Electronic structure and exchange interactions from ab initio theory : New perspectives and implementations

Sammanfattning: In this thesis, the magnetic properties of several materials were investigated using first principle calculations. The ab initio method named real space linear muffin-tin orbitals atomic sphere approximation (RS-LMTO-ASA) was used to calculate the electronic structure and magnetic properties of bulk systems, surface and nanostructures adsorbed on surfaces.We have implemented new features in the RS-LMTO-ASA method, such as the calculation of (a) Bloch Spectral Function (BSF), (b) orbital resolved Jij and (c) Dzyaloshinskii-Moriya interaction (DMI). Using (a), we have shown that one can calculate the dispersion relation for bulk systems using a real space method. Furthermore, the dispersion relation was revealed to be existent even for finite one-dimensional structures, such as the Mn chain on Au(111) and Ag(111) surfaces. With (b), we have investigated the orbital resolved exchange coupling parameter Jij for 3d metals. It is demonstrated that the nearest neighbor (NN) interaction for bcc Fe has intriguing behavior, however, the contribution coming from the T2g orbitals favours the anti-ferromagnetic coupling behavior. Moreover, the Fermi surface for bcc Fe is formed mostly by the T2g orbitals and these are shown to be highly Heisenberg-like, i.e. do not depend significantly on the magnetic configuration. Later, the same approach was used to study other transition metals, such as Cr, Mn, Co and Ni. In the end, we have presented the results obtained with the implementation (c). Our results have shown the large dependence of the DMI values, both the strength and direction, with respect to which magnetic configuration they are calculated from. We argue that, for the investigated systems, the non-collinearity induces currents (spin and charge) that will influence directly the DMI vectors.