Magnetic exchange coupling mediated by bound states

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Abstract
Usually indirect exchange coupling is mediated by unbound, noncorrelated intermediate states (Ruderman-Kittel-Kasuya-Yosida–like mechanisms) or by unbound, correlated intermediate states (superexchange-like mechanisms). Here, we investigate the possibility of indirect magnetic exchange coupling mediated by bound, correlated intermediate states. As a concrete example we study the magnetic coupling between two magnetic impurities embedded in a semiconductor matrix. The importance of long-ranged attractive Coulomb interactions between electrons and holes is emphasized. This attraction leads to exciton bound states which act as mediators of the effective exchange interaction between the two impurities. The resulting exchange interaction presents strong temperature dependence and can be analyzed in terms of the symmetry of the internal wave function of the exciton bound states. Possible applications of these results may include recent experimental results on ferromagnetic metal-semiconductor multilayers.