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We analyze critical temperature of ferromagnetic ordering in diluted magnetic semiconductors in the mean-field approximation. Using a general field-theoretical approach we formulate a description of magnetic phase transition, which takes into account the interaction of magnetic fluctuations. We also propose a mechanism of exchange interaction between Mn impurity moments in GaMnAs alloys via the Mn-induced acceptor levels. This mechanism may lead to low temperature ferromagnetism at vanishing concentration of free carriers.

High and low field magnetotransport measurements, as well as SQUID measurements of magnetization, were carried out on Ga 1-x Mn x As epilayers grown by low temperature molecular beam epitaxy, and subsequently annealed under various conditions. We observe a large enhancement of ferromagnetism when the samples are annealed at an optimal temperature, typically about 280°C. Such optimal annealing leads to an increase of Curie temperature, accompanied by an increase of both the conductivity and the saturation magnetization. A decrease of the coercive field and of magnetoresistivity is also observed for Ga 1-x Mn x As annealed at optimal conditions. We suggest that the experimental results reported in this paper are related to changes in the domain structure of Ga 1-x Mn x As.

The Anomalous Hall Effect (AHE) was investigated in IV–VI ferromagnetic semimagnetic semiconductors of Sn1−xMnxTe codoped with either Eu or Er. The analysis of experimental data is as follows. Hall resistivity and magnetization showed that AHE coefficient Rs depends on temperature and its value decreases with thetemperature increase. We observe that above ferromagnet–paramagnet transition temperature Rs changes sign. We discuss the possible physical mechanisms responsible for observed temperature dependence of Rs, particularly change of the sign.