Spin waves and instabilities in the collinear four component antiferromagnetic materials

TL;DR

This paper investigates spin wave excitations and instabilities in four-component antiferromagnetic materials with specific equilibrium states, deriving dispersion relations and analyzing effects of anisotropy and interactions.

Contribution

It derives dispersion equations for spin waves in four-component antiferromagnets, including effects of anisotropy and different interaction approximations, highlighting conditions for instabilities.

Findings

At least one spin wave mode has negative frequency squared, indicating instability.

Dispersion relations depend on the orientation of equilibrium spins relative to anisotropy axis.

Differences between nearest-neighbour and continuous medium approximations are clarified.

Abstract

The small amplitude perturbations of spins are considered in the four component antiferromagnetic materials with the equilibrium state of form up-up-down-down (uniaxial samples). Other configurations for the four component antiferromagnetic materials and two component antiferromagnetic materials are briefly considered for the comparison with main regime. Dispersion dependencies of two spin waves existing in the system are found if equilibrium spins are parallel to the anisotropy axis. Dispersion equation leading to a possibility of four spin waves is derived if equilibrium spins are perpendicular to the anisotropy axis. It is found that at least one solution has negative frequency square for all possible modules and signs of the anisotropy constants. Calculations are made for the one dimensional chain of classical spins in the approximation of the nearest neighbours interaction. Next, we also addressed the nearest neighbours interaction approximation in the limit of the continuous medium (for the Landau--Lifshitz--Gilbert equation). Mostly applied form of the Landau--Lifshitz--Gilbert equation goes beyond the nearest neighbours interaction approximation. The difference is described. Required assumptions are described.