This title appears in the Scientific Report : 2019 

Large-scale Investigations of Non-trivial Magnetic Textures in Chiral Magnets with Density Functional Theory
Bornemann, Marcel (Corresponding author)
Quanten-Theorie der Materialien; PGI-1
Quanten-Theorie der Materialien; IAS-1
Jülich Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag 2019
Dissertation, RWTH Aachen University, 2019
Dissertation / PhD Thesis
Controlling Configuration-Based Phenomena
Controlling Spin-Based Phenomena
Schriften des Forschungszentrums Jülich Reihe Schlüsseltechnologien / Key Technologies 195
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The large-scale Density Functional Theory (DFT) code KKRnano allows one to performab initio simulations for thousands of atoms. In this thesis an extension of KKRnanois presented and utilized which facilitates the investigation of exotic non-collinearmagnetic textures in bulk materials on huge length scales. Such an undertakinginevitably involves the utilization of High Performance Computing (HPC) which isitself a scientific field. The work in this context includes the adaptation of new codingparadigms and the optimization of codes on constantly changing hardware architectures.In KKRnano, the runtime of a simulation scales linearly with the number of atoms dueto an advanced Korringa-Kohn-Rostoker (KKR) scheme that is applied, in which thesparsity of the matrices in the multiple-scattering equations is exploited. This enablesus to investigate phenomena that occur on a length scale of nanometers involvingthousands of atoms.The main purpose of this thesis was to generalize the KKR formalism in KKRnanoin such a way that a non-collinear alignment of the atomic spins can be treated. Inaddition to this, the relativistic coupling of spin and orbital degrees of freedom, whicharises from the Dirac equation, was introduced to the code. This coupling gives rise tothe Dzyaloshinskii-Moriya interaction (DMI) from which the formation of non-collinearmagnetic textures usually originates. Other methodological features that were addedto KKRnano or were re-established in the context of this thesis are the GeneralizedGradient Approximation (GGA), Lloyd’s formula and a semi-core energy contourintegration. GGA is known to be a better approximation to the exchange-correlationenergy in DFT than the still very popular Local Density Approximation (LDA), Lloyd’sformula allows to determine the charge density exactly, despite the angular momentumexpansion of all quantities, and the semi-core energy contour integration facilitates thetreatment of high-lying electronic core states. Furthermore, an experimental port of themultiple-scattering solver routine to Graphics Processing Unit (GPU) architectures isdiscussed and the large-scale capabilities of KKRnano are demonstrated by benchmarkcalculations on the supercomputer JUQUEEN that include more than 200.000 atoms.The new version of KKRnano is used to investigate the magnetic B20 compoundsB20-MnGe and B20-FeGe as well as alloys of B20-Mn1−xFexGe type with variedconcentration of Mn and Ge. These compounds are well-known for exhibiting helicalstates. Recently reported observations of topologically protected magnetic particles,also known as skyrmions, make them promising candidates for future spintronic devices.Initially, the known pressure-induced transition from a high-spin to a low-spin state inB20-MnGe is reproduced with KKRnano and an examination of the magnetocrystallineanisotropy yields unexpected results