Magnetic properties of an easy-plane trigonal NdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> antiferromagnet

The field and temperature dependences of magnetization and the temperature dependences of the initial magnetic susceptibility have been theoretically studied for three crystallographic directions in a trigonal NdFe₃(BO₃)₄ antiferromagnetic crystal. The calculations were performed using a molecular field approximation and a crystal field model for the rare-earth subsystem. The obtained theoretical expressions are applied to the interpretation of recent experimental data [1–4] on the magnetic properties of NdFe₃(BO₃)₄. The results of calculations show a good agreement with experiment. The proposed theory adequately describes (i) anomalies of the Schottky type in the temperature dependence of the magnetic susceptibility, (ii) nonlinear curves of magnetization in the basal plane in a magnetic field up to 1 T (showing evidence of the first-order phase transitions) and their evolution with the temperature, and (iii) the field and temperature dependences of magnetization in a magnetic field up to 9 T.     

Magnetic properties of DyFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The magnetic properties of an easy-axis trigonal DyFe₃(BO₃)₄ antiferromagnetic crystal have been theoretically studied. On this basis, recent experimental data [1] on the field and temperature dependences of magnetization and the temperature dependence of the initial magnetic susceptibility for three crystallographic directions in this antiferromagnet have been interpreted. The characteristics of the trigonal crystal field for the rare earth ion and the parameters of the Fe-Fe and Fe-Dy exchange interactions are determined. Limitations imposed by features of the magnetic characteristics (anisotropic magnetization in the three crystallographic directions, Schottky-type anomalies in the magnetic susceptibility, etc.) on the possible splitting of the ground-state multiplet in the crystal field and the splitting of the lowest doublet due to the f-d interaction for Dy³⁺ ions are established.     

Magnetic properties of the quasi-two-dimensional crystal (CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>)<Subscript>2</Subscript>CuBr<Subscript>4</Subscript>

The magnetic properties of (CH₃NH₃)₂CuBr₄ quasi-two-dimensional crystals were studied experimentally. The magnetic-field and temperature dependences of magnetization were measured for various magnetic field orientations relative to the crystallographic axes. Possible reasons for features in the behavior of the magnetization are discussed.     

Magnetic anisotropy of the VBO<Subscript>3</Subscript> and CrBO<Subscript>3</Subscript> transition-metal borates

Temperature and field dependences of the magnetization of VBO₃ and CrBO₃ single crystals with the magnetic field applied parallel and perpendicular to the (111) basal plane were measured. VBO₃ was found to have a considerable uniaxial anisotropy with a field H_a≈6.25 T. CrBO₃ was shown to exhibit not only uniaxial but also hexagonal anisotropy. The experimental anisotropy constants were estimated, and their temperature dependences are presented.     

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.     

Thermal stability of EuMnO<Subscript>3</Subscript> compound

We determine the low-oxygen border of the region of homogeneity for the compound showing the pressure of oxygen and temperrature at which it is possible to obtain materials based on EuMnO₃ with various useful properties. It is established that under conditions of low oxygen pressures, EuMnO₃ dissociates according to the reaction EuMnO₃ = 1/2Eu₂O₃ + MnO + 1/4O₂ in the temperature range of 973–1190 K. We determine temperature dependences of the equilibrium pressure of oxygen and the changes in free Gibbs energy for this reaction. The thermodynamic parameters for the formation of EuMnO₃ from its elements are calculated on the basis of our experimental data.     

Antiferromagnetic resonance and phase diagrams of gadolinium ferroborate GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Antiferromagnetic resonance in single crystals of rhombohedral gadolinium ferroborate GdFe₃(BO₃)₄ was studied. The frequency-field dependences of antiferromagnetic resonance over the frequency range 26–70 GHz and the temperature dependences of resonance parameters for magnetic fields oriented along the crystal axis and in the basal plane were determined. It was found that the iron subsystem, which can be treated as a two-sublattice antiferromagnet with anisotropy of the easy-plane type, experienced ordering at T=38 K. At temperatures below 20 K, the gadolinium subsystem with the opposite anisotropy sign strongly influenced the anisotropic properties of the crystal. This resulted in a spontaneous spin-reorientation transition from the easy-plane to the easy-axis state at 10 K. Below 10 K, magnetic field-induced transitions between the states were observed. Experimental phase diagrams on the temperature-magnetic field plane were constructed for fields oriented along the crystal axis and in the basal plane. A simple model was used to calculate the critical transition fields. The results were in close agreement with the experimental values measured at T=4.2 K for both field orientations.     

