Phase transition in a vortice lattice in a Bi2212:Pb single crystal

The magnetooptical method was used to investigate the penetration of a magnetic flux into a single crystal of a high-temperature superconductor (Bi_0.84Pb_0.16)_2.2Sr₂CaCu₂O₈ in crossed magnetic fields. It is shown that at low temperatures the penetration of the magnetic flux is anisotropic: the flux moves preferentially along the magnetic field applied in the plane of the sample, and the anisotropy grows as the temperature increases. At a temperature T_m = 54±2 K, there occurs a sharp change in the character of penetration of the magnetic field into the superconductor; the direction of the flux ceases be dependent on the direction and magnitude of the magnetic field applied in the plane of the sample. In this case, the transition temperature T_m is independent of the applied magnetic field. The effect is interpreted in terms of the concepts of a phase transition in the system of vortices, which is related to a sharp decrease in the correlations in the position of vortices in various CuO planes, i.e., with the transition from three-dimensional to two-dimensional behavior of the vortex structure.     

Ground-state magnetism of chromium-substituted LiMn2O4 spinel

Comparative crystal structure and magnetic properties studies have been conducted on quaternary powder spinel samples LiMn_1.82Cr_0.18O₄ obtained by two different synthesis methods, glycine-nitrate (GN) and ultrasonic spray-pyrolysis (SP). Although both samples possess the same spinel structure of the cubic space group Fd3¯m, their low-temperature magnetic properties display significant differences. While the SP sample undergoes only spin-glass transition at the freezing temperature T_f=20 K, the GN sample possesses more complicated low-temperature magnetic behavior of the reentrant spin-glass type with the Néel temperature T_N=42 K and freezing temperature T_f=22 K. High-temperature magnetic susceptibility of both samples is of the Curie–Weiss type with the effective magnetic moments in agreement with the nominal compositions. This fact together with the results of the chemical analysis discards the existence of the diversity in chemical compositions as a possible cause for the observed differences in the low-temperature magnetism. On the other hand, the crystal structure analysis done by the Rietveld refinement of the X-ray powder diffraction data points to the strong influence of the cation distribution on the ground-state magnetism of these systems. An explanation of this influence is proposed within the framework of a collective Jahn–Teller effect.     

Anomalous magnetism and 209Bi nuclear spin relaxation in Bi4Ge3O12 crystals

The quadrupole ²⁰⁹Bi spin–spin and spin–lattice relaxation were studied within 4.2–300 K for pure and doped Bi₄Ge₃O₁₂ single crystals which exhibit, as was previously found, anomalous magnetic properties. The results revealed an unexpectedly strong influence of minor amounts of paramagnetic dopants (0.015–0.5 mol.%) on the relaxation processes. Various mechanisms (quadrupole, crystal electric field, electron spin fluctuations) govern the spin–lattice relaxation time T ₁ in pure and doped samples. Unlike T ₁, the spin–spin relaxation time T ₂ for pure and Nd-doped samples was weakly dependent on temperature within 4.2–300 K. Doping Bi₄Ge₃O₁₂ with paramagnetic atoms strongly elongated T ₂. The elongation, although not so strong, was also observed for pure and doped crystals under the influence of weak (～30 Oe) external magnetic fields. To confirm the conclusion about strong influence of crystal field effects on the temperature dependence of T ₁ in the temperature range 4.2–77 K, the magnetization vs. temperature and magnetic field was measured for Nd- and Gd-doped Bi₄Ge₃O₁₂ crystals using a SQUID magnetometer. The temperature behavior of magnetic susceptibility for the Nd-doped crystal was consistent with the presence of the crystal electric field effects. For the Gd-doped crystal, the Brillouin formula perfectly fitted the curve of magnetization vs. magnetic field, which pointed to the absence of the crystal electric field contribution into the spin–lattice relaxation process in this sample.     

