Effect of compensation on magnetic ordering in (Ga,Mn)As

Abstract

The real mechanism of ferromagnetism in III-V diluted magnetic semiconductors such as Ga$_{1-x}$Mn$_x$As is still debated. The experimental results point out the realization of appreciable ferromagnetism at filling of the impurity band close to one-half that can be expressed in terms of compensation degree. Based on our recent results on the existence of two competing mechanisms of pair indirect exchange (with ferro- and antiferro alignment of Mn $d$ shells), a theory of magnetic ordering in (Ga,Mn)As is presented. The molecular-field-like equation is derived in which the effective exchange constant contains all necessary information about material, including the Mn content, the compensation degree, the crystal coordination, and the parameters of two competing pair exchange interactions. The Curie temperature depends crucially on the compensation degree. Depending on Mn content and compensation degree, three possible magnetic phases (paramagnetic, ferromagnetic, and spin-glass-like) are predicted. However, the spin-glass-like phase in (Ga,Mn)As is apparently not realized due to the additional inevitable compensation caused by As antisites. The phase transition into the ferromagnetic state occurs not only with Mn content variation but through the change of the compensation degree as well. The new mechanism of photoferromagnetic effect is predicted and discussed in the context of possible clarification of the magnetic ordering mechanism. It is assumed that the main obstacle to a possible increase in the Curie temperature is the increasing contribution of the antiferromagnetic superexchange between the nearest-neighbor Mn's, rather than compensatory (electric or magnetic) effects from Mn interstitials, as was previously stated. The comparison of our predictions with available experimental data is presented.