Papers by Author: V. Marchenkov

Abstract: We report on the effect of severe plastic deformation by torsion under the Bridgman anvil pressure (SPDT) on structure and physical properties of the Ni54Mn21Ga25 and Ni54Mn20Fe1Ga25 alloys. Two types of samples were studied: ordered (cast) samples and nanocrystalline samples disordered by SPDT. The thermal expansion, thermo EMF, electro- and magnetoresistivity, Hall effect, and magnetization were measured from 4.2 to 800 K and in magnetic fields of up to 15T. We show that the deviation from the stoichiometric Ni50Mn25Ga25 composition shifts the temperatures of the martensitic transition TM and the magnetic (Curie) transition TC to room temperature. Large changes in the physical properties near TM and TC were discovered. The alloys become amorphous and nanocrystalline after the SPDT treatment. This process is accompanied by the disappearance of the martensitic transformations and of the nanocrystalline modulated substructures and, consequently, by the suppression of the magnetic shape-memory effect. The physical properties of the disordered alloys are also strongly changed. Subsequent annealing at Т ≥ 700 K leads to a practically full restoration of the structure and all properties characteristic of the initial cast alloys.

Abstract: We studied the electrical and optical properties of X2YZ (X = Co, Fe; Y = Cr, Mn, Ti; Z = Ga, Al, Si) Heusler alloys. We showed that the electrical and optical properties of Co2TiAl and Fe2TiAl are typical of metals. An abnormal behavior of the resistivity and the optical properties in the IR range of the other alloys was observed, i.e., the resistivity exhibits a semiconductor-like behavior and the optical conductivity has no contribution from the intraband absorption of the conduction electrons. The experimental data analysis, based on existing band calculations, allowed us to conclude that the anomalous behavior of their properties is determined by electronic states near to and at the Fermi level. The electronic states in the subband with spin up give the main contribution to the interband transitions in the IR range, but they practically do not contribute to the static conductivity (resistivity). There are almost no charge carries in the other electronic subband (with spin down), and hence, the interband transitions occur only at the energies above the semiconductor gap values.

Abstract: Polycrystalline Nd1-xSrxMnO3 was subjected to a thermobaric treatment, that is, a quenching under a high quasihydrostatic pressure of 9 GPa , starting from the temperature 1000 °C. This results in a strong change of the electroresistive and magnetoresistive properties. The changes in the electrical transport might result from the increase of oxygen deficient regions in the boundaries between the nanosized grains.