Double-exchange mechanisms for Mn-doped III-V ferromagnetic semiconductors

Abstract

A microscopic model of indirect exchange interaction between transition metal impurities in dilute magnetic semiconductors (DMS) is proposed. The hybridization of the impurity $d$-electrons with the heavy hole band states is mainly responsible for the exchange of electrons between the impurities, whereas the Hund rule for the electron occupation of the impurity $d$-shells makes it spin selective. The model is applied to such systems as $n$-type (Ga,Mn)N and $p$-type (Ga,Mn)As, $p$-type (Ga,Mn)P. In $n$-type DMS with ${\mathrm{Mn}}^{2+∕3+}$ impurities the exchange mechanism is rather close to the kinematic exchange proposed by Zener for mixed-valence Mn ions. In $p$-type DMS ferromagnetism is governed by the kinematic mechanism involving the kinetic energy gain of the heavy hole carriers caused by their hybridization with $3d$ electrons of ${\mathrm{Mn}}^{2+}$ impurities. Using the molecular field approximation, the Curie temperatures $T_C$ are calculated for several systems as functions of the impurity and hole concentrations. Comparison with the available experimental data shows a good agreement.