Concentration phase transition near the stoichiometric composition of vanadium monoxide VO<Subscript>1.00</Subscript>

The concentration dependences of the magnetic susceptibility and lattice parameter of cubic vanadium monoxide have been measured in the composition range from VO_0.81 to VO_1.07. Near the stoichiometric composition VO_1.00, the concentration dependences exhibit a stepwise increase in the specific magnetic susceptibility (by approximately 0.7 × 10^?6 cm³/g) and lattice constant (by about 0.002 nm). These effects can be related to the concentration phase transition, which occurs in vanadium monoxide with a change from a substoichiometric composition to superstoichiometric. At such a transition, along with a decrease in the concentration of oxygen vacancies, tetrahedrally coordinated vanadium interstitials are formed, as a result of which the B1 structure changes to a more complex cubic phase structure with the same space group Fm-3m.     

Magnetic susceptibility and nuclear magnetic resonance of vanadium sulfides

Measurements of the magnetic susceptibility in the temperature range 4–1000°K are reported for the vanadium sulfides VS, V₃S₄-V₂S₃ and V₅S₈. At higher temperatures the susceptibility of all compounds approaches a constant, positive value, indicating a Pauli-paramagnetic contribution of itinerant electrons. At lower temperatures an increase of the susceptibility is observed. This effect is particularly pronounced in V₅S₈, and is taken as evidence for the presence of localized magnetic moments.Nuclear magnetic resonance spectra of V₅S₈ show two lines, one with a temperature-independent Knight shift, and one with a positive Knight shift which increases strongly at lower temperatures. The two lines are attributed to vanadium atoms with itinerant d electrons, and to vanadium atoms with localized d electrons, respectively. This interpretation is consistent with the magnetic susceptibility data. The difference in properties between two types of vanadium atoms in V₅S₈ can be understood by considering the crystal structure.     

Disintegration of a coarse-grained vanadium monoxide VO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> powder

A coarse-grained powder of nonstoichiometric cubic vanadium monoxide VO _y is disintegrated in a Retch PM 200 planetary ball mill. Milling of the coarse-grained vanadium monoxide powder VO _y at a rate of rotation of 500 rpm for 2 h significantly broadens diffraction lines, and the crystal structure of vanadium monoxide VO_1.00 after milling remains the same. High-resolution scanning electron microscopy and X-ray diffraction studies of the microstructure of vanadium monoxide demonstrate that high-energy milling can produce vanadium monoxide powders with an average crystallite size of 23 ± 10 nm. The vanadium monoxide produced by milling has a crystallite size that is half the crystallite size in the titanium monoxide produced by severe plastic deformation.     

Pseudo-gap and possible Spin-Peierls transition in the vanadium oxide VOSb 2 O 4

We report on the magnetic susceptibility and electron spin resonance measurements on polycrystalline samples of the vanadium oxide VOSb₂O₄, a quasi-one-dimensional S = 1/2 Heisenberg system. The susceptibility vanishes at zero temperature, but there is no cusp at the onset of the susceptibility drop, and the ESR linewidth exhibits an increase characteristic of a phase transition at a much lower temperature. We show that this behaviour is consistent with the formation of a pseudo-gap in a spin-Peierls system in the adiabatic limit. Received 7 February 2001 and Received in final form 24 April 2001     

The influence of imperfection of the crystal lattice on the electrokinetic and magnetic properties of disordered titanium monoxide

The conductivity and magnetic susceptibility of disordered titanium monoxide TiO_y (0.920≤y≤1.262) containing vacancies in titanium and oxygen sublattices are investigated. For TiO_y monoxides with an oxygen content y≤1.069, the temperature dependences of the conductivity are described by the Bloch-Grüneisen function at a Debye temperature ranging from 400 to 480 K and the temperature dependences of the magnetic susceptibility are characterized by the contribution from the Pauli paramagnetism due to conduction electrons. The behavior of the conductivity and magnetic susceptibility of TiO_y monoxides with an oxygen content y≥1.087 is characteristic of narrow-gap semiconductors with nondegenerate charge carriers governed by the Boltzmann statistics. The band gap ΔE between the valence and conduction bands of TiO_y monoxides with y≥1.087 falls in the range 0.06–0.17 eV.     

Spin glass phase in the iron vanadium oxide bronze θ − Fe0.35V2O5

Low temperature magnetic susceptibility measurements performed on Fe_0.35V₂O₅ and Al_0.35V₂O₅ indicate for the first time for vanadium oxide bronzes that the magnetically ordered spin glass phase occurs in the iron bronze below 8 K.     

Paramagnetic susceptibility of nickel-germanium alloys

Magnetic susceptibilities of nickel-rich nickel-germanium solid solution (8 compositions) have been studied as a function of temperature up to 1500°K. The results support the previous conclusion, obtained on nickel binary alloys with vanadium, niobium, and tantalum, that the magnetic susceptibility does not follow the modified Curie-Weiss law commonly assumed to hold for nickel alloys. The inadequacy of this law is briefly discussed. From electrical restivity measurements at low temperatures it is inferred that residual resistivity contribution of germanium is about 2.8μΩ cm per atom percent.     

