Sebastian Schneider1 Daniel Wolf1 Matthew Stolt2 Song Jin2 Darius Pohl1 Bernd Rellinghaus1 Marcus Schmidt3 Bernd Büchner1 Sebastian Goennenwein4 Kornelius Nielsch1 Axel Lubk1

1, IFW Dresden, Dresden, , Germany
2, University of Wisconsin-Madison, Madison, Wisconsin, United States
3, Max-Planck Institute for Chemical Physics of Solids, Dresden, , Germany
4, Technische Universität Dresden, Dresden, , Germany

Skyrmions [1] are topologically non-trivial vortex-like spin textures, anticipated for application in spintronic technologies, referred to as skyrmionics, in next generation magnetic data processing and storage due to their facile manipulation by spin-polarized currents of very low magnitude [2, 3]. Moreover, the unique features of skyrmions, e.g., their dynamics, topological structure, competing magnetic interactions, are generally of great interest from a fundamental physics point of view, understanding emerging magnetic field-like interactions induced by topologically non-trivial chiral spin structures. Envisaged applications of skyrmions in magnetic memory and logic devices crucially depend on the stability and mobility of these topologically non-trivial magnetic textures in thin films. In particular knowledge about the full three-dimensional spin texture including its coupling to surfaces and interfaces, ubiquitous in thin film technology, is of fundamental importance.

We combine transport of intensity (TIE) holography, focal series inline electron holography (EH), and off-axis EH to quantitatively reconstruct the projected magnetic field pertaining to both the helical and the skyrmion lattice phase of chiral magnet Fe0.95Co0.05Ge. By combining these investigations with electron tomography and magnetostatic simulations of the fields, we extract quantitative information on the 3D spin texture of skyrmions. Experiments are conducted in a double corrected FEI Titan3 80-300 microscope operated in imaging corrected Lorentz mode. A single crystal Fe0.95Co0.05Ge nanoplates is cooled to 90 K using a Gatan double tilt liquid nitrogen cooling holder. The objective lens is excited in such a manner, that an out-of-plane magnetic field of 0 mT and 43 mT is present at the sample, to study the helical and skyrmion phase respectively.

Our experiments yield an in-plane magnetic flux of up to 0.3 T in the skyrmions, which does not agree with the field values expected for z-invariant Bloch skyrmions. The analysis of a cryogenic tilt series of the helical phase results in the observation of a sinusoidal modulation of the Lorentz contrast [4]. Both findings provide for the first time experimental evidence for a characteristic 3D modulation of the skyrmionic spin texture that was theoretically predicted [5]. Instead of a z-invariant Bloch skyrmion, Néel like surface states surround an unmodulated Bloch skyrmion core. Our findings highlight the relevance of surfaces for the formation of skyrmions in thin film geometries and may pave the way towards a surface-induced tailoring of the skyrmion structure.

[1] A.N. Bogdanov and A. Hubert, J. Magn. Magn. Mater. 138, 255 (1994).
[2] N. Nagaosa and Y. Tokura, Nature Nanotech 8, 899 (2013).
[3] N. Kanazawa et al., “Noncentrosymmetric Magnets Hosting Magnetic Skyrmions” (2017).
[4] S. Schneider et al., arXiv:1710.08322 [cond-mat.mtrl-sci] (2017)
[5] F.N. Rybakov et al., New J. Phys. 18, 045002 (2016).