Magnetic polaron: Theory and experiment

We present a general thermodynamic model of the bound magnetic polaron which takes into account the f$\stackrel{\mathrm{̇}}{\mathrm{r}}$ee energies of the localized spins, of the free carriers, of the electrons localized in the bound magnetic polarons, and of the electrons trapped on impurities which play the role of compensators. In addition to the configurational entropy, a magnetic entropy is associated with the bound magnetic polarons. It is shown that this magnetic entropy characterizes the bound magnetic polaron, and most physical properties are closely related to its order of magnitude. In particular, it is shown that this entropy is responsible for the stability of the bound magnetic polaron at high temperature both in EuO and in semimagnetic semiconductors such as ${\mathrm{Cd}}_{1-x}{\mathrm{Mn}}_x\mathrm{Te}$ and ${\mathrm{Cd}}_{1-x}{\mathrm{Mn}}_x\mathrm{Se}$. Our model accounts well for the physical properties of these materials. The extension of the model to other magnetic semiconductors is also discussed.