Transport characteristics of phonons and the specific heat of Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:ZrO<Subscript>2</Subscript> solid solution single crystals

The temperature dependences of the specific heat and transport characteristics of phonons in single crystals of yttrium-stabilized zirconium dioxide Y₂O₃:ZrO₂ solid solutions have been studied. It has been shown that the temperature dependences of the specific heat at T > 5 K are almost identical at the degree of stabilization of a solid solution with an Y₂O₃ content of 5–20 mol %. Differences in the temperature dependences of the specific heat of samples from different sources at T < 5 K are due to the presence of low-energy two-level systems. The features of the transport characteristics of thermal phonons at liquid helium temperatures reflect not only the presence of two-level systems but also the scattering of phonons on low-dimensional domains of another phase coherently conjugate to the main phase of the Y₂O₃:ZrO₂ solid solution.     

A study of the structure and properties of nanostructured ZrO<Subscript>2</Subscript>-Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite powders

The correlation between temperature treatment conditions and the ratio of components in nanostructured fibrous powders with a composition of ZrO₂-Y₂O₃-Al₂O₃ and their porous crystal structure and physicochemical properties is studied. The dependences of the ratio between zirconia tetragonal and monoclynic phases on the treatment temperature and the alumina content are found to have a nonmonotonic character. The growth of zirconia crystallite size is suppressed by introduced nanocrystalline alumina in a temperature range of 600–1200°C, which is caused by the processes of ternary solid solution formation. The bulk and picnometric density values of materials are proportional to the temperature of heat treatment. The temperature dependence of the specific surface and the size of oxide grain particles has an inversely proportional character. With increasing alumina content in the powders, the specific surface increases, while the picnometric and bulk densities decrease.     

Electromechanical properties of Pb<Subscript>3</Subscript>Ga<Subscript>2</Subscript>Ge<Subscript>4</Subscript>O<Subscript>14</Subscript> piezoelectric crystals grown from solution in a melt

Single crystals of lead gallium germanate Pb₃Ga₂Ge₄O₁₄ are grown from their own solution melts. The propagation of bulk acoustic waves is investigated, and the elastic, piezoelectric, and dielectric constants are calculated. The temperature dependences of the dielectric constants of this compound are analyzed.     

Optical studies of delayed relaxation in Pb<Subscript>0.94</Subscript>Ba<Subscript>0.06</Subscript>Sc<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> (PBSN-6) solid solutions

The temperature dependences of the permittivity, birefringence, optical transmittance, and small-angle light scattering and their variations with time are studied for single crystals of the Pb_0.94Ba_0.06Sc_0.5Nb_0.5O₃ relaxor (PBSN-6) in the heterophase region of coexistence of different phases. It is shown that an electric field induces a phase transition to the ferroelectric state, which manifests itself within some time (delay time τ) after application of the electric field to the crystal. The observed dependence of the temperature of this transition on the heating rate of the sample and the changes in the birefringence and small-angle light scattering intensity with time confirm the kinetic character of the induced transition. Temperature dependences of the delay time τ for different electric fields are constructed. It is revealed that, at low temperatures, the delay time τ decreases with increasing temperature. This agrees with the behavior of τ in classical relaxors. At the Vogel-Fulcher temperature, however, one observes that dτ/dT reverses sign and τ increasing as the temperature continues to increase. This anomalous behavior of τ in the heterophase region is accounted for by the coexistence of the cubic relaxor and rhombohedral macrodomain phases.     

Magnetoelectric effect in ytterbium aluminum borate YbAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The anisotropic magnetoelectric properties of an ytterbium aluminum borate YbAl (BO single crystal having noncentrosymmetric crystal structure (space group R32) are studied, including the orientational, field, and temperature dependences of the polarization in magnetic fields up to 5 T in the temperature range of 2–300 K. It has been shown experimentally for the first time that the symmetry of the observed magnetoelectric effects exactly corresponds to the trigonal structure of the crystal and is characterized by two quadratic magnetoelectric constants. The polarization in the basal plane P _{a, b} is a quadratic function of the field at low fields and reaches 250–300 μC/m² in a field of 5 T at a temperature of 2 K, almost an order of magnitude exceeding the previously reported values. A theoretical model based on the spin Hamiltonian of the ground Kramers doublet of Yb³⁺ ions in the crystal field is proposed including magnetoelectric interactions allowed by the symmetry. This model makes it possible to quantitatively describe all observed magnetic and magnetoelectric properties of YbAl₃(BO₃)₄.     

