Thermodynamic study of compounds of the Nd-Ba-Cu-O system

Standard enthalpies of formation for solid solutions of composition Nd_{1 + x} Ba_{2 ? x} Cu₃O _y (x = 0–0.8, y = 6.65–7.24) from oxides were determined by solution calorimetry. The heat capacity of NdBa₂Cu₃O_6.87 phase was measured in the range 5–320 K by low-temperature adiabatic calorimetry. The absolute entropy S ^o(T), the difference of enthalpies H ^o(T)-H ^o(0 K), and the reduced Gibbs energy Φ^o(T) = S ^o(T)–[H ^o(T)–H ^o(0)]/T were calculated on the basis of smoothed dependence C _p (T) in the 0–320 K range. An assessment was made for the heat capacities and the absolute entropies of solid solutions Nd_1+x Ba_2?x Cu₃O _y . The obtained set of thermodynamic parameters can be used for the calculation of phase equilibria in the Nd-Ba-Cu-O system.     

Peculiarities of magnetic field-induced ferromagnetic ordering in narrow-band manganites

The temperature dependences of the residual magnetization in narrow-band manganites (Pr_0.67Ca_0.33MnO₃, Sm_0.55Sr_0.45Mn¹⁸O₃, Sm_0.55Sr_0.45Mn¹⁶O₃, and (NdEu)_0.55Sr_0.45Mn¹⁸O₃) have been studied. All compounds studied are characterized by a fairly high residual magnetization M _R (about 0.5 μ_B/Mn) at 4.2 K, which vanishes upon sample heating to the temperature T _RE ≈ 30–35 K, which is much lower than the temperature T _C of the ferromagnetic transition. However, upon magnetization of the samples at T _RE < T < T _C , the residual magnetization (smaller in magnitude) remains up to T _C . For the composition (NdEu)_0.55Sr_0.45Mn¹⁸O₃, the residual magnetization remains at T < T _C , independent of the temperature of magnetization. The disappearance of the residual magnetization found at intermediate temperatures is apparently related to the destruction of the magnetic field-induced ferromagnetic ordering (which contains an additional contribution of the rare-earth sublattice).     

Optical and x-ray diffraction studies of the symmetry of distorted phases of the (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>ZrF<Subscript>7</Subscript> crystal

(NH₄)₃ZrF₇ single crystals were grown, and polarization-optical and x-ray diffraction studies were performed on powders and crystalline plates of various cuts over a wide temperature range. Phase transitions are revealed at temperatures T _1↑ = 280 K, T _2↑ = 279.6 K, T _3↑ = 260–265 K, and T _4↑ = 238 K on heating and at T _1↓ = 280 K, T _2↓ = 269–270 K, T _3↓ = 246 K, and T _4↓ = 235 K on cooling. The sequence of changes in symmetry is established to be as follows: O _h ⁵ (Z = 4) ? D _2h ²⁵ (Z = 2) ? C _2h ³ (Z = 2) ? C _i ¹ (Z = 108) ? monoclinic²(Z = 216).     

Spin state of Co<Superscript>3+</Superscript> ions in EuBaCo<Subscript>2–<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>5.5–δ</Subscript> in the paramagnetic temperature range

The magnetic studies of EuBaCo₂O_5.50, EuBaCo_1.93O_5.40, and EuBaCo_1.90O_5.36 have been performed. Layered cobaltites EuBaCo_2–x O_5.5–δ demonstrated the ferrimagnetic ordering in the temperature range 200 K < T < 650 K. The ferromagnetic Curie temperature T _C almost did not change as the number of Co³⁺–O–Co³⁺ bonds decreased. The contributions of the ferro- and antiferromagnetic interactions were nearly the same and determine the relationship T _C > T _N and T _C < T _N. At T > T _C, the antiferromagnetic order was retained in the EuBaCo1.90O5.36. As temperature increased, EuBaCo_2–xO_5.5–δ conserved the IS state in pyramids, and gradual LS → IS → HS transitions occurred in octahedron sites. The increase in the effective magnetic moment μeff in EuBaCo_1.90O_5.36 at T > 580 K demonstrated the transition of Co³⁺ ions to the HS state at lower temperatures as compared to the EuBaCo₂O_5.50 stoichiometric composition, and the structural distortions and the increase in the unit cell volume favored to this transition.     

