First-principles theory of electrically-induced spin and orbital magnetization

Sammanfattning: Spin-orbit torques (SOTs) have emerged recently as practical tools to control the magnetization in spintronic devices, but it is debated what the underlying fundamental processes are that enable fast and energy-efficient magnetization switching.In this thesis, we investigate theoretically possible means of controlling magnetization in magnetic materials and heterostructures. To this end we employ relativistic density functional theory and linear-response theory to compute electrically induced spin currents and spin polarizations. We focus initially on two effects, the spin Hall effect (SHE) and the spin Rashba-Edelstein effect (SREE) that are due to spin-orbit coupling (SOC).First, we investigate the electric-field induced local magnetization in two antiferromagnets, CuMnAs and Mn2Au. Our explicit calculations show that there is not only an SREE-induced local spin polarization, but also a surprisingly large orbital polarization, due to what we call an orbital Rashba-Edelstein effect (OREE). We show that the induced orbital polarization does not require SOC and that it exhibits a staggered, Rashba symmetry in contrast to the induced spin polarization that can have Rashba- or Dresselhaus-like symmetries.Second, we investigate heavy-metal/ferromagnetic-metal bilayers, previously proposed to be exceptionally suited for large SOTs. Calculating the induced spin currents and accumulations, we find that there is an unusual magnetic spin Hall effect (MSHE), carrying a spin current whose polarization points along the electric field. The MSHE is odd under magnetization reversal, in contrast to the conventional SHE which is magnetization invariant.We investigate further the MSHE for the ferromagnets Fe, Co, and Ni, for which we show that the size of the MSHE can be comparable to that of the SHE and therefore they can in general not be ignored for SOTs. To access their thermal counterparts, we systematically investigate the spin Nernst and orbital Nernst effect (ONE) for 40 metallic monoatomic crystals. We predict large ONE values for the group-10 elements Ni, Pd, and Pt. Lastly, we go beyond linear-response theory by using a more advanced method based on non-equilibrium Green’s functions, to compute ab initio the impact of applied bias voltages and show that induced spin and orbital accumulations have different spatial profiles. Our work emphasizes that there exist various novel spin/orbital effects that have not received much attention yet but could become harnessed in future spin-orbitronics.