Intrinsic and extrinsic spin-orbit torques from first principles

This thesis attempts to shed light on the microscopic mechanisms underlying the current-induced magnetic torques in ferromagnetic heterostructures. We have developed first principles methods aiming at the accurate and efficient calculation of the so-called spin-orbit torques (SOTs) in magnetic thin films. The emphasis of this work is on the impurity-driven extrinsic SOTs. The main part of this thesis is dedicated to the development of a formalism for the calculation of the SOTs within the Korringa-Kohn-Rostoker (KKR) method. The impurity-induced transitions rates are obtained from first principles and their effecton transport properties is treated within the Boltzmann formalism. The developed formalism provides a mean to compute the SOTs beyond the conventional constant relaxation time approximation. We first apply our formalism to the investigation of FePt/Pt and Co/Cu bilayers in the presence of defects and impurities. Our results hint at a crucial dependence of the torque on the type of disorder present in the films, which we explain by a complex interplay of several competing Fermi surface contributions to the SOT. Astonishingly, specific defect distributions or doping elements lead respectively to an increase or a sign change of the torque, which can not be explained on the basis of simple models. We also compute the intrinsic SOT induced by electrical and thermal currents within the full potential linearized augmented plane-wave method. Motivated by recent experimental works, we then investigate the microscopic origin of the SOT in a Ag$_{2}$Bi-terminated Ag film grown on ferromagnetic Fe(110). We find that the torque in that system can not be explained solely by the spin-orbit coupling in the Ag$_{2}$Bi alloy, and instead results from the spin-orbit coupling in all regions of the film. Finally, we predict a large SOT in Fe/Ge bilayers and suggest that semiconductor substrates might be a promising alternative to heavy metals for the development of SOT-based magnetic random access memories. We show the strong dependence of the SOT on the stacking direction, thereby providing important guidelines for future experimental works. We also compute the sublattice-resolved SOTs in an antiferromagnetic Fe/Ge thin film and find a large anisotropy of the torkance tensor