Magnetic susceptibility of dilute Al-Dy alloys at high temperatures

The magnetic susceptibility of Al-Dy alloys with the dysprosium content from 0.1 to 2.0 at % in the temperature range 300–1850 K has been studied. It is established for all compositions that the susceptibility begins to increase when the alloy temperature exceeds the melting temperature of the Al₂Dy compound, and the concentration curves exhibit oscillations of the same type in the solid and liquid states. The experimental results are interpreted in terms of the existence of chains composed of Al₂Dy quasi-molecules in the alloys.     

Magnetic properties of titanium vanadium hydrides

Measurements have been made of the magnetic susceptibility X in ternary titanium-vanadium hydrides Ti_1?yV_yH_x in the range 0<y< 0.5 and with a hydrogen to metal ratio from 1.6 to 1.92; they cover the temperature interval from 80 to 440 K. The x^?T curves of some of the ternary hydrides possess a maximum similar to that seen in binary hydrides. Its occurrence depends upon both hydrogen and vanadium concentration. Comparison with the lattice constants shows that this dependence is related to the transition from the cubic γ-phase to the tetragonal δ-phase. The x^?T curves of those samples showing the phase transition were extrapolated from the cubic phase down to lower temperatures. Conclusions were drawn from the measured and extrapolated susceptibility values, as to the influence of both hydrogen and vanadium concentrations on the electronic structure of these hydrides. The dependence of the magnetic susceptibility on the vanadium concentration shows that, in the cubic phase, the fermi energy lies on the increasing side of a maximum in the density of states curve. The degree of occupation of the conduction band does not depend on the hydrogen concentration. The rigid band model cannot be used in the hydrides studied here to account for the effects of varying their hydrogen concentration.     

Magnetic properties of Fe1−xCoxO

Measurements are reported for the magnetic susceptibility of solid solutions Fe_1-xCo_xO for compositions with x =0.10 to 0.95 and for the temperature range 4.2 to 600 K. It is found that the Curie-Weiss law is obeyed at high temperatures while the dependence of the susceptibility and transition temperature on composition appear to be in good agreement with the “virtual crystal” approximation.     

Electrical conductivity and magnetic susceptibility of titanium monoxide

Conductivity and magnetic susceptibility of disordered cubic titanium monoxide TiO_y(0.920≤y≤1.262) are studied. Temperature dependences of the conductivity of TiO_y monoxides with y≤1.069 are described by the Bloch-Grüneisen function with Debye temperature 400–480 K, and temperature dependences of the susceptibility include Pauli paramagnetism of conduction electrons. The behavior of conductivity and susceptibility of TiO_y with y≥1.087 is typical of semiconductors with nondegenerate charge carriers obeying Boltzmann statistics. The band gap ΔE between the valence and conduction bands of TiO_y(y≥1.087) is 0.06–0.17 eV, and effective mass of charge carriers is equal to 7–14 electron masses.     

Oscillations of the Critical Temperature in a (Fe/Cr/Fe)/V/Fe Heterostructure

The superconducting and magnetic properties of the (Fe/Cr/Fe)/V/Fe layered system with variable thickness of the chromium layer have been experimentally and theoretically studied. The magnetic properties of the system have been studied by the ferromagnetic resonance method, and the superconducting transition temperature has been measured from the jump in the magnetic susceptibility. A wide variety of magnetic states are observed in the system; in particular, the structure of small domains can arise in the iron layer placed between vanadium and chromium. It has been shown experimentally that the critical temperature T_c of the superconducting transition undergoes nonmonotonic oscillations with a noticeable amplitude in the given system with the change in the thickness of the Cr layer. The proposed model based on the proximity effect theory makes it possible to relate these T_c oscillations to the features of the magnetic structure of the samples.     

Magnetic susceptibility of single-crystal hexagonal nickel sulphide

The magnetic susceptibilities of five single-crystals of hexagonal NiS, with compositions ranging from NiS-NiS_1.02 have been measured in the temperature range 78–300°K. The susceptibility curves have a markedly different shape from that normally predicted for an antiferromagnet. This results from the first-order character of the phase transition at T_N.     

Spin-spin correlations in the insulating and metallic phases of the Mott system V 2 O 3

The magnetic response in V ₂ O ₃ has been investigated using polarised neutron scattering with polarisation analysis. Measurements were carried out at three temperatures corresponding to the antiferromagnetic insulating ground state, the metallic phase and the high temperature metallic phase. At the first order metal insulator transition there is a dramatic change in the magnetic response with the metallic and high temperature metallic phases being characterised by ferromagnetic spatial correlations of the paramagnetic response. The establishment of ferromagnetic correlations at the metal insulator transition accounts for the abrupt jump in the uniform susceptibility. It is proposed that the differentiation of the V-V distances across the edges of VO ₆ octahedra is of critical importance for the change in electronic conductivity but also for the establishment of the spatial correlations. The gradual high temperature evolution of the conductivity then occurs by the reduction in the vanadium d overlap brought about by thermal expansion. The first order reduction in atomic volume which occurs on the establishment of the metallic phase results from an instability of the vanadium local moment arising from the change in electronic structure. Received 7 April 1999     