Optical and dielectric studies of phase transitions in single crystals of NaNbO<Subscript>3</Subscript>-Gd<Subscript>1/3</Subscript>NbO<Subscript>3</Subscript> solid solutions

The temperature dependences of the permittivity ? and the false-color image patterns obtained by the rotating polarizer method for single crystals of (1 ? x)NaNbO_3?x Gd_1/3NbO₃ (x = 0.003, 0.090) solid solutions with different degrees of diffuseness of the phase transition are investigated. A multifractal analysis of the false-color images has revealed anomalies in the temperature dependences of the parameter ?_∞ of the multifractal spectrum. For a sample with a sharp phase transition (x ≈ 0.003), the temperature of this anomaly is in good agreement with the temperature of the jumps in the permittivity ?(T) and birefringence. For an NNG crystal with x ≈ 0.09, which exhibits a diffuse maximum of ?(T), the temperatures of the anomalies of ?_∞(T) differ in the central and peripheral regions, which correlates with the distribution of Gd over the crystal.     

Influence of the special features of the effective magnetic anisotropy on the temperature dependences of the magnetoimpedance of nanocrystalline Fe<Subscript>73.5</Subscript>Si<Subscript>16.5</Subscript>B<Subscript>6</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript> strips

The influence of the thermal treatment type on the temperature dependences of the magnetoimpedance of nanocrystalline Fe_73.5Si_16.5B₆Nb₃Cu₁ alloy strips is investigated. The main mechanisms determining the temperature behavior of the magnetoimpedance of strips with induced magnetic anisotropy having various special features are established. The prospects for application of the alloy strips nanocrystallized in the presence of a magnetic field as sensitive elements of temperature sensors and special magnetic field detectors are demonstrated.     

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.     

Elastic properties of a La<Subscript>0.5</Subscript>Pr<Subscript>0.2</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> single crystal

The temperature dependences of the velocity of longitudinal sound and internal friction in the ferromagnetic La_0.5Pr_0.2Ca_0.3MnO₃ single crystal with magnetic first-order phase transition were studied. It was found that the sound velocity decreases by ≈20% in transition from the ferromagnetic to paramagnetic state. In the paramagnetic region, the extended temperature hysteresis of the sound velocity and the internal friction was observed. It was shown that La_0.5Pr_0.2Ca_0.3MnO₃ has two paramagnetic phases with different sound velocities.     

Temperature dependence of the critical current of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−δ</Subscript> films

The temperature dependence of the critical current of YBa₂Cu₃O_7?δ films is studied experimentally. The performed analysis allows separating two components of the critical current owing to pinning of vortices both on defects in the volume of the superconductor and on oxygen vacancies in the CuO₂ planes. The established temperature dependences of components make it possible to correctly describe the behavior of the total critical current in the studied temperature range from 4.2 K to the irreversibility temperature.     

Phase transitions of SC(NH<Subscript>2</Subscript>)<Subscript>2</Subscript> ferroelectrics in Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-based nanoporous matrices

Temperature dependences of the linear permittivity ε' and the third harmonic amplitude γ_3ω of composites prepared by introducing ferroelectrics SC(NH₂)₂ into matrices of porous aluminum oxide Al₂O₃ with pore sizes 60 and 100 nm are determined. It is found that temperature T _c of the ferroelectric phase transition and the temperature T _i of the phase transition from incommensurable phase to the paraphrase increase significantly. The transition shifts increase as pore diameters decrease.     

Magnetic properties of aerugite Co<Subscript>10</Subscript>Ge<Subscript>3</Subscript>O<Subscript>16</Subscript>

This paper reports on the first study of the temperature and field dependences of the magnetization, heat capacity, and electrical properties of synthesized polycrystalline samples of aerugite Co₁₀Ge₃O₁₆, as well as on x-ray diffraction analysis of this compound. It is shown that the cobalt ions in this compound occupy three nonequivalent positions. The results of the experimental and theoretical studies suggest that aerugite is a ferrimagnet with two uncompensated magnetic moments of the cobalt atom per formula unit.     

Elastic and magnetic properties of the La<Subscript>0.6</Subscript>Pr<Subscript>0.1</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> single crystal

The temperature dependences of the velocity of longitudinal sound, internal friction, and magnetization of the single crystal with the nominal composition La_0.6Pr_0.1Ca_0.3MnO₃ have been measured. It has been found that the substitution of praseodymium for lanthanum in La_0.7Ca_0.3MnO₃ leads to a decrease in the velocity of sound and to an increase in the spontaneous magnetization. The method of determining the Curie temperature distribution function during a first-order transition has been proposed. It has been shown that, in the crystal under study, this function is asymmetric.