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.     

Thermophysical study of structural phase transitions in Na<Subscript>0.95</Subscript>Li<Subscript>0.05</Subscript>NbO<Subscript>3</Subscript> solid solution

Temperature dependences of specific heat C_p(T) and coefficient of thermal expansion ;(T) for Na_0.95Li_0.05NbO₃ sodium-lithium niobate ceramic samples are investigated in the temperature range of 100–800 K. The C_p(T) and α(T) anomalies at T₃ = 310 ± 3 K, T₂ = 630 ± 8 K, and T₁ = 710 ± 10 K are observed, which correspond to the sequence of phase transitions N ? Q ? S(R) ? T2(S). The effect of heat treatment of the samples on the sequence of structural distortions was established. It is demonstrated that annealing of the samples at 603 K leads to splitting of the anomaly corresponding to the phase transition Q → R/S in two anomalies. After sample heating to 800 K, the only anomaly is observed in both the C_p(T) and ;(T) dependence. Possible mechanisms of the observed phenomena are discussed.     

On electron transport in ZrB<Subscript>12</Subscript>, ZrB<Subscript>2</Subscript>, and MgB<Subscript>2</Subscript> in normal state

We report on measurements of the temperature dependence of resistivity, ρ(T), for single-crystal samples of ZrB₁₂, ZrB₂, and polycrystalline samples of MgB₂. It is shown that the cluster compound ZrB₁₂ behaves as a simple metal in the normal state, with a typical Bloch-Grüneisen ρ(T) dependence. However, the resistive Debye temperature, T_R=300 K, is three times smaller than T_D obtained from specific heat data. We observe the T² term in ρ(T) of all these borides, which could be interpreted as an indication of strong electron-electron interaction.     

Frustrated antiferromagnetism in the Sr<Subscript>2</Subscript>YRuO<Subscript>6</Subscript> double perovskite

The spins of Ru⁵⁺ ions in Sr₂YRuO₆ form a face-centered cubic lattice with antiferromagnetic nearest neighbor interaction J≈25 meV. The antiferromagnetic structure of the first type experimentally observed below the Néel temperature T _N =26 K corresponds to four frustrated spins of 12 nearest neighbors. In the Heisenberg model in the spin-wave approximation, the frustrations already cause instability of the antiferromagnetic state at T=0 K. This state is stabilized by weak anisotropy D or exchange interaction I with the next-nearest neighbors. Low D/J～I/J～10^?3 values correspond to the experimental T _N and sublattice magnetic moment values.     

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.     

Magnetic and thermal properties of single-crystal NdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The neodymium ferroborate NdFe₃(BO₃)₄ undergoes an antiferromagnetic transition at T _N = 30 K, which manifests itself as a λ-type anomaly in the temperature dependence of the specific heat C and as inflection points in the temperature dependences of the magnetic susceptibility χ measured at various directions of an applied magnetic field with respect to the crystallographic axes of the sample. Magnetic ordering occurs only in the subsystem of Fe³⁺ ions, whereas the subsystem of Nd³⁺ ions remains polarized by the magnetic field of the iron subsystem. A change in the population of the levels of the ground Kramers doublet of neodymium ions manifests itself as Schottky-type anomalies in the C(T) and χ(T) dependences at low temperatures. At low temperatures, the magnetic properties of single-crystal NdFe₃(BO₃)₄ are substantially anisotropic, which is determined by the anisotropic contribution of the rare-earth subsystem to the magnetization. The experimental data obtained are used to propose a model for the magnetic structure of NdFe₃(BO₃)₄.     

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.     

