This title appears in the Scientific Report : 2018 

Topological properties of complex magnets from an advanced ab-initio Wannier description
Hanke, Jan-Philipp (Corresponding author)
Quanten-Theorie der Materialien; IAS-1
Quanten-Theorie der Materialien; PGI-1
Jülich Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag 2018
xi, 173
Dissertation, RWTH Aachen University, 2018
Dissertation / PhD Thesis
Controlling Spin-Based Phenomena
Schriften des Forschungszentrums Jülich Reihe Schlüsseltechnologien / Key Technologies 183
Please use the identifier: in citations.
Berry phases impart an elegant interpretation of fundamental condensed-matter phenomenaas a direct consequence of the electrons' adiabatic evolution under the variationof control parameters. This thesis develops advanced ab initio methods based ondensity functional theory and applies them to investigate Berry phase effects in complexmagnets, rooting in the global properties of two distinct types of phase spaces.The non-trivial geometry of momentum space manifests in intrinsic contributionsto the anomalous Hall effect as well as orbital magnetism in solids. While the formerhas been subject to intensive research in the past decades, our understanding oforbital magnetism in periodic systems is still at a rather premature stage. Even itsquantum-mechanical description was elusive until the recent advent of a rigorous butinvolved Berry phase theory, the overall importance of which is unclear. To resolvethis open question, we implement the modern theory of orbital magnetization withinthe full-potential linearized augmented-plane-wave method that is known for its highprecision. By comparing to a commonly applied but simple local approximation,we uncover in this thesis that the Berry phase theory is crucial to predict reliablyorbital magnetism in systems studied extensively in spintronics, including thin magneticheterostructures and topological magnets. Remarkably, we demonstrate thatthe emergent magnetic field due to the chiral spin structure of non-coplanar antiferromagnetsconstitutes an effcient mechanism to lift the orbital degeneracy evenin the absence of spin-orbit coupling. In a new class of materials to which we referas topological orbital ferromagnets, the macroscopic magnetization originates solelyfrom pronounced orbital magnetism due to non-local charge currents. We identifypromising candidates of film and bulk systems that realize the predicted topologicalorbital magnetization, without any reference to correlation or spin-orbit effects.Paving the road towards innovative device architectures, the burgeoning researcheld of spin-orbitronics exploits relativistic phenomena to control electrically magnetismby means of spin-orbit torques. Only recently, these torques and the relatedDzyaloshinskii-Moriya interaction were recognized as innately geometrical effects thatoriginate from the global properties of a mixed phase space entangling the crystal momentumwith the magnetization direction. However, the effcient treatment of suchcomplex higher-dimensional phase spaces sets a central challenge for ab initio theory,calling for advanced computational methods. This demand is met by a generalizedWannier interpolation that we develop here in order to describe Berry phase effectsin generic parameter spaces precisely. Using the scheme for spin torques and chiralinteractions in magnetic heterostructures, we correlate their microscopic origin withthe electronic structure, and elucidate the role of chemical composition and disorder.In addition, the developed formalism enables us to evaluate effciently the dependenceof these phenomena on the magnetization direction, revealing large anisotropies in thestudied systems. Considering the interplay of magnetism and topology, we uncoverthat magnetically induced band crossings manifest in prominent magneto-electric responsesin magnetic insulators. We introduce the concept of mixed Weyl semimetalsto establish novel guiding principles for engineering large spin-orbit torques in topologicallycomplex ferromagnets. Moreover, we show that topological phase transitionsin these materials are accompanied by drastic changes of the local orbital chemistry