The influence of chromium impurity centers on the critical properties of a weakly polar ferroelectric Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript>

The Cr³⁺ EPR spectra of Li₂Ge₇O₁₅ (LGO) crystals are analyzed in the temperature range of the ferroelectric phase transition. The temperature dependence of the local order parameter is determined from the measured splittings of the EPR lines in the polar phase. The experimental critical exponent of the order parameter β=0.31 in the range from the phase transition temperature T _C to (T _C -T) ～ 40 K corresponds to the critical exponent of the three-dimensional Ising model. Analysis of the available data demonstrates that, away from the phase transition temperature T _C , the macroscopic and local properties of LGO crystals are characterized by a crossover from the fluctuation behavior to the classical behavior described in terms of the mean-field theory. The temperature dependence of the local order parameter for LGO: Cr crystals does not exhibit a crossover from the Ising behavior (β=0.31) to the classical behavior (β=0.5). This is explained by the defect nature of Cr³⁺ impurity centers, which weaken the spatial correlations in the LGO host crystal. The specific features of the critical properties of LGO: Cr³⁺ crystals are discussed within a microscopic model of structural phase transitions.     

EPR study of copper ions in Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript> crystals

The EPR spectra of Cu²⁺ ions (² D _5/2) located at two structurally nonequivalent positions Cu1 and Cu2 in crystals of lithium heptagermanate Li₂Ge₇O₁₅ are recorded. The angular dependences of the EPR spectrum are measured in the paraelectric phase of the Li₂Ge₇O₁₅ compound (T = 300 K). The components of the g factor and the hyperfine interaction tensor A are determined, and the orientation of the magnetic axes with respect to the crystallographic basis is established. The EPR spectra are recorded in the temperature range in the vicinity of the temperature T _C = 283 K of the transition from the paraelectric phase to the ferroelectric phase. The position symmetry of the Cu1 and Cu2 centers is determined at temperatures above and below the phase transition temperature T _C . The localization of paramagnetic centers in the structure is discussed, An analysis of the results obtained demonstrates that the Cu1 and Cu2 centers in the Li₂Ge₇O₁₅ crystal lattice replace lithium ions located at two structurally nonequivalent positions with the symmetries described at temperatures above T _C by the triclinic C _i and monoclinic C ₂ point groups, respectively.     

Structure and stability of two-dimensional systems of C<Subscript>20</Subscript> fullerenes

Two-dimensional systems of C₂₀ fullerenes connected to each other by strong covalent bonds have been investigated. Several isomers differing in the type of intercluster bonds have been revealed. The lifetimes τ of the (C₂₀) _{M × M} complexes with M = 2 and 3 at T = 1800–3300 K have been directly calculated using the molecular dynamics method. It has been shown that these complexes lose their periodic cluster structure due usually to the coalescence of two or several neighboring C₂₀ fullerenes. The activation energy of this process determined by analyzing the τ(T) dependence appears to be E _a ≈ 2.5 eV in agreement with the calculations of the heights of the potential barriers preventing the coalescence. At high temperatures T > 2400 K, the decay of C₂₀ fullerenes entering into the complex is possible.     

Specific heat of La(Fe<Subscript>0.873</Subscript>Co<Subscript>0.007</Subscript>Al<Subscript>0.12</Subscript>)<Subscript>13</Subscript> compound in antiferromagnetic and ferromagnetic states

The low-temperature specific heat C _p of La(Fe_0.873Co_0.007Al_0.12)₁₃ compound has been measured in two states: (i) antiferromagnetic (AFM) with a Néel temperature of T _N = 192 K and (ii) ferromagnetic (FM). The FM order appears at T = 4.2 K in a sample exposed to an external magnetic field with induction B _C ≥ 2.5 T and is retained for a long time in a zero field at temperatures up to T*_C = 23 K. The coefficient γ_FM in the low-temperature specific heat C = γT + βT ³ in the FM state differs quite insignificantly from that (γ_AFM) in the AFM state. Contributions to the low-temperature specific heat, which are related to a change in the elastic and magnetoelastic energy caused by magnetostrictive deformations, are considered.     

