Anomalies of the specific heat near the quantum critical point in Tm<Subscript>0.74</Subscript>Yb<Subscript>0.26</Subscript>B<Subscript>12</Subscript>

The behavior of the specific heat near the quantum critical point x ∼ 0.3 in the Tm_{1 − x} Yb _x B₁₂ system has been studied. Detailed measurements have been performed on high-quality single-crystalline Tm_0.74Yb_0.26B₁₂ samples within a wide temperature range of 1.9–300 K in a magnetic field up to 9 T. The temperature dependence of the magnetic contribution to the specific heat has a logarithmic divergence of the form C/T ∼ lnT at T < 4 K, which can be attributed to the quantum critical behavior regime suppressed by the external magnetic field. The Schottky anomaly of the magnetic contribution to the specific heat in Tm_0.74Yb_0.26B₁₂ has been analyzed.     

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.     

Modulated magnetic structure in quasi-one-dimensional clinopyroxene NaFeGe<Subscript>2</Subscript>O<Subscript>6</Subscript>

The magnetic structure of the NaFeGe₂O₆ monoclinic compound has been experimentally investigated using the elastic scattering of neutrons. At a temperature of 1.6 K, an incommensurate magnetic structure has been observed in the form of an antiferromagnetic helix formed by a pairs of the spins of the Fe³⁺ ions with helical modulation in the ac plane of the crystal lattice. The wave vector of the magnetic structure has been determined and its temperature behavior has been studied. The analysis of the temperature dependences of the specific heat and susceptibility, as well as the isotherms of the field dependence of the magnetization, has revealed the existence of not only the order-disorder magnetic phase transition at the point T _N = 13 K, but also an additional magnetic phase transition at the point T _c = 11.5 K, which is assumingly an orientation phase transition.     

Magnetic properties of the Nd<Subscript>0.5</Subscript>Gd<Subscript>0.5</Subscript>Fe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystal

The magnetic properties of the Nd_0.5Gd_0.5Fe₃(BO₃)₄ single crystal have been studied in principal crystallographic directions in magnetic fields to 90 kG in the temperature range 2–300 K; in addition, the heat capacity has been measured in the range 2–300 K. It has been found that, below the Néel temperature T _N = 32 K down to 2 K, the single crystal exhibits an easy-plane antiferromagnetic structure. A hysteresis has been detected during magnetization of the crystal in the easy plane in fields of 1.0–3.5 kG, and a singularity has been found in the temperature dependence of the magnetic susceptibility in the easy plane at a temperature of 11 K in fields B < 1 kG. It has been shown that the singularity is due to appearance of the hysteresis. The origin of the magnetic properties of the crystal near the hysteresis has been discussed.     

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.     

Magnetocaloric effect in the Sm<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript> manganite

The magnetocaloric effect ΔT has been studied by a direct method in two samples of the manganite Sm_0.55Sr_0.45MnO₃, namely, a single crystal (sample A) and a ceramic sample (sample C). The temperature dependences of the ΔT effect of both samples exhibit a maximum at T _max = 143.3 K for the sample A and T _max = 143 K for the sample C. In these maxima, the values of the ΔT effect are 0.8 and 0.4 K in the magnetic field H = 14.2 kOe for the samples A and C, respectively. In addition, the ΔT(T) curve of the sample A has a minimum at T _min = 120 K, in which ΔT = −0.1 K. The maximum value of the ΔT effect increases with an increase in the magnetic field H in the range of magnetic fields up to 14.2 kOe, and the rate of this increase at H > 8 kOe is higher than that at H < 8 kOe. These features of the ΔT effect are explained by the presence of ferromagnetic and antiferromagnetic A- and CE-type clusters in the samples.     

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.     

