This title appears in the Scientific Report : 2019 

Topological transport in non-Abelian spin textures from first principles
Buhl, Patrick (Corresponding author)
Quanten-Theorie der Materialien; PGI-1
Quanten-Theorie der Materialien; IAS-1
Jülich Forschungszentrum Jülich GmbH Zetralbibliothek, Verlag 2019
vii, 158
Dissertation, RWTH Aachen University, 2019
Dissertation / PhD Thesis
Controlling Spin-Based Phenomena
Schriften des Forschungszentrums Jülich Reihe Schlüsseltechnologien / Key Technologies 197
Please use the identifier: in citations.
Recently, skyrmions attracted huge attention due to their topological character which ensuressurprisingly stable, particle-like magnetic excitations on small scales with distinctive dynamicalproperties. Their characteristic transport signature—the topological Hall effect—has becomean established tool for detection of topologically non-trivial ferromagnetic textures. However,this attribute vanishes when considering degenerate antiferromagnetic structures as theassociated emergent magnetic field is spin-dependent. This thesis demonstrates the emergenceof an alternative transport signature in case of antiferromagnetic skyrmion textures—thetopological spin Hall effect.Firstly, a computational scheme is developed which estimates the topological spin Hall effectbased on semiclassical wave-packet dynamics. In the adiabatic limit, their equations of motionallow to treat large-scale magnetic textures on top of locally collinear, small-scale Hamiltonians,here based on density functional theory. Transport expressions are extracted by combinationof the equations of motion and the Boltzmann formalism. While the analogous procedureis straightforward for ferromagnetic materials, the wave-packet’s SU(2)-nature, caused bydegenerate bands, results in additional spin dynamics and non-abelian Berry curvatures whichinhibit direct transport evaluation. While the reciprocal-space dynamics are treated on theBoltzmann level, the spin and real-space dynamics are solved iteratively starting from multipleinitial positions. Evaluation of the traversed paths results in integrated expressions for thetopological spin Hall effect.Sizable topological spin Hall responses are predicted in simulations for the exemplaryFe/Cu/Fe-trilayers and thin chromium layers when artificially imprinting synthetic and intrinsicantiferromagnetic skyrmions, respectively. The importance of the non-abelian dynamics isdemonstrated by large differences relative to comparative calculations of decoupled antiparallelferromagnets. While the spin evolution results in surprisingly homogeneous transportmodifications, the k-resolved intra-band overlap has a particularly unpredictable distributionrequiring precise density functional theory calculations. Further numerical thoroughness isrequired because of extreme sensitivity with respect to small reciprocal-space modificationssuch as slight Fermi energy changes. Furthermore, the evolution of the k-dependent transportand overlap properties is shown with respect to thickness variations demonstrating richtuning potential. Conversely, multiple calculations modifying the skyrmion-radius, -shape,and -density demonstrate the topological invariance of the topological spin Hall effect.Overall, the topological spin Hall effect is an interesting phenomenon with rich applicationpossibilities. Foremost, it facilitates the discovery of the so far undetected antiferromagneticskyrmions, but also might provide efficient spin-current generation as required in spintronicapplications. Alternatively, it could serve as read-out mechanism of more complex deviceslike antiferromagnetic, skyrmion-based racetrack memory. Hence, the developed versatileand readily applicable computational scheme is a great addition for future antiferromagneticskyrmion studies.