sd-type Exchange Interactions in Nonhomogeneous Ferromagnets

TL;DR

This paper develops a quantum field theoretical framework to analyze spin-momentum transport and sd-type exchange interactions in nonhomogeneous ferromagnets, deriving coupled equations for magnetization and spin currents.

Contribution

It introduces a non-perturbative, first-principles approach to model sd interactions and magnetization dynamics in nonuniform ferromagnets, extending beyond linear response.

Findings

Derived a generalized Landau-Lifshitz equation including sd interactions.

Showed conduction electron polarization obeys a diffusion equation with a magnetization-dependent tensor.

Demonstrated magnetization fluctuations follow a diffusion-type equation.

Abstract

Motivated by a need to understand spin-momentum transport in CPP (current perpendicular to the plane) structures, a quantum field theoretical treatment of spin-spin interactions in ferromagnets is presented. The sd interaction of the conduction electrons and the magnetic medium is treated non-perturbatively from first principles in real space. The localized magnetic moments also interact with each other through a Heisenberg exchange potential. To take into account correlation effects, a second quantized formulation is used. The semi-classical limit is taken by using a coherent-state path-integral technique which also allows us to go beyond a linear-response approach. We derive a set of coupled equations of motion for the nonuniform magnetization, the spin current and the two-point correlation functions of the magnetization. The rate of change of the magnetization is shown to obey a generalized Landau-Lifshitz equation that takes into account interaction with the conduction electrons. Within the relaxation time approximation it is shown that the polarization of the conduction electrons obeys a diffusion equation. The diffusion tensor, which has off-diagonal terms due to the sd exchange interaction, is now explicitly dependent on the magnetization of the medium. We also show that the magnetization fluctuations satisfy a diffusion-type equation. The derived equations are used in two illustrative examples.