Spin clustering and ferromagnetic couplings in a dilute magnetic semiconductor

TL;DR

This paper demonstrates that positional disorder of Mn impurities in dilute magnetic semiconductors significantly enhances ferromagnetic couplings and Curie temperature, with implications for magnetic properties and spin-wave modes.

Contribution

It reveals that disorder in impurity placement enhances ferromagnetic interactions and explains experimental magnetization data in GaMnAs.

Findings

Ferromagnetic couplings are stronger in disordered impurity arrangements.

Positional disorder enhances cluster-localized spin-wave modes.

Model reproduces temperature-dependent magnetization data.

Abstract

Ferromagnetic couplings in spin clusters are shown to be strongly enhanced compared to those for an ordered impurity arrangement, even for the same spin separation and hole doping. The consequent energy-enhancement of the cluster-localized spin-wave modes indicates a potentially significant role of positional Mn disorder in enhancing $T_{\rm C}$. Within a simple model involving two spin-excitation energy scales corresponding to weakly and strongly coupled spins, the temperature dependence of magnetization is found to be in good agreement with the SQUID magnetization data for Ga$_{1-x}$Mn$_x$As.