Enhancement of the ferromagnetic coupling in doped ferromagnetic and antiferromagnetic semiconductors

The Neel and Curie temperatures and paramagnetic Curie points for lightly doped antiferromagnetic and ferromagnetic insulators are calculated. The impurities are nonionized donors or acceptors, whose electrons or holes are not Mott-delocalized. The theory is applicable also to heavily doped magnetic semiconductors and manganites displaying a high resistivity peak in the vicinity- of TC. The impurities increase TC and decrease TN. As for the paramagnetic Curie points Θ, impurities increase them. In antiferromagnetic semiconductors Θ can even change the sign from the negative to the positive. In ferromagnetic semiconductors with "weak" impurities the paramagnetic Curie point coincides with the true that. But in the case of "strong" impurities, the temperature dependence of the inverse magnetic susceptibility is essentially nonlinear. One can speak about the "low-temperature" and "high-temperature" paramagnetic Curie points with the former lower than the latter. These theoretical results correspond qualitatively to the experimental data.