Mechanism of carrier-induced ferromagnetism in magnetic semiconductors

TL;DR

This paper models carrier-induced ferromagnetism in GaMnAs, showing that holes are localized near impurities and strongly couple with Mn spins, leading to ferromagnetic order via a double-exchange mechanism, with results matching experiments.

Contribution

It introduces a model accounting for impurity distribution and thermal fluctuations, revealing the role of localized holes and double-exchange in ferromagnetism in GaMnAs.

Findings

Holes are localized near impurity sites rather than free carriers.

Carrier spin couples strongly with Mn localized spins.

Calculated Curie temperature aligns with experimental data.

Abstract

Taking into account both random impurity distribution and thermal fluctuations of localized spins, we have performed a model calculation for the carrier (hole) state in Ga$_{1-x}$Mn$_x$As by using the coherent potential approximation (CPA). The result reveals that a {\it p}-hole in the band tail of Ga$_{1-x}$Mn$_x$As is not like a free carrier but is rather virtually bounded to impurity sites. The carrier spin strongly couples to the localized {\it d} spins on Mn ions. The hopping of the carrier among Mn sites causes the ferromagnetic ordering of the localized spins through the double-exchange mechanism. The Curie temperature obtained by using conventional parameters agrees well with the experimental result.