Atomic-vacancy ordering and magnetic susceptibility of nonstoichiometric hafnium carbide

Experimental data on the magnetic susceptibility of nonstoichiometric hafnium carbide HfC_y(0.6< y<1.0) are presented. Anomalies are found in the temperature dependences of the magnetic susceptibility of the carbides HfC_0.71, HfC_0.78, and HfC_0.83 in the temperature range 870–930 K. These anomalies are due to the formation of a superstructural short-range order in their nonmetallic sublattice. It is shown that the short-range order in the carbides HfC_0.71 and HfC_0.78 corresponds to the ordered phase Hf₃C₂, while in the carbide HfC_0.83 it corresponds to the ordered phase Hf₆C₅. It is found that the magnetic susceptibility of the carbide HfC_0.78 in the temperature interval 910–980 K is zero. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 4, 296–301 (25 February 1999)     

Electrical and magnetic properties of titanium diselenide intercalated with cobalt

This paper discusses experimental studies of the temperature dependence of the magnetic susceptibility, electrical conductivity, and Seebeck coefficient of the compounds Co_xTiSe₂ (0.1⩽x⩽0.5), along with the structural behavior exhibited by these compounds. For Co_0.5TiSe₂, the observed temperature dependence of the magnetic susceptibility is characteristic of an antiferromagnet with a Néel temperature of 510 K. The data obtained on the structural characteristics and physical properties of the intercalated phases are interpreted using a model that assumes the formation of bands of localized states near the Fermi level. Fiz. Tverd. Tela (St. Petersburg) 39, 1618–1621 (Sepember 1997)     

Colossal magnetoresistance of the Sm1−x SrxMnO3 system

The results of investigating the temperature dependence of the resistivity, the differential magnetic susceptibility, and the magnetoresistance of a partially substituted perovskite Sm_1−x Sr_xMnO₃ (x=0.16–0.4) are presented. Colossal intrinsic magnetoresistance, reaching 90% in an external magnetic field of 30 kOe, is discovered in the compound with x=0.30 at 77 K. Fiz. Tverd. Tela (St. Petersburg) 39, 1831–1832 (October 1997)     

Effect of intercalation by silver on the charge-density-wave state in TiSe2

The temperature dependence of electrical conductivity σ (77–300 K) and magnetic susceptibility χ (2–300 K) of Ag_xTiSe₂ in the Ag_1/4TiSe₂ phase has been studied in order to determine the possibility of preserving the charge-density-wave state in silver-intercalated TiSe₂. The behavior of χ and σ in this phase is compared with that of the starting compound TiSe₂. Fiz. Tverd. Tela (St. Petersburg) 40, 2165–2167 (December 1998)     

Positive magnetoresistivity of Fe0.95Co0.05Ge2 single crystals inside the region of a magnetic phase transition

We report on the detection of two narrow peaks of positive magnetoresistivity in the temperature dependence of the magnetoresistivity of a Fe_0.95Co_0.05Ge₂ single crystal in the vicinity of a “smeared” first-order phase transition. The position of these peaks correlates with the position of singularities in the temperature dependence of the temperature derivative of the electrical resistivity and magnetic susceptibility. We show that these singularities in the transport and magnetic properties are, probably due to the presence of two percolation transitions with temperature in the magnetic subsystem of the crystal. Zh. éksp. Teor. Fiz. 112, 690–697 (August 1997)     

Imperfection of the Sm sublattice and valence instability in compounds based on SmB6

The magnetic susceptibility, electrical resistance, specific heat, and thermal expansion coefficient of SmB₆, Sm_0.8B₆, and Sm_1–x La_xB₆ (x=0.1 and 0.2) are measured in the temperature range T=4–300 K. The dispersion curves of the acoustic phonon branches in lanthanum-doped samples are studied. A combined analysis of the results confirms the existence of an activation gap in the electron density of states in both nonstoichiometric and lanthanum-doped compounds. The anomalies in the electronic component of the thermal expansion coefficient are associated to a considerable degree with the temperature variation of the valence and, like the magnetic susceptibility, reflect features of the f-electron excitation spectrum. It is found that lanthanum doping does not lead to significant changes in the anomalies in the phonon spectrum of SmB₆. It is established that the homogeneous intermediate-valent state of the samarium ion is fairly stable and is maintained when the perfection of the Sm sublattice is violated. Zh. éksp. Teor. Fiz. 115, 1024–1038 (March 1999)     

Modification of several physicochemical properties of a high-temperature superconducting yttrium ceramic upon doping with gold

