Anomalous magnetocaloric effect in van vleck paramagnet HoVO<Subscript>4</Subscript> near energy level crossing

The anomalous magnetocaloric effect in singlet rare-earth paramagnets near energy level crossing has been experimentally observed for the first time. The magnetization and differential magnetic susceptibility of Ho_{1 ? x} Y _x VO₄ crystals (x = 0, 0.5) measured along the tetragonal axis in a pulsed magnetic field with various change rates near the crossover field, dH/dt, from 3 × 10³ to 2.5 × 10² T/s are compared to the data in static magnetic fields down to 0.1 K. These data indicate the large negative magnetocaloric effect in the pulsed magnetic field, so that the crystal with the initial temperature T ₀ = 4.2 K is cooled much lower than 1 K. The experimental data are qualitatively described in the crystal field model with various interactions including hyperfine interactions and with the known interaction parameters.     

The oxygen content and its ordering in the chain planes in fine-grained HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

A comparative analysis of the results of the X-ray and Mösbauer studies of the high-temperature superconductor (HTSC) YBa₂Cu₃O _y and YBa₂Cu_{3 ? x} ⁵⁷Fe _x O _y (x = 0.015, T _c ≈ 91.5 K) samples with different average grain sizes <D> in the micron and submicron ranges has been performed. The regularities in the change in the lattice parameter c and in the degree of occupation of different oxygen sites in the CuO_δ chain planes taking place at the decrease in <D> have been studied. The quantitative interrelation between the parameter c and the oxygen content δ in the CuO_δ planes exceeding the amount of the mobile oxygen due to the interplane oxygen redistribution is established.     

Effect of heterovalent substitution of rare-earth elements on the magnetic and transport properties of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript>

The Y_{(1 ? x} Ce _x Ba₂Cu₃O₇ system with low cerium concentrations has been synthesized. The cerium solubility limit measured using x-ray powder diffraction analysis is about 2.4 at. %. The temperature dependences of the magnetization M(T) are measured for samples cooled in a magnetic field (FC) and in a zero field (ZFC). The difference between the magnetizations M _ZFC-M _FC at 77.4 K, which is proportional to the pinning potential, passes through a maximum at x = 0.0156. This concentration corresponds to the average distance (equal to eight lattice constants) between the impurity ions in the plane of the rare-earth elements, which is comparable to the diameter of Abrikosov vortices in YBa₂Cu₃O₇.     

The Nernst-Ettingshausen coefficient in the normal phase of doped HTSCs of the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>y</Subscript> system

The temperature dependence of the Nernst-Ettingshausen coefficient Q(T) in the normal phase of doped HTSCs of the yttrium system was studied. The main features characterizing the behavior of this coefficient were revealed, and the character and mechanism of the effect that various nonisovalent substituents exert on the Q(T) dependence were analyzed. It is shown that the narrow-band model permits one not only to describe all the specific features observed in the Q(T) curves but also to perform a simultaneous quantitative analysis of the temperature dependences of four kinetic coefficients (the electrical resistivity and the Seebeck, Hall, and Nernst-Ettingshausen coefficients) with the use of a common set of model parameters characterizing the band structure and carrier system in the normal phase of an HTSC. This approach was employed to determine the carrier mobilities and the asymmetry of the dispersion curve in the systems studied (YBa₂Cu₃O_y, y = 6.37–6.91; YBa₂Cu_3?xCo_xO_y, x = 0–0.3; Y_1?xCa_xBa₂Cu₃O_y, x = 0–0.25; Y_1?xCa_xBa_2?xLa_xCu₃O_y, x = 0–0.5) and to analyze the effect of the substitutions involved on the variation of these parameters.     

Effect of low-temperature annealing on the magnetization curve of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6 + <Emphasis Type="Italic">x</Emphasis></Subscript> ceramics

The effect of low-temperature annealing on the magnetization curve of YBa₂Cu₃O_{6 + x} ceramics in the superconducting state (x ≈ 0.9) is investigated. When the annealing time is fairly long, the field dependence of magnetic moment M exhibits a feature in the form of a plateau, where the value of M remains almost constant. The evolution of this feature in the magnetization curves of annealed samples with annealing time and temperature is studied. It is assumed that low-temperature annealing gives rise to metastable ferromagnetic clusters in YBa₂Cu₃O_{6 + x} ceramics, the contribution of which to the magnetic moment accounts for the feature in the magnetization curves of the annealed samples.     

