Theory of spin waves in ferromagnetic (Ga,Mn)As

TL;DR

This paper develops a theoretical framework for understanding spin waves and magnetic anisotropy in (Ga,Mn)As, linking atomic interactions to macroscopic magnetic properties and explaining experimental observations.

Contribution

It introduces a variational-perturbation calculus approach to connect atomic quantum mechanics with macroscopic magnetic behavior in (Ga,Mn)As.

Findings

Dzyaloshinskii-Moriya interaction causes cycloidal spin structures.

Surface-like anisotropy appears in thicker films.

The theory matches some experimental stiffness values but not all.

Abstract

The collective behavior of spins in a dilute magnetic semiconductor is determined by their mutual interactions and influenced by the underlying crystal structure. Hence, we begin with the atomic quantum-mechanical description of this system using the proposed variational-perturbation calculus, and then turn to the emerging macroscopic picture employing phenomenological constants. Within this framework we study spin waves and exchange stiffness in the p-d Zener model of (Ga,Mn)As, its thin layers and bulk crystals described by the spds* tight-binding approximation. Analyzing the anisotropic part of exchange, we find that the Dzyaloshinskii-Moriya interaction may lead to the cycloidal spin arrangement and uniaxial in-plane anisotropy in thin layers, resulting in a surface-like anisotropy in thicker films. We also derive and discuss the spin-wave contribution to magnetization and Curie temperature. Our theory reconstructs the values of stiffness determined from the temperature dependence of magnetization, but reproduces only partly those obtained from analyzing precession modes in (Ga,Mn)As thin films.