The influence of additions of gold (5 and 10 at. %) on the lattice parameters, the superconducting transition temperature, and the diamagnetic susceptibility of the high-temperature superconducting ceramic YBa₂Cu₃O_7−δ is investigated. The influence of gold on the kinetics of the release of weakly bound oxygen during heating of the ceramic in a vacuum is also studied. It is shown that doping the ceramic with gold causes uniaxial expansion of the lattice of the material, a significant decrease in the diamagnetic susceptibility, and nonmonotonic variation of the kinetic parameters of the oxygen release, while the superconducting transition temperature remains nearly unchanged. Zh. Tekh. Fiz. 67, 31–34 (February 1997)     

Magnetic susceptibility of multilayered carbon nanotubes

A theoretical study of the magnetic susceptibility of multilayered carbon nanotubes in fields both parallel, H _∥, and perpendicular, H _⊥, to the tube axis has been carried out disregarding electron-electron interaction. The temperature dependences of the magnetic susceptibility obtained exhibit a nontrivial form for H _∥, which is related to the quasi-one-dimensionality of such a system as the nanotube. The dependences of the magnetic susceptibility on chemical potential χ(μ) have also been derived. At low temperatures, χ(μ) has sharp peaks in fields H _∥, which is connected with the presence of singularities in the density of states of nanotubes. The effect of interlayer coupling on magnetic susceptibility of small-radius tubes has been investigated numerically. Fiz. Tverd. Tela (St. Petersburg) 40, 1950–1954 (October 1998)     

Atomic structure and transport and magnetic properties of the Sm1−x SrxMnO3 system

This is the first study of the temperature dependences of the atomic structure by neutron diffraction, as well as of the resistivity, differential magnetic susceptibility, and magnetoresistance of the ceramic system ¹⁵⁴Sm_1−x Sr_xMnO₃ (x∼0.16–0.4). Samples (x⩾0.3) having an initially orthorhombic structure transfer upon cooling from the insulating to the metallic state and exhibit giant magnetoresistance, which at liquid-helium temperature reaches as high as 90% in magnetic fields up to 30 kOe. At lower doping levels (x⩽0.25), the compound has monoclinic structure. The resistivity of such compounds in zero magnetic field displays insulating behavior upon lowering the temperature to 77 K. Fiz. Tverd. Tela (St. Petersburg) 40, 1271–1276 (July 1998)     

Thermal ionization of impurity centers in Fe-doped Bi12SiO20 and Bi12GeO20 crystals

The spectral and temperature dependence of the optical absorption and thermally stimulated depolarization currents in Fe-doped Bi₁₂SiO₂₀ and Bi₁₂GeO₂₀ crystals are investigated in the photon energy range 1.36–3.46 eV and temperature 85–750 K. The results show thermally induced electron redistribution between donor and acceptor levels and defect association-dissociation processes and are discussed using the configuration-coordinate model. Fiz. Tverd. Tela (St. Petersburg) 41, 1006–1011 (June 1999)     

A new quasi-Ising magnet

We have performed measurements of the magnetization and differential magnetic susceptibility of Dy_0.62Y_2.38Fe₅O₁₂ single crystals in pulsed magnetic fields up to 45 T at liquid-helium temperature for three orientations of the external field: H‖[100], H‖[110], and H‖[111]. It was found that the magnetization reversal in the rare-earth magnetic subsystem occurs via several phase transitions, whose number depends on the direction of the external field, as is characteristic for Ising magnets. The anomalies in the field dependences of the magnetization are interpreted on the assumption of quasi-Ising ordering of the rare-earth ions. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 8, 552–556 (25 April 1998)     

Manifestation of ferromagnetism in lightly oxygen-doped La2CuO4 single crystals in magnetic fields H<50 Oe

The temperature and field dependences χ(T,H) in La₂CuO_4+δ single crystals with δ<0.015 have been investigated in magnetic fields 0.1<H<450 Oe by the differential magnetic susceptibility method. It was found that under oxygen doping conditions ferromagnetic regions are formed. These regions produce a characteristic curve of the magnetic susceptibility χ(T,H), which is observed only in magnetic fields of less than 50 Oe. This can be explained by the formation of ferrons [A. Aharony et al. Phys. Rev. Lett. 60, 1330 (1988); L. I. Glazman and A. S. Ioselevich, Z. Phys. B 80, 268 (1990)] in an antiferromagnetic matrix. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 152–155 (10 August 1996)     

On the nature of the Faraday effect in the rare-earth ortho-aluminate TbAlO3

The magnetic susceptibility χ, the absorption and luminescence spectra, and the Verde constant V along different crystallographic axes of the rhombic crystal TbAlO₃ have been investigated experimentally in the temperature interval 78–300 K. It is found that, in contrast to the magnetic susceptibility χ measured along the [110] axis, the Verde constant V varies in inverse proportion to the temperature in the given range. It is shown that this temperature dependence of the Verde constant measured along the [110] axis is connected with the absence of a contribution to the Faraday rotation of the van Vleck mechanism (in an external field H) of “mixing” of the states of the rare-earth ion Tb³⁺. From a comparison of the optical and magnetic measurements, we have determined the wave functions and magnitudes of the Stark intervals between the lower Stark sublevels for the ⁷ F ₆ ground multiplet of the Tb³⁺ ion in the ortho-aluminate TbAlO₃. Fiz. Tverd. Tela (St. Petersburg) 41, 2047–2052 (November 1999)     