Structural and magnetic phase transformations in the BiFeO<Subscript>3</Subscript>-CaFe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> system

The crystal structure and magnetic properties of the Bi_{1 ? x} Ca _x Fe_{1 ? x/2}Nb _x/2O₃ system were studied. It is shown that, at x ≤ 0.15, the unit-cell symmetry of solid solutions is rhombohedral (space group R3c). Solid solutions with x ≥ 0.3 have an orthorhombic unit cell (space group Pbnm). The rhombohedral compositions are antiferromagnetic, while the orthorhombic compositions exhibit a small spontaneous magnetization due to Dzyaloshinski?-Moriya interaction. In CaFe_0.5Nb_0.5O₃, the Fe³⁺ and Nb⁵⁺ ions are partially ordered and the unit cell is monoclinic (space group P2₁/n). In the concentration range 0.15 < x < 0.30, a two-phase state (R3c + Pbnm) is revealed.     

Absorption spectra of some radiation-doped A<Subscript>3</Subscript>B<Subscript>5</Subscript> compounds

Temperature dependences of the absorption coefficient in A₃B₅ crystals before and after irradiation by electrons with an energy of 6 MeV and a dose of Ф = 2 × 10¹⁷ electron/cm² are studied. A low-lying E_v + 0.4 eV center of a nonimpurity origin is found in both undoped GaAs crystals and those doped with various impurities (Te, Zn, Sn, Ga_1–xIn_xAs, InP, and InP〈Fe〉).     

Nonstoichiometric high-pressure phase of Tm<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>3</Subscript>V<Subscript>4</Subscript>O<Subscript>12</Subscript>

Tm _x Cu₃V₄O₁₂, a perovskite-like oxide (space group, Im-3; Z = 2; a = 7.279–7.293 Å) containing vacancies in its cationic sublattice, was obtained barothermally (P = 7.0–9.0 GPa, t = 1000–1100°C) for the first time. The temperature dependences on the electrical resistivity (10–300 K) and the magnetic susceptibility (0–300 K) were investigated. It was shown that the oxide Tm _x Cu₃V₄O₁₂ is characterized by metal-type conductivity and paramagnetic properties.     

Low-temperature thermal-expansion anomaly in Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>CuO<Subscript>6</Subscript>

Single-crystal samples of the Bi_{2 + x}Sr_{2 ? x ? y}Cu_{1 + y}O_{6 + δ} system revealed anomalous (negative) thermal expansion in the temperature range 10–20 K. Magnetic fields of 1–3 T were found to strongly affect the position and width of the anomaly region. A thermal-expansion singularity was detected at temperatures T≈30–50 K, which may be related to the formation of a pseudogap.     

In-plane torque and gap symmetry of untwinned YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript> crystals

The reversible magnetic torque of untwinned YBa₂Cu₃O₇ single crystals shows the four-fold symmetry in thea-b plane. The irreversible torque indicates evidence for a novel intrinsic pinning along thea andb axes. These facts mean that the free energy of the four-fold symmetry has a minimum when the field is applied along thea orb axis. The results are consistent with those expected from thed _x ²?y ² symmetry and rule out the possibility of thed _xy symmetry. The Fermi surface anisotropy is not responsible for the observed anisotropy. This is firstbulk evidence for thek-dependent gap anisotropy on the Fermi surface. The two-fold anisotropy parameter is found as\(\gamma _{ab} = \sqrt {{{m_a } \mathord{\left/ {\vphantom {{m_a } {m_b }}} \right. \kern-\nulldelimiterspace} {m_b }}} = 1.18 \pm 0.14\).     

Oxygen isotope effect in chromium-doped Pr<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> manganites

The effect of oxygen isotope substitution on the properties of Pr_0.5Ca_0.5Mn_{1 ? x} Cr _x O₃ manganites (x = 0, 0.02, 0.05) have been studied. The introduction of chromium favors (i) the decomposition of a charge-ordered state and (ii) the appearance of a ferromagnetic metallic phase in Pr_0.5Ca_0.5Mn_{1 ? x} Cr _x ^16–18O₃. The isotope substitution ¹⁶O → ¹⁸O leads to a decrease in the content of the ferromagnetic phase, an increase in the charge-ordering transition temperature (T _CO), and a decrease in the ferromagnetic transition temperature (T _FM). The isotope mass exponent is evaluated.     

