Lifetimes of Magnons in Two-Dimensional Diluted Ferromagnetic Systems

TL;DR

This paper investigates the lifetimes of magnons in two-dimensional disordered ferromagnetic systems using a Green's functions approach, revealing a $q^4$ scaling law for linewidths due to disorder, and clarifying previous ambiguities.

Contribution

It provides the first clear demonstration of the $q^4$ linewidth scaling in 2D disordered ferromagnets and critiques methods based on spectral moments for evaluating magnon properties.

Findings

Magnon linewidth scales as $q^4$ due to disorder.

Green's functions approach effectively determines magnon lifetimes.

Spectral moments are ineffective for accurate magnon dispersion evaluation.

Abstract

Spin dynamics in low dimensional magnetic systems has been of fundamental importance for a long time and has currently received an impetus owing to the emerging field of nanoelectronics. Knowledge of the spin wave lifetimes, in particular, can be favorable for future potential applications. We investigate the low-temperature spin wave excitations in two-dimensional disordered ferromagnetic systems, with a particular focus on the long wavelength magnon lifetimes. A semi-analytical Green's functions based approach is used to determine the dynamical spectral functions, for different magnetic impurity concentrations, from which the intrinsic linewidth is extracted. We obtain an unambiguous $q^4$ scaling of the magnon linewidth which is ascribed to the disorder induced damping of the spin waves, thereby settling a longstanding unresolved issue on the wave-vector dependence. Our findings are also in good agreement with previous theoretical studies on Heisenberg ferromagnets. Additionally, we demonstrate the futility of using the low moments associated with the spectral densities to evaluate the magnon dispersions and lifetimes.