Papers by Author: A.S. Morozov

Abstract: In Sm_0.55Sr_0.45MnO₃ single crystals, grown by the floating-zone method with the cooling in oxygen, has been observed the spontaneous generation of voltage (SGV). Its maximum reaches 60 μV and occurs in the temperature range where simultaneous decay of the CE-type antiferromagnetic order and the charge order take place in some clusters. The SGV peak becomes lower by about 45% if magnetic field 14.2 kOe applied. Also we observed the SGV in La_0.75Ba_0.25MnO₃ single crystals. Maximum value of SGV occurs in a vicinity of the Curie temperature. The SGV value seems to be almost independent from the cooling or heating rate and crystallographic direction, but in La_0.75Ba_0.25MnO₃ it is smaller than in Sm_0.55Sr_0.45MnO₃ by an order of magnitude. La_0.75Ba_0.25MnO₃ compound contains ferromagnetic clusters, in which the electrical charges are localized according to gain in the s-d exchange energy. They are distributed in the paramagnetic lattice, impoverished of an electrical charge. It is shown that SGV stems from the presence of regions with different electrical charges in both samples.

Abstract: Magnetocaloric effect (T-effect) was studied by direct method on three samples of Sm0.55Sr0.45MnO3 manganite: ceramic (C) sample and two single crystals, annealed in oxygen (O) or in air atmosphere (A). The temperature dependence of T-effect T(T) of all the samples has maximum at Tmax equal to 143.3 K for A-sample, 244 K for O-sample, and 143 K for C-sample. In these maxima T values are 0.8 K, 0.41 K, and 0.4 K for A-, O- and C-samples respectively. In addition, the T(T) curve of A-sample has minimum at Tmin = 120 K and T-value in minimum is equal to - 0.1 K. The maximum value of T-effect increases with H up to the maximum field of measurement 14.2 kOe. When this takes place the rate of this increasing is higher at H > 8 kOe than at H < 8 kOe. The above listed peculiarities of T-effect are explained by the presence in the samples of ferromagnetic, antiferromagnetic A-type and antiferromagnetic CE-type clusters.

Abstract: Based on the Mn-doped chalcopyrites CdGeAs2, ZnGeAs2 and ZnSiAs2, new dilute magnetic semiconductors with the p-type conductivity were produced. Magnetization, electrical resistivity and Hall effect of these compositions were studied. Their temperature dependences of magnetization are similar in form in spite of a complicated character, which is controlled by the concentration and mobility of the charge carriers. Thus, for T < 15 K, these curves are characteristic of superparamagnets and for T > 15 K, of a frustrated ferromagnet. In compounds with Zn these two states are diluted by a spinglass-like state. This specific feature is ascribed to attraction of Mn ions occupying neighboring sites and to competition between the carrier-mediated exchange and superexchange interactions. The Curie temperatures of these compounds are above room temperature. These are the highest Curie temperatures in the AIIBIVCV2:Mn systems.