Structural,magnetic, and magnetothermal properties of the Tb<Subscript>0.3</Subscript>Dy<Subscript>0.7</Subscript>Co<Subscript>2</Subscript> compound

A complex investigation of the structural, magnetic, and magnetothermal properties of the Tb_0.3Dy_0.7Co₂ compound synthesized with the use of high-purity rare-earth metals has been performed. The phase composition has been controlled using the X-ray structural analysis, and the topology of the alloy surface has been investigated using atomic-force microscopy. It has been established that the Tb_0.3Dy_0.7Co₂ compound is single-phase, while the samples selected for measurements possess a clearly pronounced texture. The magnetization has been measured using a vibrating-sample magnetometer in the fields up to 100 kOe in a temperature range from 4.2 to 200 K. The Curie temperature of the compound is 170 K. The data on the temperature dependence of heat capacity of Tb_0.3Dy_0.7Co₂ have been obtained. The magnetocaloric effect ΔT has been measured by a direct method in the fields up to 18 kOe applied both along and perpendicularly to the texture axis. The anisotropic behavior of the magnitude ΔT for this compound, which possesses the cubic structure, has been found. The maximum value of the magnetocaloric effect ΔT = 2.3 K (ΔH = 18 kOe) has been observed upon applying the magnetic field along the texture axis.     

Colossal magnetodielectric effect in SmFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> multiferroic

The colossal (more than threefold) decrease in the dielectric constant ɛ in the easy-plane SmFe₃(BO₃)₄ ferroborate in a magnetic field of ∼5 kOe applied in the basal ab plane of the crystal has been found. A close relation of this effect to anomalies in the field dependence of the electric polarization has been established. It has been shown that this magnetodielectric effect is due to the contribution to ɛ from the electric susceptibility, which is related to the rotation of spins in the ab plane, arises in the region of the antiferromagnetic ordering T < T _N = 33 K, and is suppressed by the magnetic field. A theoretical model describing the main features of the behavior of ɛ and electric polarization in the magnetic field has been proposed, taking into account the additional anisotropy in the basal plane induced by the magnetoelastic stresses.     

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.     

Normal-state electrical resistivity and superconducting magnetic penetration depth in Eu<Subscript>0.5</Subscript>K<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> polycrystals

We report measurements of the temperature dependence of the electrical resistivity, ρ(T), and magnetic pen-etration depth, λ(T), for polycrystalline samples of Eu_0.5K_0.5Fe₂As₂ with T _c = 31 K. ρ(T) follows a linear temperature dependence above T _c and bends over to a weaker temperature dependence around 150 K. The magnetic penetration depth, determined by radio frequency technique displays an unusual minimum around 4 K which is associated with short-range ordering of localized Eu³⁺ moments. The article is published in the original.     

Low-temperature specific heat of GdCu,GdCu<Subscript>2</Subscript>, and GdCu<Subscript>5</Subscript> antiferromagnets

It is found that the expansion of the low-temperature specific heat C _p of the antiferromagnetic metallic compounds GdCu, GdCu₂, and GdCu₅ contains a large term that is proportional to the square of temperature (δT ²). The value of δ is inversely proportional to the Néel temperature T _N. The GdCu₂ compound exhibits a strong dependence of the specific heat anomaly at T _N on an external magnetic field. The results obtained are compared with the data for other metal antiferromagnets, for example, the CuMnSb Heusler alloy.     

Magnetic phases in UNi<Subscript>2</Subscript>Si<Subscript>2</Subscript>

The tetragonal compound UNi₂Si₂ exhibits in zero magnetic field three different antiferromagnetic phases belowT _N =124 K. They are formed by ferromagnetic basal planes, which are antiferromagnetically coupled along thec-axis with the propagation vectorq=(0, 0, q _z ). Two additional order-order magnetic phase transitions are observed below T _N , namely atT ₁=108 K and T ₂=40 K in zero magnetic field. All three phases exhibit strong uniaxial anisotropy confining the U moments to a direction parallel to the c-axis. UNi₂Si₂ single crystals were studied in detail by measuring bulk thermodynamic properties, such as thermal expansion, resistivity, susceptibility, and specific heat. A microscopic study using neutron diffraction was performed in magnetic fields up to 14.5 T parallel to the c-axis, and a complex magnetic phase diagram has been determined. Here, we present the analysis of specific-heat data measured in magnetic fields up to 14 T compared with the results of the neutron-diffraction study and with other thermodynamic properties of UNi₂Si₂.     

