Density Functional Theory for Magnetism and Magnetic Anisotropy

Density functional theory and its application for the simulation of magnetic properties of condensed matter is introduced. This includes vector-spin density functional theory for the evaluation of spin-spin interactions and relativistic extensions to capture effects like the magnetocrystalline anisotropy. The role of the different approximations to the exchange-correlation functional, e.g., the local density approximation, or the generalized gradient approximation, is investigated, showing successes and limitations of the present functionals. Special techniques to determine, e.g., the magnetic ground state or finite temperature properties based on density functional theory are shortly discussed

Spectral Density Functional Approach to Electronic Correlations and Magnetism in Crystals

A novel approach to electronic correlations and magnetism of crystals based on realistic electronic structure calculations is reviewed. In its simplest form it is a combination of the ``local density approximation'' (LDA) and the dynamical mean field theory (DMFT) approaches. Using numerically exact QMC solution to the effective DMFT multi-orbital quantum-impurity problem, a successful description of electronic structure and finite temperature magnetism of transition metals has been achieved. We discuss a simplified perturbation LDA+DMFT scheme which combines the T-matrix and fluctuation-exchange approximation (TM-FLEX). We end with a discussion of cluster generalization of the non-local DMFT scheme and its applications to the magnetism and superconductivity of high-Tc superconductors.Comment: 37 pages, to be published in: "Electron Correlations and Materials Properties 2" ed. by A. Gonis (Kluwer, NY

Spin Excitations in Solid from Many-Body Perturbation Theory

Electronic spin excitations are low-energy excitations that influence the properties of magnetic materials substantially. Two types of spin excitations can be identified, single-particle Stoner excitations and collective spin-wave excitations. They can be treated on the same footing within many-body perturbation theory. In this theory, the collective spin excitations arise from the correlated motion of electron-hole pairs with opposite spins. We present the theory in detail and discuss several aspects of an implementation within the full-potential linearized augmented plane-wave method. The pair propagation is described by the transverse magnetic susceptibility, which we calculate from first principles employing the ladder approximation for the T matrix. The four-point T matrix is represented in a basis of Wannier functions. By using an auxiliary Wannier set with suitable Bloch character, the magnetic response function can be evaluated for arbitrary k points, allowing fine details of the spin-wave spectra to be studied. The energy of the acoustic spin-wave branch should vanish in the limit k →0, which is a manifestation of the Goldstone theorem. However, this condition is often violated in the calculated acoustic magnon dispersion, which can partly be traced back to the choice of the Green function. In fact, the numerical gap error is considerably reduced when a renormalized Green function is used. As an alternative simple correction scheme, we suggest an adjustment of the Kohn-Sham exchange splitting. We present spin excitation spectra for the elementary ferromagnets Fe, Co, and Ni as illustrative examples and compare to model calculations of the homogeneous electron gas

Many-Body Spin Excitations in Ferromagnets from First Principles

Electronic spin excitations are low-energy excitations that influence the properties of magnetic materials substantially. Two types of spin excitations can be identified, single-particle Stoner excitations and collective spin-wave excitations. They can be treated on the same footing within many-body perturbation theory. In this theory, the collective spin excitations arise from the correlated motion of electron-hole pairs with opposite spins. We present the theory in detail and discuss several aspects of an implementation within the full-potential linearized augmented plane-wave method. The pair propagation is described by the transverse magnetic susceptibility, which we calculate from first principles employing the ladder approximation for the T matrix. The four-point T matrix is represented in a basis of Wannier functions. By using an auxiliary Wannier set with suitable Bloch character, the magnetic response function can be evaluated for arbitrary k points, allowing fine details of the spin-wave spectra to be studied. The energy of the acoustic spin-wave branch should vanish in the limit k →0, which is a manifestation of the Goldstone theorem. However, this condition is often violated in the calculated acoustic magnon dispersion, which can partly be traced back to the choice of the Green function. In fact, the numerical gap error is considerably reduced when a renormalized Green function is used. As an alternative simple correction scheme, we suggest an adjustment of the Kohn-Sham exchange splitting. We present spin excitation spectra for the elementary ferromagnets Fe, Co, and Ni as illustrative examples and compare to model calculations of the homogeneous electron ga

Spin polarization in half-metals probed by femtosecond spin excitation

Müller GM, Walowski J, Djordjevic M, et al. Spin polarization in half-metals probed by femtosecond spin excitation. NATURE MATERIALS. 2009;8(1):56-61.Knowledge of the spin polarization is of fundamental importance for the use of a material in spintronics applications. Here, we used femtosecond optical excitation of half-metals to distinguish between half-metallic and metallic properties. Because the direct energy transfer by Elliot-Yafet scattering is blocked in a half-metal, the demagnetization time is a measure for the degree of half-metallicity. We propose that this characteristic enables us vice versa to establish a novel and fast characterization tool for this highly important material class used in spin-electronic devices. The technique has been applied to a variety of materials where the spin polarization at the Fermi level ranges from 45 to 98%: Ni, Co2MnSi, Fe3O4, La0.66Sr0.33MnO3 and CrO2