X-ray Microscopy of Magnetic Topological Spin Textures – From van der Waals Magnets to Bulk Chiral Systems

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Scanning transmission x-ray microscopy (STXM) exploits the effects of x-ray magnetic circular dichroism to achieve magnetic contrast (Fig. 1). In this talk, I will highlight the capabilities of STXM – in particular the MAXYMUS microscope at the synchrotron BESSY II in Berlin – for the investigation of magnetic spin textures. I will draw comparisons to typical electron-based magnetic imaging techniques, demonstrating that STXM can provide complementary information, while offering advantages such as the ease of applying arbitrary external magnetic fields to the sample, the possibility for cryogenic cooling, and the opportunity for stroboscopic time-resolved imaging of dynamics. I will then showcase the recent works that we have achieved with this method on the topic of topological spin textures, including magnetic skyrmions, hosted in both van der Waals systems and bulk chiral magnets. Firstly, we shall look at results on the van der Waals magnets Fe3GeTe2 (F3GT) and Fe5GeTe2 (F3GT), where we observed the formation of stripe and skyrmion states primarily stabilized by the dipolar interaction and strong out-of-plane magnetocrystalline anisotropy (Fig. 2). The cryogenic capabilities of MAXYMUS allowed us to fully map the magnetic phase diagram of exfoliated flakes of these compounds [1,2], and led us to a method of inducing the formation of higher-order topological spin textures such as skyrmionium and skyrmion bag states (Fig. 2) [3,4]. Furthermore, I will detail our efforts to apply current stimuli, which allowed us to investigate current-induced skyrmion nucleation and motion in FGT flakes [5]. Secondly, we shall explore the imaging of the elongated string-like structure of skyrmions [6] in the bulk chiral magnet system Co8Zn9Mn3, made possible by the arbitrary orientation of the magnetic field achievable with x-ray microscopy. I shall highlight the associated current-induced Bloch-point toggling dynamics of this 3D topological object [7]. Finally, I will show our recent attempts at investigating topological spin textures in non-trivial sample geometries (beyond typical lamellae), achieved by novel focused ion beam fabrication methods. Schematic illustration of the scanning transmission x-ray microscopy (STXM) method. X-rays from the synchrotron are focused to a ∼20 nm spot size by a Fresnel zone plate (FZP) and order selecting aperture (OSA). An image of the sample (in this case an exfoliated FGT flake) is achieved by scanning the sample in the focal plane, and detecting the transmitted intensity with the photodiode. The optical micrograph shows the typical van der Waals magnet sample construction: an Fe3GeTe2 flake is stamped on top of a Si3N4 membrane for transmission measurements, and capped by a flake of hexaboron nitride to prevent oxidation. A selection of topological spin textures observed in the FGT flakes are shown, including stripe domain, skyrmion and high-order topological states such as a skyrmionium and a skyrmion bag.