Bound-magnetic-polaron molecule in diluted magnetic semiconductors.

We formulate a complete microscopic theory of a coupled pair of bound magnetic polarons, the bound-magnetic-polaron molecule (BMPM) in a diluted magnetic semiconductor by taking into account both the proper two-body nature of the impurity-electron wavefunction and within the general spin-rotation-invariant approach to the electronic states. Also, the model takes into account both the Heisenberg and the antiferromagnetic kinetic-exchange interactions, as well as the ferromagnetic coupling within the common spin BMPM cloud. In this manner, we correct, unify and extend the weakly interacting BMP pair models of Wolff-Bhatt-Durst (2002 Phys. Rev. B 65 235205) and the model of nonoverlapping polarons considered by Angelescu and Bhatt (2002 Phys. Rev. B 65 75211). The resulting BMPM Hamiltonian is solved within the continuum-medium and the effective-mass approximations for the donor case and the thermodynamics is derived. In our approach the thermodynamic fluctuations of magnetization of the spins within BMPM are taken as Gaussian. It appears that the fluctuations can strongly stabilize the spin-triplet state, which may constitute a precursor effect of a ferromagnetic ordering in a many-impurity system.