Crystal structure and magnetic susceptibility of (CuInSe<Subscript>2</Subscript>)<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>(2MnSe)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

The crystal structure of samples in the (CuInSe₂)_{1 ? x} (2MnSe) _x system at room temperature and their magnetic susceptibility in the temperature range 77–1000 K are investigated. It is established that compositions with concentrations 0≤ x ≤ 0.2 form solid solutions with a tetragonal structure, space group I \(\bar 4\)2d (122). The specific magnetic susceptibility χ of samples with 0.1 ≤ x ≤ 0.4 at 77 K lies in the range 9 × 10^?4?1.6 × 10^t-3cm³/g. The temperature dependence of the inverse magnetic susceptibility of the sample with x = 0.4 suggests the presence of a component with antiferromagnetic ordering and a reliably measured Néel temperature that is characteristic of MnSe. The dependences χ = f(T) of the compositions with x = 0.1, 0.2, 0.3, and 0.4 indicate the occurrence of magnetic phase transitions with a change in the spin state.     

Anisotropic susceptibility of (Ti1−xVx)2O3 single crystals

Anisotropy of the susceptibility, which depends on vanadium concentration, was observed for (Ti_1?xV_x)₂O₃(x=0.005, 0.015, 0.02) single crystals. The anisotropy is hard to interpret by localized magnetic states of vanadium ions.     

Evidence for strong Coulomb correlations in the metallic phase of vanadium dioxide

The influence of Coulomb correlations on magnetic and spectral properties in the metallic rutile phase of vanadium dioxide is studied by state of the art LDA + DMFT method. Calculation results in strongly correlated metallic state with an effective mass renormalization m*/m ≈ 2. Uniform magnetic susceptibility shows Curie-Weiss temperature dependence with effective magnetic moment, p _eff^theor = 1.54μ_B, in a good agreement with the experimental value p _eff^exp = 1.53μ_B that is close to ideal value for V⁴⁺ ion with the spin S = 1/2, p _eff = 1.73μ_B. Calculated spectral function shows well developed Hubbard bands observable in the recent experimental photoemission spectra. We conclude that VO₂ in rutile phase is strongly correlated metal with local magnetic moments formed by vanadium d-electrons.     

Atomic displacements in the V<Subscript>52</Subscript>O<Subscript>64</Subscript> superstructure and the short-range order in superstoichiometric cubic VO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> vanadium monoxide with metal vacancies

Atomic displacements in the lattice of the tetragonal V₅₂O₆₄ superstructure have been experimentally determined. It has been found that atomic displacement waves, which are attributed to the formation of the short-range displacement order, appear in the vanadium and oxygen sublattices of this superstructure. It has been shown that the V₅₂O₆₄ superstructure is formed on the basis of disordered superstoichiometric cubic vanadium monoxide with the short-range order in the metallic sublattice. The character of the short-range order is such that vanadium atoms occupying tetrahedral positions are in the environment of four vacant sites of the vanadium sublattice. This means that the superstoichiometric VO_>1.0 vanadium monoxide has a cubic structure differing from the B1-type structure characteristic of most of the strongly nonstoichiometric cubic compounds MX _y (X = C, N, O) of transition metals.     

Influence of particle size,stoichiometry, and degree of long-range order on magnetic susceptibility of titanium monoxide

In situ measurements of the magnetic susceptibility of ordered and disordered titanium monoxides TiO_y in the temperature range from 300 to 1200 K have revealed that it depends on the size of crystals, their stoichiometry, and long-range order parameters. Analysis of the data for both the ordered and disordered TiO_y has demonstrated that the dependence of the Van Vleck paramagnetism on the nanocrystal size is inversely proportional due to the breaking of symmetry of the local environment of titanium and oxygen atoms near the surface of nanocrystals. It has been found that the Van Vleck contribution from the atomic vacancy disorder in monoxide nanocrystals of superstoichiometric composition, as well as in the crystalline stoichiometric monoxide, is proportional to the deviation of the degree of long-range order from the maximum value.     

Magnetic behavior of amorphous Fe−Ni−Zr alloys and their response to radiation damage

The magnetic properties of two amorphous Fe?Ni?Zr alloys, Fe_89.7Ni_0.03Zr₁₀ and Fe₇₀Ni₂₀Zr₁₀, both in the “as cast” and neutron irradiated states were investigated by Mössbauer spectroscopy and dc magnetic susceptibility measurements. The upper magnetic ordering temperatures of Fe_89.7Ni_0.03Zr₁₀ are 232K and 246K for the “as cast” and irradiated samples, respectively. The magnetic ordering temperature for Fe₇₀Ni₂₀Zr₁₀ was about 478K for both the “as cast” and irradiated samples. Both compositions yield magnetic hyperfine spectra, which show a considerable relaxation effect that must be explicitly considered in the calculation of the average local Fe moments. When this is done, these values derived from Mössbauer spectra are in good agreement with the dc susceptibility values. The effects of neutron irradiation on the magnetic properties of these alloys are small.