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

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 Mn2+∕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 Mn²⁺ impurities. Using the molecular field approximation, the Curie temperatures TC are calculated for several systems as functions of the impurity and hole concentrations. Comparison with the available experimental data shows a good agreement.