textures on crystal surfaces, overcoming previous limitations related to crystal size and diffraction range.

Contribution

The authors demonstrate a new XRMS technique in specular reflection that enables surface-sensitive analysis of chiral spin textures in small crystals without extensive sample preparation.

Findings

Successfully observed diffraction peaks from helical and conical spin modulations.

Probed the chirality contributions to the dichroic scattered intensity.

Showed potential for studying noncentrosymmetric systems with soft x-rays.

Abstract

Resonant elastic soft x-ray magnetic scattering (XRMS) is a powerful tool to explore long-periodic spin textures in single crystals. However, due to the limited momentum transfer range imposed by long wavelengths of photons in the soft x-ray region, Bragg diffraction is restricted to crystals with the large lattice parameters. Alternatively, small angle x-ray scattering has been involved in the soft energy x-ray range which, however, brings in difficulties with the sample preparation that involves focused ion beam milling to thin down the crystal to below a few hundred nm thickness. We show how to circumvent these restrictions by using XRMS in specular reflection from a sub-nanometer smooth crystal surface. The method allows observing diffraction peaks from the helical and conical spin modulations at the surface of a Cu$_2$OSeO$_3$ single crystal and probing their corresponding chirality as contributions to the dichroic scattered intensity. The results suggest a promising way to carry out XRMS studies on plethora of noncentrosymmetric systems hitherto unexplored with soft x-rays due to the absence of the commensurate Bragg peaks in the available momentum transfer range.