Effect of magnetic field on the TC and magnetic properties for the aligned MnBi compound

In the compound MnBi, a first-order transition from the paramagnetic to the ferromagnetic state can be triggered by an applied magnetic field and the Curie temperature increases nearly linearly with an increase in magnetic field by ∼2 K/T. Under a field of 10 T, T_C increases by 20 and 22 K during heating and cooling, respectively. Under certain conditions a reversible magnetic field or temperature induced transition between the paramagnetic and ferromagnetic states can occur. A magnetic and crystallographic H-T phase diagram for MnBi is given. Magnetic properties of MnBi compound aligned in a Bi matrix have been investigated. In the low temperature phase MnBi, a spin-reorientation takes place during which the magnetic moments rotate from being parallel to the c-axis towards the basal plane at ∼90 K. A measuring Dc magnetic field applied parallel to the c-axis of MnBi suppresses partly the spin-reorientation transition. Interestingly, the fabricated magnetic field increases the temperature of spin-reorientation transition T_s and the change in magnetization for MnBi. For the sample solidified under 0.5 T, the change in magnetization is ∼70% and T_s is ∼91 K.     

Transition De Phase Metamagnetique Du Bromure Ferreux

In a magnetic field parallel to the magnetization axis of an antiferromagnetic Fe Br₂ single crystal, a caracteristic metamagnetic behaviour is observed. The transition from an antiferromagnetic phase to a paramagnetic phase is studied by help of magnetization measurements in a steady field (H < 60 kOe). The measurement precision has allowed a detailed study of the magnetization isotherms, caracteristic of a first order magnetization phase transition (T < T_c = 4, 7 K) and of a second order phase transition (T_c < T < T_N = 14, 2 K).We have observed an original phase diagram. In a certain temperature and field range, the ordered phase is stable on the high temperature side of the transition point. Some theoretical studies in an Ising model, or in the hypothesis of a strong magnetoelastic coupling forecast the existence of such a magnetic phase diagram.At present, we proceed to a theoretical study, in a molecular field approximation, of the magnetic phase diagram of compounds similar to Fe Br₂ where we take into account the relative values of parameters J₁, J₂ and D associated with ferromagnetic and antiferromagnetic interactions and crystalline anisotropy.     

Crystal structural, magnetic and electrical transport properties of CeKFeMoO6 double perovskite

The crystal structural, magnetic and electrical transport properties of double perovskite CeKFeMoO₆ have been investigated. The crystal structure of the compound is assigned to the monoclinic system with space group P2₁/n and its lattice parameters are a=0.55345(3) nm, b=0.56068(2) nm, c=0.78390(1) nm, β=89.874(2). The divergence between zero-field-cooling and field-cooling M-T curves demonstrates the anisotropic behavior. The Curie temperature measured from C_p-T curve is about 340 K. Isothermal magnetization curve shows that the saturation and spontaneous magnetization are 1.90 and 1.43 μ_B/f.u. at 300 K, respectively. The electrical behavior of the sample shows a semiconductor. The electrical transport behavior can be described by variable range hopping model. Large magnetoresistance, −0.88 and −0.18, can be observed under low magnetic field, 0.5 T, at low and room temperature, respectively.     

Anomalous thermal expansion of InSe layered semiconductors in the low-temperature region

The temperature dependence of the linear thermal expansion coefficient (TEC) of an InSe single crystal in the (001) plane is measured in the temperature range 7–50 K. A peak in the thermal expansion is detected near T = 10 K, after which the sample shrinks upon heating. The effect of an external magnetic field of up to 6 T, which is parallel to the (001) plane, on the TEC is investigated. The observed partial suppression of the peak and crystal compression by the field indicates the relation of these anomalies to possible electron ordering in InSe layers.     

Ferromagnetic resonance observation of a phase transition in magnetic field-aligned Fe2O3 nanoparticles

Fe₂O₃ hematite (alpha) nanoparticles suspended in the liquid phase of the liquid crystal 4,4-azoxyanlsole (PAA) are cooled below the freezing temperature (397 K) in a 4000 G dc magnetic field. The in field solidification locks the direction of maximum magnetization of the particles parallel to the direction of the applied dc magnetic field removing the effects of dynamical fluctuations of the nanoparticles on the magnetic properties allowing a study of the intrinsic magnetic properties of the nanoparticles as well as the anisotropic behavior of the ferromagnetic resonance (FMR) signal. Freezing in PAA allows temperature-dependent measurements to be made at much higher temperature than previous measurements. The field position, line width and intensity of the FMR signal as a function of temperature as well as the magnetization show anomalies in the vicinity of 200 K indicative of a magnetic transition, likely the previously observed Morin transition shifted to lower temperature due to the small particle size. Weak ferromagnetism is observed below T_c in contrast to the bulk material where it is antiferromagnetic below T_c. The Raman spectrum above and below 200 K shows no evidence of a change in lattice symmetry associated with the magnetic transition.     