Magnetic and structural correlations in EuMnO and BiMnO crystals in the paramagnetic temperature range

The magnetic and dielectric properties of EuMn₂O₅ and BiMn₂O₅ crystals are investigated over a wide range of temperatures 4.2–250 K, including the range T≫T _N ≃40 K. Significant departures from the Curie-Weiss law are observed in both crystals for the magnetic susceptibility in the paramagnetic range; they are attributed to the presence of correlated domains of magnetic order over a wide range of temperatures. Anomalies in the dielectric properties of the crystals are observed in the same temperature range T>T _N and, as in the case T<T _N , are correlated with the magnetic properties. Zh. éksp. Teor. Fiz. 112, 284–295 (July 1997)     

Anisotropy of the superconducting properties of YBa2Cu3O7−x single crystals with reduced oxygen content

In an investigation of the resistivity anisotropy of YBa₂Cu₃O_7−x single crystals with suboptimal oxygen content it is observed that the superconducting transition for the component ρc of the resistivity tensor is shifted to lower temperatures with respect to the transition for the component ρab. A similar shift is also observed for the transition in the temperature dependence of the dynamic magnetic susceptibility. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 309–313 (25 August 1998)     

Effect of doping on the magnetic properties of the low-dimensional antiferromagnet CuO

The effect of doping with Li⁺, Zn²⁺, Ni²⁺, and Ga³⁺ ions on the magnetic susceptibility of the low-dimensional antiferromagnet CuO (T _N=230 K) has been studied within a broad temperature range of 77–600 K. The solubility of impurity ions in the CuO lattice is low, ⩽3%. Impurity ions, similar to intrinsic defects, distort antiferromagnetic coupling and can shift the long-and short-range magnetic-order regions toward lower T. Fiz. Tverd. Tela (St. Petersburg) 40, 1876–1880 (October 1998)     

Magnetic susceptibility and parameters of electronic structure of Al<Subscript>2</Subscript>REM (Gd,Dy, and Ho) intermetallic compounds at high temperatures

The magnetic susceptibility of Al₂REM (REM = Gd, Dy, and Ho) intermetallic compounds is experimentally investigated by the Faraday method in a wide temperature interval (290–2000 K) in different magnetic fields (0.3–1.3 T). In the crystalline state, the temperature dependences of the susceptibility follow the generalized Curie–Weiss law. In the liquid phase, the magnetic susceptibility of these intermetallic compounds above the melting point increases for all examined samples. The parameters of the electronic structure of the compounds are calculated based on the experimental data. It is established that the effective magnetic moment per rareearth metal atom is smaller than that characteristic of the free REM⁺ ion.     

Phase transitions in the ferromagnetic alloys Ni2+x Mn1−x Ga

The phase diagram of the cubic ferromagnetic shape-memory alloy Ni_2+x Mn_1−x Ga is constructed theoretically for the case when the Curie temperature is close to the structural transition temperature. This diagram agrees well with the experimental data obtained from resistance and magnetic susceptibility measurements. It is shown that the transition from the paramagnetic cubic phase to the ferromagnetic tetragonal phase can be second-order or first-order. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 3, 212–216 (10 February 1998)     

Existence of an incommensurate phase in a Li2B4O7 crystal

The x-ray diffraction spectra of Li₂B₄O₇ single crystals are investigated in the temperature range 80–300 K, and the lattice parameter c is determined in the same temperature range in the presence of a periodically varying temperature field. An incommensurate phase is not observed anywhere in the temperature range investigated, regardless of whether the crystals are subjected to a periodic temperature field. Fiz. Tverd. Tela (St. Petersburg) 39, 1461–1463 (August 1997)     

Electrically active defects in BiSiO crystals undoped and doped with Cr and Mn ions

The influence of doping of Bi₁₂SiO₂₀ (BSO) with chromium and manganese ions on the thermal depolarization currents (TDCs) is investigated. Measurements are performed in the temperature interval 300–800 K as the preliminary polarization temperature is varied in the range T _p=300–523 K. It is shown that doping significantly alters the structure of the TDC spectra. The Cr and Mn ions produce a set of new peaks over the entire investigated temperature range. The thermal activation energies are 0.85–1.98 eV (BSO:Cr) and 0.58–1.72 eV (BSO:Mn). Another consequence of doping is an increase in the amplitudes of the peaks and the charge accumulated during preliminary polarization. Fiz. Tverd. Tela (St. Petersburg) 40, 472–474 (March 1998)