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 semicon ductors containing Mn ions. In the case of a carrier or exciton trappe d by an impurity or defect, the exchange interaction leads to zero-fie ld spin-splitting, The current theory of such complexes, known as boun d magnetic polarons, describes correctly their spectroscopic and therm odynamic properties as well as their formation dynamics. At the same t ime, a free magnetic polaron a delocalized carrier accompanied by a tr aveling cloud of polarized spins is not expected to exist for the actu al values of the coupling constants. However, hole scattering by therm odynamic and static fluctuations is shown to affect significantly its energy. For a strong coupling, the corresponding renormalization has t o be described by a non-perturbative approach. Finally, the influence of the carrier liquid upon the Mn spins is discussed. Here, either opt ical pumping or p-type doping may lead to a ferromagnetic order, both in bulk and layered structures. Because of a long-range character of t he carrier mediated interactions, this ordering is not destroyed by th e fluctuations, even in the reduced dimensionality systems. PACS numbe rs: 78.47.+p, 75.70.Cn, 78.55.Et, 78.66.Hf.