Phase transitions and magnetoelectric coupling in BiFe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript> multiferroics

Multiferroic BiFe_1?xZn _x O₃ ceramics were prepared by solution combustion method. Their structure, magnetoelectric, dielectric, magnetic, thermal characteristics were studied. The magnetic M(T) and heat capacity C _p (T) measurements demonstrate an antiferromagnetic to paramagnetic phase transition (T _N ) around 635 K. The anomaly on the temperature dependence of the dielectric constant near T _N was observed, which could be induced by the magnetoelectric coupling between electric and magnetic ordering. The magnetoelectric behavior was also confirmed by the linear relation between Δε and M², which is in the agreement of the Ginzburg-Landau theory for the second-order phase transition.     

The 2<Superscript>1/3</Superscript> rule and other properties of ferromagnets near the temperature of the maximum of magnetic susceptibility

The behavior of the magnetization M and the magnetic susceptibility χ is theoretically analyzed for ferromagnets at the temperature T=T _m corresponding to the maximum of the function χ(T). Four new methods of determining the Curie temperature T_C with the use of the derived relationships are proposed. One of these methods is based on the relationship χ(T _m ) =2^1/3χ(T_C) (the 2^1/3 rule). The results are applied for processing experimental data obtained for lanthanum manganite of composition La_0.85Sr_0.15MnO₃.     

Long-range and short-range magnetic order in new compound NaVGe<Subscript>2</Subscript>O<Subscript>6</Subscript>

New metal oxide pyroxene compound NaVGe₂O₆ containing isolated edge-sharing VO₆ (S=1) chains undergoes transition into a long-range antiferromagnetic state at T _N =16 K. The broad maximum in the temperature dependence of magnetic susceptibility at T _M =26 K indicates the low-dimensional character of the magnetic subsystem. Even though the antiferromagnetic ordering is accompanied by a sharp peak of specific heat, significant magnetic entropy is released above the Néel temperature.     

Nature of low-temperature phase transitions in CaMn<Subscript>7</Subscript>O<Subscript>12</Subscript>

New data on the specific heat, thermal expansion, and magnetization of the CaMn₇O₁₂ phase require a revision of the current concepts of the sequence of phase transitions in this compound. It is found that a spin-glass phase transition occurs in CaMn₇O₁₂ at T _M = 49 K, whereas the transition at T _S = 89 K exhibits the features of a first-order phase transition and thereby is apparently of structural origin. In the range T _M < T< T _S , the CaMn₇O₁₂ compound exhibits negative thermal expansion, which is also indicative of structural changes.     

Mictomagnetic State in an EuBaCo<Subscript>2–<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>5.5–δ</Subscript> Single Crystal

The magnetic properties of an EuBaCo_1.9O_5.36 single crystal are studied in the temperature range T = 2–300 K and the magnetic field range H ≤ 90 kOe. This binary layered cobaltite single crystal has vacancies in the cobalt and oxygen sublattices, in contrast to the stoichiometric EuBaCo₂O_5.5 composition. All cobalt ions in EuBaCo1.9O5.36 are in a trivalent state. The single crystal has an orthorhombic structure with space group Pmmm, and its unit cell parameters are a = 3.883 Å, b = 7.833 Å, and c = 7.551 Å. The field and temperature dependences of the magnetization of the single crystal demonstrate that it is ferrimagnet below T_C = 242 K. At T < 300 K, all three spin states of the Co³⁺ ions are present. The nearest-neighbor interactions give antiferromagnetic (AFM) and ferromagnetic (FM) contributions to the exchange energy. The ratio of the AFM to the FM contributions changes when temperature decreases because of a change in the spin state of the Co³⁺ ions. The single crystal exhibits signs of mictomagnetism at low temperatures in high magnetic fields. At T = 2 K and H = 90 kOe, the zero-field and nonzero-field magnetizations are strongly different because of a uniaxial magnetic anisotropy, which tends to set magnetization along the magnetic field applied in cooling throughout the crystal volume. As a result, a complex ferrimagnetic structure with a noncollinear direction of Co³⁺ spins appears. The following phenomena characteristic of mictomagnets are also observed in the EuBaCo_1.9O_5.36 single crystal: a shift in a magnetization hysteresis loop when temperature decreases, retained hysteretic phenomena and no magnetization saturation in high magnetic fields, and an orientation transition. The mictomagnetic state in EuBaCo_1.9O_5.36 is shown to be caused by the structural distortions induced by vacancies in the cobalt and oxygen sublattices and by the frustration of AFM and FM exchange interactions.     

