Ab initio investigation of the anomalous Hall effect in thin films of transition metals
Zusammenfassung der Projektergebnisse
The main objective of my research in the group of Prof. Ivo Souza at the University of California, Berkeley (USA) was to pick up the expertise in efficient calculations of the anomalous Hall conductivities (AHC) in realistic materials and apply it to the studies of the anomalous Hall effect (AHE) in thin films of transition metals. In the course of my work in the United States I have successfully performed two code implementations. The first one concerns interfacing of the Wannier interpolation program developed by Ivo Souza et al., which he and his group intensively apply to study the anomalous Hall effect from first principles, with Jülich full-potential linearized augmented plane-wave code FLEUR. The former code, designed in Jülich Forschungszentrum by the group of Prof. Stefan Blügel, is optimally suited for accurate ab initio studies of complex magnetic materials. Another implementation dealt with modifying the Wannier interpolation procedure and the corresponding program so that an efficient computation of the anomalous Hall conductivity for thin magnetic films without employing the super cell approach would be possible. During my four months stay in the group of Prof. Ivo Souza, besides implementational work, I have applied the FLEUR code to the studies of the anomalous Hall effect in various materials. At first, the FLEUR-based Wannier interpolation scheme was carefully tested versus calculations of Souza group for hcp Co, bcc Fe and fcc Ni bulk ferromagnets. At the next step, starting off from the ideas of Prof. Ivo Souza we went beyond current studies of the AHE in the field, which are severely restricted computationally, by investigating a completely unexplored issue of the magnetocrystalline anisotropy of the anomalous Hall effect. The magnetocrystalline anisotropy of the AHE arises due to dependency of the electronic structure in the crystal on the direction of its magnetization via spin-orbit coupling, and manifests itself in a dependency of the absolute value and the direction of the AHC on the direction of the magnetization. Being a subclass of anisotropic magnetotransport phenomena, anisotropic anomalous Hall effect can be utilized for the purposes of future spintronics by providing an additional degree of freedom for controlling spin-dependent transport properties. By performing extensive ab initio calculations we investigated the AHE anisotropy in bcc Fe, fcc Ni, hcp Co and FeX (X = Ni, Pd, Pt) alloys, and were able to built a phenomenological model of the AHE anisotropy in cubic and uniaxial crystals, as well as its temperature dependence. Concerning thin films of transition metals, in these four months I have successfully finished above mentioned implementation and carefully tested it against the super cell computational scheme. After that I have started extensive calculations on the AHE in monolayers of Fe, Co and Ni, as well as the thickness dependence of the AHE in thin films of late transition metals – these calculations are also being continued currently. In this report, however, I would like to concentrate in more detail on the main bulk of my calculations I performed in Berkeley, which constitute a consistent and complete study of the AHE anisotropy in hcp Co and layered FeX alloys.
