Abstract
From the perspective of surface science, only the topmost atomic layers usually exhibit physical properties that are different to those of the bulk material, whereas the deeper layers are assumed to be bulk-like and remain largely unexplored. Going beyond conventional diffraction and imaging techniques, we have determined the depth dependence of the full 3D spin structure of magnetic skyrmions below the surface of a bulk Cu2OSeO3 sample using the polarization dependence of resonant elastic x-ray scattering (REXS). While the bulk spin configuration showed the anticipated Bloch type structure, it was found that the skyrmion lattice changes to a Néel twisting (i.e., with a different helicity angle) at the surface within a distance of several hundred nm. The exact surface helicity angle and penetration length of this twist have been determined, revealing the detailed internal structure of the skyrmion tube. It was found that the experimental penetration length of the Néel twisting is 7× longer than the theoretical value given by the ratio of J/D. This indicates that apart from the considered spin interactions, i.e., the Heisenberg exchange interaction J and the Dzyaloshinskii-Moriya interaction D, as well as the Zeeman interaction, other effects must play an important role. The findings suggest that the surface reconstruction of the skyrmion lattice is a universal phenomenon, stemming from the breaking of translational symmetry at the interface.
Highlights
A sharp distinction is drawn between surface and bulk properties, and it is only seldom specified what happens further below the surface, and how deep the transition from surface to bulk properties extends below the surface
We describe how the polarization dependence of resonant elastic x-ray scattering (REXS) can be used to probe the spin twisting of skyrmion tubes at depths of hundreds of nm below the surface
We have demonstrated that circular dichroism (CD)-REXS unambiguously measures the helicity angle of skyrmions near the surface by using the universal dichroism extinction rule (DER)
Summary
A sharp distinction is drawn between surface and bulk properties, and it is only seldom specified what happens further below the surface, and how deep the transition from surface to bulk properties extends below the surface. Skyrmions are topologically nontrivial whirls of the magnetization which were theoretically proposed by Bogdanov et al.1 They were experimentally observed by Mühlbauer et al in the chiral magnet MnSi using small angle neutron scattering (SANS), where they form a hexagonal lattice with a periodicity of ∼18 nm.. The well-defined spin topology in real space enables the magnetic skyrmions to many intriguing quantum mechanical phenomena including emergent electromagnetic dynamics, effective magnetic monopole, topological/skyrmion Hall effects.. The well-defined spin topology in real space enables the magnetic skyrmions to many intriguing quantum mechanical phenomena including emergent electromagnetic dynamics, effective magnetic monopole, topological/skyrmion Hall effects.6 Due to their topological properties, magnetic skyrmions can behave as metastable quasiparticles and have been proposed as information carriers for ultralow power nonvolatile spintronics. They were experimentally observed by Mühlbauer et al in the chiral magnet MnSi using small angle neutron scattering (SANS), where they form a hexagonal lattice with a periodicity of ∼18 nm. This was soon followed by real-space observation using Lorentz transmission electron microscopy (LTEM). The well-defined spin topology in real space enables the magnetic skyrmions to many intriguing quantum mechanical phenomena including emergent electromagnetic dynamics, effective magnetic monopole, topological/skyrmion Hall effects. due to their topological properties, magnetic skyrmions can behave as metastable quasiparticles and have been proposed as information carriers for ultralow power nonvolatile spintronics.
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