Augmented Planewaves, Developments and Applications to Magnetism

Abstract: The present thesis concerns method development and applications in the field of first principles electronic structure calculations.Augmented planewaves combine the simple planewaves with exact solutions of the Schrödinger equation for a spherical potential. This combination yields a very good set of basis functions for describing the electronic structure everywhere in a crystal potential. In the present work, developments of the original augmented planewave (APW) method are presented. It is shown that the exact APW eigenvalues can be found using information from the eigenvalues of the APW secular matrix. This provides a more efficient scheme to solve the APW eigenvalue problem, than the traditional evaluation of the secular determinant. Further, a new way of linearizing the APW method is presented and compared to the traditional linearized APW method (LAPW). Using a combination of the original APW basis functions and the so called local orbitals (lo), the APW+lo linearization is found to reproduce the results of the LAPW method, but already at a smaller basis set size. Another advantage of the new linearization is a faster convergence of forces, with respect to the basis set size, as compared to the LAPW method.The applications include studies of the non-collinear magnetic configuration in the fcc-based high-temperature phase of iron, ?-Fe. The system is found to be extremely sensitive to volume changes, as well as to a tetragonal distortion of the cubic unit cell. A continuum of degenerate spin spiral configurations, including the global energy minimum, are found for the undistorted crystal. The in-plane anisotropy of the ideal interface between a ferromagnetic layer of bcc Fe and the semiconducting ZnSe crystal is also investigated. In contrast to the four-fold symmetric arrangement of the atoms at the interface, the in-plane magnetic anisotropy displays a large uniaxiality. The calculated easy axes are in agreement with experiments for both Se and Zn terminated interfaces. In addition, calculations of the hyperfine parameters were performed for Li intercalated battery materials.