Elastic,magnetic, and magnetoelectric properties of the CaBaCo<Subscript>4</Subscript>O<Subscript>7</Subscript> multiferroic

The structural, elastic, magnetic, and magnetoelectric properties of the CaBaCo₄O₇ multiferroic are experimentally studied and compared with the properties of the related YBaCo₄O₇ cobaltite, where Y³⁺ ions substitute for Ca²⁺ ions. Unlike the frustrated YBaCo₄O₇ magnet, the softening of Young’s modulus and the hysteresis in the ΔE(T)/E ₀ curve of ferrimagnetic CaBaCo₄O₇ in the paramagnetic region are weak, and the anomaly during the magnetic transition increases by almost an order of magnitude. This difference can point to different characters of the development of a long-range magnetic order in these two cobaltites. The distortion of the crystal structure that removes the frustrations of exchange interactions is found to correlate with the magnetic behavior of the cobaltites under study. The magnetization curves of the Ca cobaltite have two steps below 15 K, which can point to the presence of a metastable state in a high magnetic field. The study of the longitudinal and transverse magnetoelectric effects in a pulsed magnetic field demonstrates that their magnitudes are maximal near T _C and change their character from linear to quadratic during passage through this temperature.     

Anomalous low-temperature behavior of the thermal characteristics of MgB<Subscript>2</Subscript>

We have studied the behavior of the thermal expansion coefficient α(T) (in a zero magnetic field and at H≈4 T), the heat capacity C(T), and the thermal conductivity κ(T) of magnesium boride (MgB₂) in the vicinity of T_c and at lower temperatures. It was established that MgB₂, like oxide-based high-temperature superconductors, exhibits a negative thermal expansion coefficient at low temperatures. The anomaly of α(T) in MgB₂ is significantly affected by the magnetic field. It was established that, in addition to the well-known superconducting transition at T_c≈40 K, MgB₂ exhibits an anomalous behavior of both heat capacity and thermal conductivity in the region of T≈10–12 K. The anomalies of C(T) and κ(T) take place in the same temperature interval where the thermal expansion coefficient of MgB₂ becomes negative. The low-temperature anomalies are related to the presence of a second group of charge carriers in MgB₂ and to an increase in the density of the Bose condensate corresponding to these carriers at T_c2≈10–12 K.     

Magnetic and EPR studies of the EuFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystal

Magnetic and electron paramagnetic resonance (EPR) properties of EuFe₃(BO₃)₄ single crystals have been studied over the temperature range of 300–4.2 K and in a magnetic field up to 5 T. The temperature, field and orientation dependences of susceptibility, magnetization and EPR spectra are presented. An antiferromagnetic ordering of the Fe subsystem occurs at about 37 K. The easy direction of magnetization perpendicular to the c axis is determined by magnetic measurements. Below 10 K, we observe an increase of susceptibility connected with the polarization of the Eu sublattice by an effective exchange field of the ordered Fe magnetic subsystem. In a magnetic field perpendicular to the c axis, we have observed an increase of magnetization at T < 10 K in the applied magnetic field, which can be attributed to the appearance of the magnetic moment induced by the magnetic field applied in the basal plane. According to EPR measurements, the distance between the maximum and minimum of derivative of absorption line of the Lorentz type is equal to 319 Gs. The anisotropy of g-factor and linewidth is due to the influence of crystalline field of trigonal symmetry. The peculiarities of temperature dependence of both intensity and linewidth are caused by the influence of excited states of europium ion (Eu³⁺). It is supposed that the difference between the g-factors from EPR and the magnetic measurements is caused by exchange interaction between rare earth and Fe subsystems via anomalous Zeeman effect.     

