Spin waves in disordered magnetic systems

TL;DR

This paper investigates how magnetic disorder affects spin wave modes in disordered magnetic systems, revealing the emergence of a longitudinal mode and its impact on surface spin wave dispersion, with implications for magnetic nanocomposites.

Contribution

It introduces a model showing the influence of magnetic disorder on spin wave dispersion, including the appearance of a longitudinal mode in disordered systems.

Findings

Magnetic disorder causes a decrease in the DE mode frequency and slope.

A new longitudinal spin wave mode appears due to disorder.

The model explains the double-peak FMR spectra in nanocomposites.

Abstract

Long-wavelength spin waves in disordered magnetic systems have been investigated. In the framework of the Heisenberg model with magnetic dipole and exchange interactions between spins it is found that an additional longitudinal spin wave mode appears. This mode is characterized by variations of the value of the magnetic moment density. In order to analyse influence of the magnetic disorder on spin wave dispersion relations, the special case of volume and surface spin waves in the Damon-Eshbach (DE) geometry in films with magnetic disorder is considered. It is revealed strong influence of the magnetic disorder on surface spin waves, which consist of two branches -- the DE mode and the longitudinal spin wave mode. Decrease of the ordering parameters leads to a decrease of the initial frequency and the curve slope of the DE mode dispersion curve and to an increase of the dispersion curve slope of the longitudinal mode. If the spin noncollinearity is high, then the DE mode dispersion assumes a curve with the backward character. It is found that the dispersion relation of the longitudinal mode is weak temperature dependent. The developed model can explain the observed double-peak structure of FMR spectra in magnetic nanocomposites.