Zeeman effect and stark splitting of the electronic states of the rare-earth ion in the paramagnetic terbium garnets Tb<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> and Tb<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>

The Zeeman effect in the ⁷ F ₆ → ⁵ D ₄ absorption band of the Tb³⁺ ion in the paramagnetic garnets Tb₃Ga₅O₁₂ and Tb₃Al₅O₁₂ was studied. The field dependences of the Zeeman splitting of some absorption lines are found to exhibit unusual behavior: as the magnetic field increases, the band splitting decreases rather than increases. Symmetry analysis relates these lines to 4f → 4f electron transitions of the doublet-quasi-doublet or quasi-doublet-doublet type, for which the field dependences of the splitting differ radically from the well-known field dependences of the Zeeman splitting for quasi-doublet-quasi-doublet or quasi-doublet-singlet transitions in a longitudinal magnetic field.     

Microstructure and superconducting properties of the DyBaCuO ceramic doped with Na<Subscript>2</Subscript>CO<Subscript>3</Subscript>, NaCl,and KClO<Subscript>3</Subscript>

This paper reports on a study of the effect of doping with Na₂CO₃, NaCl, and KClO₃ salts on the microstructure and superconducting characteristics of ceramics with nominal compositions Dy_{1 ? x} M _x Ba₂Cu_{3 ? y} O_{7 ? δ} and DyBa₂Cu_{3 ? y} O_{7 ? δ} : M _x for M = Na, K; x = 0.2, 0.3, and y = 0, 0.2. The microstructure was characterized by transmission electron microscopy with local energy-dispersive x-ray in situ analysis (probe size ～1 nm). An analysis shows that none of the doping elements (Na, K, or Cl) enters into 123 grains in sizable amounts and that, as a result, the critical temperature of the superconducting transition remains practically constant in the range 90.0–93.5 K. Potassium and chlorine segregate at grain boundaries. It is shown that grain-boundary segregation of chlorine leads to a substantial increase in the superconducting critical current (by a factor 3–5 at 70 K) as compared to the undoped sample. The possible mechanisms accounting for the effect of Cl on intergrain critical current are discussed.     

Mobility and diffusion of oxygen isotopes in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6+<Emphasis Type="Italic">x</Emphasis></Subscript>: Monte carlo simulation

The self-diffusion (D₀) and isotope diffusion (D*) coefficients of oxygen in YBa₂Cu₃O_6+x are calculated as functions of the temperature (600–1200 K) and the oxygen content (0<x<1). The Monte Carlo simulation is performed with due regard for both the interaction of oxygen ions at lattice sites in the basal planes of YBa₂Cu₃O_6+x and the interaction between a jumping ion at a saddle point and the environment. Equilibrium thermodynamic characteristics (including the phase diagram and the heat capacity) are calculated in terms of the Hamiltonian of interaction between oxygen ions at the lattice sites. It is found that an increase in the oxygen content leads to a decrease in the diffusion coefficients D₀ and D*, an increase in the effective activation energies for diffusion by 0.3–0.5 eV, and a decrease in the Haven ratio from 1 to ～0.5.     

CaCuMn<Subscript>6</Subscript>O<Subscript>12</Subscript> vs. CaCu<Subscript>2</Subscript>Mn<Subscript>5</Subscript>O<Subscript>12</Subscript>: A comparative study

The ferrimagnetic compounds Ca(Cu_xMn_3?x)Mn₄O₁₂ of the double distorted perovskites AC₃B₄O₁₂ family exhibit a rapid increase of the ferromagnetic component in magnetization at partial substitution of square coordinated (Mn³⁺)_C for (Cu²⁺)_C. In the transport properties, this is seen as a change of the semiconducting type of resistivity for the metallic one. The evolution of magnetic properties of Ca(Cu_xMn_3?x)Mn₄O₁₂ is driven by strong antiferromagnetic exchange interaction of (Cu²⁺)_C with (Mn³⁺/Mn⁴⁺)_B coordinated octahedra. The competing interactions of (Mn³⁺)_C with (Mn³⁺/Mn⁴⁺)_B lead to the formation of noncollinear magnetic structures that can be aligned by magnetic fields.     

Electrospun Dy-doped SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript> nanofibers: microstructure and magnetic properties

SrDy _x Fe_12?x O₁₉ (x ≤ 0.08) nanofibers have been synthesized by the electrospinning method followed by calcinations process. The partial substitution of rare earth ions Dy³⁺ (10.5 μ _B of magnetic moments) mainly occupying 12k sublattice sites in the SrFe₁₂O₁₉ crystal structure is investigated and discussed in this work. An enhanced coercivity of 7155 Oe has been obtained when the doped content reached to 0.08 at a relative low calcination temperature of 800 °C. As a result, we believe the synthesized SrDy _x Fe_12?x O₁₉ nanofibers can potentially be useful in high-density recording media as well as permanent magnets.     

