Papers by Author: L.I. Koroleva

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: Definite regularity in the distribution of ferromagnetic, antiferromagnetic and superconducting elements is observed in the periodic table starting with the 4-th period. These trends were explained by distinction of degree of division of the d (f)- or р-orbitals of neighboring atoms in the crystal. We calculated also the radii of the external d (f)- and р-orbitals and the nearest to them orbitals with the Slater’s method. It is demonstrated that in the superconducting crystals the d-shells approach the nucleus of neighboring atoms are much closely those for ferromagnetic or antiferromagnetic crystals.