Synthesis and structural,magnetic, and resonance properties of the LiCuFe<Subscript>2</Subscript>(VO<Subscript>4</Subscript>)<Subscript>3</Subscript> compound

Complex studies have been performed for the structural, static magnetic, and resonance properties of a new magnet LiCuFe₂(VO₄)₃ prepared by solid-phase synthesis. The temperature dependence of the susceptibility has an anomaly at temperature T_max = 9.6 K. At high temperatures, the LiCuFe₂(VO₄)₃ sample is in the paramagnetic state described by the Curie–Weiss law at T > 50 K and mainly determined by iron ions with effective magnetic moment μ_eff(exp) = 8.6μ_B per formula unit. At low temperatures, a long-range magnetic order is observed in the magnetic subsystem of the sample; the order is predominantly characterized by the antiferromagnetic exchange interaction and high frustration level. The exchange interaction parameters are estimated in a six-sublattice representation of the LiCuFe₂(VO₄)₃ magnet. It is shown that the LiCuFe₂(VO₄)₃ compound is an antiferromagnet with strong intrachain and frustrating interchain exchange interactions.     

Magnetic and dielectric response of cobalt-chromium spinel CoCr<Subscript>2</Subscript>O<Subscript>4</Subscript> in the terahertz frequency range

The nature of the phonon and magnon modes in the CoCr₂O₄ multiferroic with a cubic spinel structure has been studied using submillimeter spectroscopy and infrared Fourier spectroscopy. This paper reports on the first measurement of the evolution with temperature of the exchange optical magnon in the ferrimagnetic (T _C = 94 K) and two low-symmetry (T _S ≈ 26 K, T _lock-in = 14.5 K) phases of CoCr₂O₄ down to T = 5 K in zero magnetic field. It has been shown that the detected magnon is not a ferrimagnetic order parameter and originates, most probably, from spin precession in the cobalt sublattices. At the points of the magnetic phase transitions, the oscillator parameters of the two lowest-frequency phonon modes reveal an anomalous temperature behavior, thus evidencing the presence of significant interaction between the magnetic and phonon subsystems. The increase by 25% of the damping parameter of the phonon mode originating from vibrations of the CoO₄ tetrahedra during the transition of CoCr₂O₄ to the multiferroic state (T < T _S ) suggests structural changes in the lattice involving loss of spatial central symmetry of the medium.     

Magnetic and magnetoelectric properties of the Tb<Subscript>0.75</Subscript>Ho<Subscript>0.25</Subscript>Fe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> ferroborate

Single crystals of the Tb_0.75Ho_0.25Fe₃(BO₃)₄ ferroborate have been grown by the group method from a solution–melt based on bismuth trimolybdate. The magnetic and magnetoelectric properties of the ferroborate single crystals have been investigated in the temperature range from 4.2 to 300 K and in magnetic fields up to 9 T. Magnetically, this material is an antiferromagnet with the Néel temperature T _N = 38.8 K and easy-axis anisotropy. The magnitude of the magnetoelectric polarization has been found to be more than 1.5–2.0 times greater than the sum of the polarizations induced by the magnetic field for the ferroborates TbFe₃(BO₃)₄ and HoFe₃(BO₃)₄ taken in the corresponding shares.     

Effect of a magnetic field on the thermal and kinetic properties of the Sm<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3.02</Subscript> manganite

The temperature and magnetic-field dependences of the heat capacity, thermal conductivity, thermopower, and electrical resistivity of the Sm_0.55Sr_0.45MnO_3.02 ceramic material are studied in the temperature range 77–300 K and in magnetic fields up to 26 kOe. It is revealed that the quantities under investigation exhibit anomalous behavior due to a magnetic phase transition at the Curie temperature T_C. An increase in the magnetic field strength H leads to an increase in the Curie temperature T_C and a jump in the heat capacity ΔC_p at T_C. The temperature dependences of the measured quantities are characterized by hystereses that are considerably suppressed in a magnetic field of 26 kOe and depend neither on the thermocycling range nor on the rate of change in the temperature. The thermal conductivity K at temperatures above T_C shows unusual behavior for crystalline solids (dK/dT>0) and, upon the transition to a ferromagnetic state, drastically increases as a result of a decrease in the phonon scattering by Jahn-Teller distortions. It is demonstrated that the hystereses of the studied properties of the Sm_0.55Sr_0.45MnO_3.02 manganite are caused by a jumpwise change in the critical temperature due to variations in the lattice parameters upon the magnetic phase transition.