Enhancement of pseudogap anomalies induced by nanoscale structural inhomogeneity in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.93</Subscript> high-<Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> superconductor

The magnetization M(H) in the superconducting state, dc magnetic susceptibility χ(T) in the normal state, and specific heat C(T) near the superconducting transition temperature T _c have been measured for a series of fine-crystalline YBa₂Cu₃O _y samples having nearly optimum values of y = 6.93 ± 0.3 and T _c = (91.5 ± 0.5) K. The samples differ only in the degree of nanoscale structural inhomogeneity. The characteristic parameters of superconductors (the London penetration depth and the Ginzburg–Landau parameter) and the thermodynamic critical field H _c are determined by the analysis of the magnetization curves M(H). It is found that the increase in the degree of nanoscale structural inhomogeneity leads to an increase in the characteristic parameters of superconductors and a decrease in H _c(T) and the jump of the specific heat ΔC/T _c. It is shown that the changes in the physical characteristics are caused by the suppression of the density of states near the Fermi level. The pseudogap is estimated by analyzing χ(T). It is found that the nanoscale structural inhomogeneity significantly enhances and probably even creates the pseudogap regime in the optimally doped high-T _c superconductors.     

Elastic properties of La<Subscript>0.82</Subscript>Ca<Subscript>0.18</Subscript>MnO<Subscript>3</Subscript> single crystal

The temperature dependences of the velocity of longitudinal sound waves and the internal friction in a La_0.82Ca_0.18MnO₃ single crystal with the Curie temperature T _C = 181 K have been studied. As temperature decreases, the single crystal is shown to undergo the transition from the pseudocubic O* to the Jahn–Teller O’ phase at T ~ 254 K and the reverse transition from O’ to O* phase at T ~ 84 K. The velocity of sound and the internal friction in the O’ phase are found to be significantly smaller than those in the O* phase.     

Superconductor-insulator transition tuned by annealing in Bi-film on top of Co-clusters

We deposited amorphous Bi films with a thickness between 3 and 6.5 nm at 4.2 K on top of previously deposited Co clusters having a mean size of ～4.5 nm. The Co cluster layers thickness was between 2.3 and 5 nm. In-situ electrical transport measurements were performed between 2 and 100 K. Measurements on as-prepared samples having a Bi layer thickness of 3.0 nm show hopping (tunneling) conductivity as σ(T) = σ ₀ exp[?(T ₀/T)^1/2] above the superconducting transition temperature T _C and re-entrance behavior again with hopping (tunneling) conductivity below T _C . Annealing of films having a Bi layer thickness of 5.5 nm results in a decrease of resistivity, with variable-range hopping conduction behavior as σ(T) = σ ₀ exp[?(T ₀/T)^1/3 ]. Quite different are the findings for films having a Bi layer thickness of 6.5 nm: annealing of these films results in a power-law behavior as σ(T) = σ ₀ T ^α with α = 2/3, indicating that these films are close to a quantum critical point separating superconducting and insulating phases. A phase diagram including all experimental observations is proposed.     

The influence of magnetic and structural heterogeneity on thermopower,magnetothermopower, electrical resistivity,and magnetoresistivity of Nd<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> manganite

