Solid State Phenomena Vols. 168-169

Abstract: Optical properties of the LaNi5-xCux (x = 0, 0.6, 1, 1.2) compounds were studied. It was shown that substitution of copper for nickel led to noticeable changes in the optical conductivity spectra. Calculations of the electronic structure of compounds with x = 0, 1, 2 were performed using a generalized gradient approximation. The interband optical conductivity of these intermetallics was calculated. The optical ellipsometrical measurements and theoretical calculations testify to the appearance of a broad absorption structure associated with the Cu 3d  Ni 3d electron transitions and increasing with the growth of copper content.

Abstract: Magnetic and magneto-optical properties of multilayers based on the (Co45Fe45Zr10)Z(Al2O3)100-Z composite and amorphous hydrogenated silicon with various thicknesses both of magnetic and semiconductor layers have been investigated. The nonlinear dependence of magnetic and MO characteristics of the nanostructures on the thickness of layers was found. The interface formed on the boundary of two phases (the ferromagnetic granules and the semiconductor ones) strongly influences the magnetic and MO properties of the structures with thin Si layers.

Abstract: Magnetoresistivity and Hall effect measured in magnetic fields up to B=9T (B ║ c, J ║ ab) in electron-doped Nd2−xCexCuO4+δ single crystal films with x = 0.15 and different oxygen content (δ) were studied in a temperature range of 0.4-4.2 K. The resistivity and Hall coefficient behaviors in the mixed state are discussed in the framework of flux-flow model with the inclusion of the back-flow of vortices owing to the pinning forces.

Abstract: The peculiarities of the Faraday rotation in the imperfect garnet-type crystals for which the lattice irregularities are represented by the mixed valence clusters of transition metal ions are analyzed. The model takes into account the cubic and low symmetry crystal fields acting on the metal ions and migration of the “extra” hole. The electric dipole transitions in these types of clusters with migrating hole coupled to the spin core through the double exchange mechanism are shown to lead to a significant magnetooptical activity in the visible range of spectrum. The intensity of these transitions can be comparable or higher than those for the magnetic dipole ones in the same range of frequencies.

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: Optical absorption of orthorhombic and hexagonal manganites RMnO3 (R= Pr3+, Gd3+, Sm3+, Eu3+, Ho3+, Tm3+, Er3+, and Yb3+) were studied in polarized light in the near- and mid-infrared range for the temperature range 80 - 300 K. Several spectral features associated with 4f-4f transitions of R3+ ions were found in the transparency window of RMnO3. Anisotropy of the absorption spectra in the range of multiplet 4f-4f transitions and changes in populations of the multiplet levels with temperature were observed.

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: The absorption and magnetic circular dichroism spectra of f-f transitions 6H15/2 → 6F3/2, 6F5/2 , 6(F7/2 +H5/2) and 6(F9/2+H7/2) in Dy3+ ion were studied in the glasses: Dy2O3-P2O5-SiO2-GeO2 and Dy2O3-La2O3-Al2O3-B2O3-SiO2-GeO2 with different concentrations of Dy3+ ions in temperature range 100-300 K. Basing on this data, absolute values and temperature dependences of the paramagnetic magneto-optical activity (MOA) of the transitions were determined. The origin of the MOA of f-f transitions was analyzed with the help of two models. In the first model, MOA is considered as a consequence of admixture of states with opposite parity by odd components of crystal field. The second approach is based on the Judd-Ofelt theory. The same f-f transitions reveal different values and temperature dependences in different glasses, what testifies to a different symmetry of the nearest environment of Dy3+ ions in these glasses.

Abstract: The abnormal “normal” state and origin of high temperature superconductivity in cuprates are still not clear [1, 2]. The strong electron correlations (SEC) are known to be one of the main difficulties for the theory of high- cuprates. The conventional LDA (local density approximation) approach to the band structure fails in the regime of SEC. Various realistic multiband models of a layer at low energy result in the effective Hubbard or models [3 7]. The hybrid LDA+GTB scheme [8] generates the low-energy effective model with all parameters calculated ab initio.