Investigation of the field-tuned quantum critical point in CeCoIn<Subscript>5</Subscript>

The main properties and the type of the field-tuned quantum critical point in the heavy-fermion metal CeCoIn₅ that arise upon application of magnetic fields B are considered within a scenario based on fermion condensation quantum phase transition. We analyze the behavior of the effective mass, resistivity, specific heat, charge, and heat transport as functions of applied magnetic fields B and show that, in the Landau Fermi liquid regime, these quantities demonstrate critical behavior, which is scaled by the critical behavior of the effective mass. We show that, in the high-field non-Fermi liquid regime, the effective mass exhibits very specific behavior, M*～ T^{? 2/3}, and the resistivity demonstrates T^2/3 dependence. Finally, at elevated temperatures, it changes to M*～T^?1/2, while the resistivity becomes linear in T. In zero magnetic field, the effective mass is controlled by temperature T and the resistivity is also linear in T. The obtained results are in good agreement with recent experimental facts.     

Adiabatic calorimetric study of the intense magnetocaloric effect and the heat capacity of (La<Subscript>0.4</Subscript>Eu<Subscript>0.6</Subscript>)<Subscript>0.7</Subscript>Pb<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>

The temperature dependences of the intense magnetocaloric effect ΔT _AD(T, H) and the heat capacity C _p (T) of the (La_0.4Eu_0.6)_0.7Pb_0.3MnO₃ manganite are directly measured using adiabatic calorimetry. The experimental dependences ΔT _AD(T) are in satisfactory agreement with those calculated from the data on the behavior of the magnetization. The factors responsible for the absence of an anomaly in the experimental temperature dependence of the heat capacity C _p (T) in the range of the magnetic phase transition are discussed.     

Antiferromagnetic Resonance in GdB<Subscript>6</Subscript>

The electron spin resonance has been measured for the first time both in the paramagnetic phase of the metallic GdB₆ antiferromagnet (T_N = 15.5K) and in the antiferromagnetic state (T < T_N). In the paramagnetic phase below T* ~ 70 K, the material is found to exhibit a pronounced increase in the resonance linewidth and a shift in the g-factor, which is proportional to the linewidth Δg(T) ~ ΔH(T). Such behavior is not characteristic of antiferromagnetic metals and seems to be due to the effects related to displacements of Gd³⁺ ions from the centrosymmetric positions in the boron cage. The transition to the antiferromagnetic phase is accompanied by an abrupt change in the position of resonance (from μ₀H₀ ≈ 1.9 T to μ₀H₀ ≈ 3.9 T at ν = 60 GHz), after which a smooth evolution of the spectrum occurs, resulting eventually in the formation of the spectrum consisting of four resonance lines. The magnetic field dependence of the frequency of the resonant modes ω₀(H₀) obtained in the range of 28–69 GHz is well interpreted within the model of ESR in an antiferromagnet with the easy anisotropy axis ω/γ = (H ₀ ² +2H_AH_E)^1/2, where H_E is the exchange field and H_A is the anisotropy field. This provides an estimate for the anisotropy field, H_A ≈ 800 Oe. This value can result from the dipole?dipole interaction related to the mutual displacement of Gd³⁺ ions, which occurs at the antiferromagnetic transition.     

Nonergodic state of relaxation ferroelectric Cd<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>7</Subscript> in a constant electric field

The temperature dependence of the residual polarization of the nonergodic relaxation state (NERS) obtained from the measurements of pyroelectric current during zero-field heating (ZFH) in the temperature interval from 10 to 295 K is investigated for the Cd₂Nb₂O₇ relaxation system in two cases: (1) after sample cooling in a constant electric field E (FC) from T = 295 K to a preset temperature, which is much lower than the “freezing” temperature of the relaxation state (T _f ≈ 182 K), field removal, and subsequent cooling in zero field (ZFC) to T = 10 K and (2) after ZFC from T = 295 K to the same temperature below T _f , application of the same field, and FC to T = 10 K. The behavior of the P _r ^FC (T) and P _r ^ZFC (T) dependences is analyzed. In the field E < 2 kV/cm, the P _r ^ZFC curves as functions of 1/T have a broad low-intensity peak in the region T ≈ T _f , which vanishes in stronger fields, when the P _r ^FC (1/T) curves intersect at T ≈ T _f and have no anomalies. The difference in the behavior of P _r ^ZFC (T) and P _r ^FC (T) indicates the difference in the nature of NERS formed during ZFC and FC of the system upon a transition through T _f . In the ZFC mode, NERS exhibits glasslike behavior; in the FC regime, features of the ferroelectric behavior even in the weak field. Analogous variations of P _r ^ZFC (T) and P _r ^FC (T) in a classical ferroelectric KDP are also given for comparison.     

