From Magnetic Polarons to Ferromagnetism

A brief overview is given on selected effects of the exchange coupling between effective mass electrons and localized spins in II-VI semiconductors containing Mn ions. In the case of a carrier or exciton trapped by an impurity or defect, the exchange interaction leads to zero-field spin-splitting. The current theory of such complexes, known as bound magnetic polarons, describes correctly their spectroscopic and thermodynamic properties as well as their formation dynamics. At the same time, a free magnetic polaron - a delocalized carrier accompanied by a traveling cloud of polarized spins - is not expected to exist for the actual values of the coupling constants. However, hole scattering by thermodynamic and static fluctuations is shown to affect significantly its energy. For a strong coupling, the corresponding renormalization has to be described by a non-perturbative approach. Finally, the influence of the carrier liquid upon the Mn spins is discussed. Here, either optical pumping or p-type doping may lead to a ferromagnetic order, both in bulk and layered structures. Because of a long-range character of the carrier mediated interactions, this ordering is not destroyed by the fluctuations, even in the reduced dimensionality systems.