Many-Body Spin Excitations in Ferromagnets from First Principles

Abstract

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.