C<Subscript>60</Subscript> heat capacity anomaly at the orientational phase transition

An orientational phase transition in C60 crystals was studied by differential scanning calorimetry with the highest resolution provided by this method. The temperature dependence of the specific heat ΔC _p (T) was found to have a double peak in the range 250–270 K. An analysis of the temperature dependences of heat capacity in the region of the peaks revealed that the lower temperature peak follows a power law of the type ΔC _p = A/(T?T₀)^1/2 characteristic of order-disorder second-order phase transitions, while the high-temperature peak can be identified with a diffuse Λ-shaped first-order phase transition.     

Low-temperature anomalies in the specific heat and thermal conductivity of MgB<Subscript>2</Subscript>

The temperature dependences of the specific heat C(T) and thermal conductivity K(T) of MgB₂ were measured at low temperatures and in the neighborhood of T _c . In addition to the well-known superconducting transition at T _c ≈40 K, this compound was found to exhibit anomalous behavior of both the specific heat and thermal conductivity at lower temperatures, T≈10–12 K. Note that the anomalous behavior of C(T) and K(T) is observed in the same temperature region where MgB₂ was found to undergo negative thermal expansion. All the observed low-temperature anomalies are assigned to the existence in MgB₂ of a second group of carriers and its transition to the superconducting state at T_c2≈10?12 K.     

Magnetic susceptibility of fine crystalline high-temperature YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.93</Subscript> superconductors. The role of nanoscale structural disordering

Static magnetic susceptibility χ(T) in the normal state (T_c ≤ T ≤ 400 K) and specific heat C(T) near temperature T_c of the transition to the superconducting state are experimentally studied for a series of fine crystalline samples of high-temperature YBa₂Cu₃O_y superconductor, having y and T_c close to optimal but differing in the degree of nanoscale structural disordering. It is shown that under the influence of structural disordering, there is enhancement of anomalous pseudogap behavior of the studied characteristics and a significant increase in the width of the pseudogap.     

Dilute ferrimagnetism of ilmenites Mn<Subscript>3</Subscript>FeTiSbO<Subscript>9</Subscript> and Mn<Subscript>4</Subscript>FeTi<Subscript>2</Subscript>SbO<Subscript>12</Subscript>

Metastable solid solutions (SS) Mn₃FeTiSbO₉ and Mn₄FeTi₂SbO₁₂ with the ilmenite structure (space group R\(\bar 3\)) have been prepared by quenching at normal conditions. The compositions of the compounds have been justified using EDX spectroscopy and X-ray diffraction. The magnetic properties of SSs have been analyzed by comparison with ferrimagnetic ilmenite Mn₂FeSbO₆ (T_N = 269 K) as a natural mineral and ceramics obtained at high pressure and high temperature. The solid solutions have been characterized as dilute magnetic systems formed as a result of substitution of nonmagnetic cations Ti⁴⁺ for a part of Fe³⁺ and Sb⁵⁺ cations. Mn₃FeTiSbO₉ is considered as a ferromagnetic with T_N = 171 K and Mn₄FeTi₂SbO₁₂ as a magnetic with the concentration of magnetic clusters below the percolation threshold.     