The thermopower, S, magnetothermopower, ΔS/S, resistivity, ρ, and magnetoresistivity, Δρ/ρ, depending on the temperature T and magnetic field H, have been studied in an Nd_0.5Sr_0.5MnO₃ single crystal consisting of three types of clusters: an antiferromagnetic CE-type with charge-orbital ordering (below the Neel temperature T_NCE ~ 145 K) and an A-type with T_NA ~ 220 K; a ferromagnetic at 234 ≤ T ≤ 252 K, and a ferromagnetic metal phase below the Curie temperature T_C = 248 K. The thermopower was found to be negative, indicating the dominance of the electronic type of conductivity. In the S(T) curves, a sharp minimum is observed in the temperature range of 100 K ≤ T ≤ 133 K, close to T_NCE, where the absolute S value attains 53 μV/K. With a further increase in temperature, the absolute S value decreases rapidly; at 200 K it is equal to 7 μV/K. It then slightly increases, reaching its maximum value of 15 μV/K at a temperature of 254 K, which is close to T_C. The absolute thermopower decreased under the influence of the magnetic field; i.e., a negative magnetothermopower occurs. In {ΔS/S}(T) curves, a sharp minimum is observed at T = 130 K close to T_NCE, where the magnetothermopower reaches a huge value of ~45% at H = 13.23 kOe. A broad minimum in the {ΔS/S}(T) curves is observed near the Curie temperature and its value is also high, viz., ~15% in the maximum measuring magnetic field of 13.23 kOe. The extremely high magnetothermopower values mean that the charge-orbital ordered nanoclusters or ferron type make the main contribution to the thermopower of the entire sample. The behavior of the ρ(T) and {Δρ/ρ}(T) curves is similar to that of the S(T) and {ΔS/S}(T) dependencies, which is in agreement with this conclusion.     

Spatial spin-modulated structure and hyperfine interactions of 57Fe nuclei in multiferroics BiFe1–x T x O3 (T = Sc,Mn; x = 0, 0.05)

The results of the Mössbauer studies on ⁵⁷Fe nuclei in multiferroics BiFe_1–x T _x O₃ (T = Sc, Mn; x = 0, 0.05) in the temperature range of 5.2–300 K have been presented. The Mössbauer spectra have been analyzed in terms of the model of an incommensurate spatial spin-modulated structure of cycloid type. Information has been obtained about the effect of the substitution of Sc and Mn atoms for Fe atoms on the hyperfine parameters of the spectrum: the shift and the quadrupole shift of the Mössbauer line, the isotropic and anisotropic contributions to the hyperfine magnetic field, and also the parameter of anharmonicity of the spatial spin-modulated structure.     

Role of structure imperfection in the formation of the magnetotransport properties of rare-earth manganites with a perovskite structure

The structure, the structure imperfection, and the magnetoresistance, magnetotransport, and microstructure properties of rare-earth perovskite La_0.3Ln_0.3Sr_0.3Mn_1.1O_3–δ manganites are studied by X-ray diffraction, thermogravimetry, electrical resistivity measurement, magnetic, ⁵⁵Mn NMR, magnetoresistance measurement, and scanning electron microscopy. It is found that the structure imperfection increases, and the symmetry of a rhombohedrally distorted R3?c perovskite structure changes into its pseudocubic type during isovalent substitution for Ln = La³⁺, Pr³⁺, Nd³⁺, Sm³⁺, or Eu³⁺ when the ionic radius of an A cation decreases. Defect molar formulas are determined for a real perovskite structure, which contains anion and cation vacancies. The decrease in the temperatures of the metal–semiconductor (T _ms) and ferromagnet–paramagnet (T _C) phase transitions and the increase in electrical resistivity ρ and activation energy E _a with increasing serial number of Ln are caused by an increase in the concentration of vacancy point defects, which weaken the double exchange 3d ⁴(Mn³⁺)–2p ⁶(O^2–)–3d ³(Mn⁴⁺)–V ^(a)–3d ⁴(Mn³⁺). The crystal structure of the compositions with Ln = La contains nanostructured planar clusters, which induce an anomalous magnetic hysteresis at T = 77 K. Broad and asymmetric ⁵⁵Mn NMR spectra support the high-frequency electronic double exchange Mn³⁺(3d ⁴) ? O^2–(2p ⁶) ? Mn⁴⁺(3d ³) and indicate a heterogeneous surrounding of manganese by other ions and vacancies. A correlation is revealed between the tunneling magnetoresistance effect and the crystallite size. A composition–structure imperfection–property experimental phase diagram is plotted. This diagram supports the conclusion about a strong influence of structure imperfection on the formation of the magnetic, magnetotransport, and magnetoresistance properties of rare-earth perovskite manganites.