Magnetic properties of the Gd<Subscript>2</Subscript>V<Subscript>0.67</Subscript>Mo<Subscript>1.33</Subscript>O<Subscript>7</Subscript> and Y<Subscript>2</Subscript>VMoO<Subscript>7</Subscript> pyrochlore compounds

It is demonstrated that 50% substitution of vanadium for molybdenum in the pyrochlore lattice of the complex oxide Y₂(V _x Mo_{1 ? x} )₂O₇ results in a transition from the spin-glass ground state (at x = 0) to the ferromagnetic state in Y₂VMoO₇ (a = 10.1645(2) Å, T _C = 55 K). The Gd₂V_0.67Mo_1.33O₇ compound (a = 10.2862(3) Å) is a ferromagnet with T _C (84 K) exceeding that of undoped Gd₂MnO₂O₇.     

Mechanisms of interaction of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript> <Emphasis Type="Italic">y</Emphasis> </Subscript> with water vapor at low-temperature annealing

Abstract—The interaction of YBa₂Cu₃O_y (123) with water vapor at temperatures T ≤ 150° has been studied. It has been shown that, with an increase in temperature, the mechanism of its interaction with water changes. Near room temperature, the main process is hydrolytic decomposition. At T ~ 100°C, the absorption of water is significantly reduced, because the role of hydrolysis becomes less important and water penetrates the structure weakly and is incorporated into oxygen vacancies mainly in the form of OH–-groups, which leads to the transition of YBa₂Cu₃O_y from the tetragonal to orthorhombic phase. With an increase in temperature to 150°C, the absorption of water increases again. In this case, the main mechanism is the penetration of water to the 123 structure, which leads to splitting of Cu–O chains and a phase transition from the 123 to pseudo-124 structure. The role of different mechanisms of interaction with water essentially depends on the oxygen content in the 123 structure. At a low oxygen index (y = 6.3), the role of hydrolysis is more important, and, at y ≥ 6.5, the incorporation of water into the structure prevails. It has been revealed that, at T = 150°C, after absorption of water, YBa₂Cu₃O_6.96 becomes a proton conductor.     

Raman scattering spectroscopy of inclusions of carbon in Al<Subscript>2</Subscript>O<Subscript>3</Subscript> films and its solid solutions with HfO<Subscript>2</Subscript>

We apply Raman scattering spectroscopy to study the nature of carbon inclusions in Al₂O₃ and (HfO₂) _x (Al₂O₃)_{1 ? x} films deposited using volatile complex compounds. Raman spectra of the films under investigation contain D and G vibrational modes, which indicate that carbon clusters of the sp ² configuration tend to form in the films. We estimate the size of clusters from the integrated intensity ratio I _D /I _G and find it to be in the range of 14–20 Å. The content of hydrogen in carbon clusters is calculated from the height of the photoluminescence pedestal and is found to vary from 14 to 30 at % depending on the regime of the film’s synthesis.     

Stabilization of Ferromagnetism in BiFeO<Subscript>3</Subscript>:Ho at Hydrostatic Pressure

The isothermal magnetization of the Bi_{1 – x}Ho _x FeO₃ (x = 0?0.2) multiferroic has been studied at a hydrostatic pressure up to 9 GPa in the range of room temperatures. A new anomaly at P_C ≈ 3.81 GPa related to intermediate phases between the structural transition R3c → Pnma has been found against the background of the pressure-induced antiferromagnetic ordering in BiFeO₃ (BFO) at P ≈ 2.59 GPa. It is established that the ferromagnetic behavior under pressure depends on the Ho impurity concentration: P_C decreases at 0.05 ≤ x ≤ 0.1 because of the decrease in R3c bond lengths in the structure, and the stabilization of ferromagnetism is implemented at 0.1 ≤ x ≤ 0.2 probably because of the coexistence of the R3c and Pnma phases. The results of studies indicate that, in Bi_{1 – x}Ho _x FeO₃ with x = 0.2, the transition pressure P_C = 3.7 GPa exceeds the values for BFO doped with other 4f elements (Eu, Y, Sm) in the region R3c → Pnma of the transition.