X-ray study of [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> at low temperatures

The unit cell parameters a, b, and c of [N(CH₃)₄]₂ZnCl₄ have been measured by x-ray diffraction in the temperature range 80–293 K. Temperature dependences of the thermal expansion coefficients α_a, α_b, and α_c along the principal crystallographic axes and of the unit cell thermal expansion coefficient α_V were determined. It is shown that the a=f(T), b=f(T), and c=f(T) curves exhibit anomalies in the form of jumps at phase transition temperatures T₁=161 K and T₂=181 K and that the phase transition occurring at T₃=276 K manifests itself in the a=f(T) and b=f(T) curves as a break. A slight anisotropy in the coefficient of thermal expansion of the crystal was revealed. The phase transitions occurring at T₁=161 K and T₂=181 K in [N(CH₃)₄]₂ZnCl₄ were established to be first-order.     

High-resolution spectroscopy of HoFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> crystal: a study of phase transitions

The transmission spectra of HoFe₃(BO₃) multiferroic single crystals are studied by optical Fourier-transform spectroscopy at temperatures of 1.7–423 K in polarized light in the spectral range 500–10 000 cm^–1 with a resolution up to 0.1 cm^–1. A new first-order structural phase transition close to the second-order transition is recorded at T_c = 360 K by the appearance of a new phonon mode at 976 cm^–1. The reasons for considerable differences in T_c for different samples of holmium ferroborate are discussed. By temperature variations in the spectra of the f–f transitions in the Ho³⁺ ion, we studied two magnetic phase transitions, namely, magnetic ordering into an easy-plane structure as a second-order phase transition at T_N = 39 K and spin reorientation from the ab plane to the c axis as a first-order phase transition at T_SR = 4.7 ± 0.2 K. It is shown that erbium impurity in a concentration of 1 at % decreases the spin-reorientation transition temperature to T_SR = 4.0 K.     

Low-energy Ce spin excitations in CeFeAsO and CeFeAsO<Subscript>0.84</Subscript>F<Subscript>0.16</Subscript>

We use inelastic neutron scattering to study the low-energy spin excitations of polycrystalline samples of nonsuperconducting CeFeAsO and superconducting CeFeAsO_0.84F_0.16. Two sharp dispersionless modes are found at 0.85 and 1.16 meV in CeFeAsO below the Ce antiferromagnetic (AF) ordering temperature of T _N ^Ce ? 4 K. On warming to above T _N ^Ce ? 4 K, these two modes become one broad dispersionless mode that disappears just above the Fe ordering temperature T _N ^Fe ? 140 K. For superconducting CeFeAsO_0.84F_0.16, where Fe static AF order is suppressed, we find a weakly dispersive mode center at 0.4 meV that may arise from short-range Ce-Ce exchange interactions. Using a Heisenberg model, we simulate powder-averaged Ce spin wave excitations. Our results show that we need both Ce spin wave and crystal electric field excitations to account for the whole spectra of low-energy spin excitations.     

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).     

Long-range magnetic order in quasi-one-dimensional NaCrSi<Subscript>2</Subscript>O<Subscript>6</Subscript> and NaCrGe<Subscript>2</Subscript>O<Subscript>6</Subscript> metal oxides

Formation of a long-range magnetic order is observed at low temperatures in NaCrSi₂O₆ and NaCrGe₂O₆ quasi-one-dimensional metal oxide compounds with a pyroxene structure. The first of these compounds, NaCrSi₂O₆, is an antiferromagnet with the Néel temperature T _N =3 K, while the second, NaCrGe₂O₆, is a ferromagnet with the Curie temperature T _C =6 K. From the measurements of magnetization and specific heat of these compounds, the main parameters of their magnetic subsystems are determined. In NaCrSi₂O₆, a spin-flip transition is observed. A change in the type of magnetic order that accompanies the replacement of Si by Ge can be attributed to a change in the parameters of the competing direct antiferromagnetic Cr-Cr and indirect ferromagnetic Cr-O-Cr interactions in isolated chains of CrO₆ octahedra.