An efficient full-potential linearized augmented-plane-wave electronic structure method for charge and spin transport through realistic nanoferronic junctions

The full-potential linearized augmented-plane-wave (FLAPW) electronic structure method, a precise relativistic all-electron method enabling the description of the electronic structure and ground-state properties of multi-component complex bulk solids and low-dimensional systems from the first principles of quantum mechanics, has been extended along two research lines: (A) An efficient order-N implementation of the FLAPW method was developed to perform ab initio calculations of electronic transport properties in magnetic tunnel junctions (MTJs) and all-metallic spin-valves within the spin-density and vector spin-density formulations of the local density approximation (LDA) and the generalized gradient approximation (GGA) to the density functional theory (DFT), whose computational effort scales not proportionally to the conventional cubic power (N), but linearly with the number of layers N of the system. The method is based on the Green-function embedding technique, which allows to treat open systems and to solve the scattering problem of electronic transport. In order to achieve the order-N scaling behavior, the system is partitioned into layers, which are calculated separately. Due to its order-N scaling, the implementation allows to profit from the high precision of the FLAPW method at a low computational cost. Computational efficiency is an important aspect in calculations of electronic transport as the scattering region often exceeds 2-5 nm in thickness and may contain several hundreds of atoms per unit cell. The applicability of the order-N FLAPW electronic structure method is much more general and allows the efficient investigation of quantities based on the selfconsistent solution of the electron charge and vector magnetization densities such as the electronic structure, the total energy and other ground state properties. The method is ideally suited to heterostructures and has been extended to treat surfaces and thin films. A particular asset is the possibility to investigate quantities slowly varying in space such as a charge or spin-density wave. The implementation was validated for the electronic, magnetic and structural properties of films and surfaces as well as their electronic transport properties. The method was applied to the investigation of the spin-transfer torque (STT), an alternative to the conventionally used Oersted field to switch the magnetization in spinvalves and tunnel junctions, whose strength scales favorably with the increasing integration density of the actual devices. Within the order-N Green function embedding method, two formulations of the spin torque were implemented: The spin torque is calculated (i) from the variation of the spin current and (ii) directly as the torque, which the exchange field exerts on the non-equilibrium spin density. The second formulation allows the calculation of the spin-torque in the presence of spin-orbit coupling. Calculations of the spin-torque were performed for Co/Cu/Co and Fe/Ag/Fe spin-valves and for the Fe/MgO/Fe MTJ. For the all-metallic spin-valves the asymmetries of the torque per current were determined and found to be in good agreement with the theory of Slonczewski and in the case of the Fe/Ag/Fe spin-valve also in good agreement with experiments. In contrast to Slonczewski’s model, the out-of-plane torque is available from the ab initio calculations of the spin-torque. It is found to be negligible for thick free layers. In the case of the Fe/MgO/Fe MTJ good quantitative agreement with experiments is found. (B) For the description of many properties in condensed matter physics, the Wannier function is superior over the Bloch function, which is the underlying concept of most electronic structure methods describing periodic solids, including the FLAPW method. The Wannier function approach to electronic structure provides a real-space description of solidstate properties and enables for example a very intuitive picture on bonding properties. In the context of electronic transport, Wannier functions have advantages in several respects: (i) They may be used to set up realistic single particle or many-body model Hamiltonians with parameters as determined from ab initio calculations. (ii) They are intimately related to the Berry phase, a quantity entering the modern theory of ferroelectric polarization, orbital magnetism and the Hall conductivities. (iii) They provide an efficient basis set of localized functions, which is optimal for the study of local correlation effects on electronic transport. Within the FLAPW formalism, maximally localized Wannier functions were implemented. The implementation was validated for bulk, films and one-dimensional systems, with and without spin-orbit coupling. The ferroelectric polarization was computed from the Wannier functions for several ferroelectric and multiferroic materials (e.g. HoMnO3) and found to be in good agreement with experimental data, where available.