Direct imaging of chiral domain walls in ferrimagnetic alloys

Abstract

Magnetic skyrmions are topological spin textures that can be stabilized by the Dzyaloshinskii-Moriya interaction [1,2]. There are particularly suitable for next generation spintronics devices, like the skyrmion-based racetrack memory [3]. Recent studies confirmed their current-driven skyrmion dynamics in ultrathin ferromagnets via spin-orbit torques [4], where the chirality and spin texture of the skyrmions are key. However, the topological Magnus effect leads to a transverse motion of ferromagnetic skyrmions due to their non-zero topological charge [5], which is disadvantageous for devices. Antiferromagnetically exchange-coupled skyrmions or compensated ferrimagnets could suppress this effect owing to an overall zero topological charge [6].Here we explore a GdFeCo-based ferrimagnet system with perpendicular magnetic anisotropy. We demonstrate that in this system the chirality of worm domains as well as a magnetic skyrmionium can be observed using scanning electron microscopy with polarization analysis (SEMPA) (Figure) [7]. The high spatial resolution magnetic imaging technique reveals the domain wall (DW) spin structure as a function of temperature, with the observed structures found to be pure Néel-type spin textures, which is promising for devices. From the images we extract the DW width as a function of the temperature and we calculate the exchange stiffness for the system.Figure: Determination of the magnetization direction inside the DW of different ferrimagnetic chiral spin textures. a) The direction of the in-plane magnetization in the DW is displayed, as defined by the color wheel in the bottom left corner of the image. Scale bar in a): 1 µm. b) Distribution of the direction of the in-plane magnetization in the DW with respect to the local tangent at RT: angle Ψ (see inset where a clockwise angle is defined as negative following the usual convention). A Gaussian fit indicates a central value around −90°. c) Absolute in-plane magnetization intensity with the DW skeleton displayed in white and d) direction of the in-plane magnetization in the DW of a ferrimagnetic skyrmionium at 320 K. Scale bars in c,d): 500 nm. Extracted from [7].