Magnetic and Magnetoresonance Properties of the Solid Solution Hg0.5Cd0.4Cr0.1Se

The results of studying the magnetic and magnetoresonance properties of the diluted magnetic semiconductor Hg_0.5Cd_0.4Cr_0.1Se are presented. Microanalysis of the samples shows that the introduction of cadmium and chromium elements into the host HgSe matrix leads to the formation in the crystal of the four-component compound HgCdCrSe with the high chromium content [Cr (18.96 %)] and the three-component compound HgCdSe. The measured temperature dependence of the crystal magnetization illustrates the transition to ferromagnetic ordering at the Curie temperature T _C = 126 K. It is noted that the measured magnetization value points out the indicates the presence of both Cr³⁺ and Cr²⁺ ions in the compound HgCdCrSe, which is responsible for the magnetic and magnetoresonance properties of the sample under test. The electron paramagnetic resonance studies are carried out on the an X-band spectrometer in the temperature range 77 K < T < 300 K. The angular dependences of electron paramagnetic resonance spectra are shown in the paramagnetic and ferromagnetic temperature ranges. As follows from the analysis of experimental data, the aforementioned transition is accompanied by the evolution of the electron paramagnetic resonance spectrum at changing the temperature and the orientation of the sample relative to the static magnetic field in the ferromagnetic temperature range. In the assumption of the g-tensor axial symmetry the components of the latter are determined and the different law of their temperature changing is revealed in the ferromagnetic ordering state of the sample.     

Magnetostructural phase transitions in manganese arsenide single crystals

The effect of an external magnetic field with a strength up to 140 kOe on the phase transitions in manganese arsenide single crystals has been investigated. The existence of unstable magnetic and crystal structures at temperatures above the Curie temperature T _C = 308 K has been established. The displacements of manganese and arsenic atoms during the magnetostructural phase transition and the shift in the temperature of the first-order magnetostructural phase transition in a magnetic field have been determined. It has been shown that the magnetocaloric effect in a magnetic field of 140 kOe near the Curie temperature T _C is equal to ??T ?? 13 K. A model of the superparamagnetic state in MnAs above the temperature T _C has been proposed using the data on the magnetic properties and structural transformation in the region of the first-order magnetostructural phase transition. It has been demonstrated that, at temperatures close to T _C, apart from the contribution to the change in the entropy from the change in the magnetization there is a significant contribution from the transformation of the crystal lattice due to the magnetostructural phase transition.     

Pressure and field dependence of the magnetic transition in GdBa2Cu3O7-x

The influence of hydrostatic pressure and of magnetic field strenght is presented for the low temperature antiferromagnetic ordering temperature (T_N=2.3 K) of GdBa₂Cu₃O_7-x. Data are presented for both superconducting and normal samples, the superconducting sample having a sharp 95 K transition and the oxygen-depleted normal sample being a semiconductor. For both systems the Néel temperatures, extrapolated to zero measuring field, are identical: T_N = (2.33±0.03) K. The effect of pressure is to raise the transition temperature slightly for both samples, dT_N/dP=+0.03 K/kbar for the superconducting sample and +0.04 K/kbar for the normal sample. The temperature dependence of the heat capacity made in several fixed external magnetic fields and the isothermal magnetization for T<T_N provide a measure of the antiferromagnetic-paramagnetic phase boundary, which shows T_N approaching T=0 K at about 2.5 T.     

Etude par spectrometrie Mössbauer (57Fe) de FeMo2S4

We have investigated FeMo₂S₄ by transmission Mössbauer spectroscopy on the ⁵⁷Fe nucleus between 4.2 and 1037 K, using both powdered and single crystal samples.The temperature dependences of the isomer shift and the quadrupole splitting indicate the existence of ferrous ions with well localized 3d electrons. The two different crystallographic Fe sites cannot be separated over the experimental temperature range. The local symmetry of the iron site is lower than axial (η ~ 0.47) and the crystal field splittings Δ₁ and Δ₂ of the Fe²⁺ ? T_2g Orbitals, estimated by the Ingalls' method, are close to 250 and 900 cm^?1.The hyperfine field makes an angle of 14° with the normal on the plane (a, b), deviating a little from the direction of the magnetic moments determined to be perpendicular to (a, b) by neutron diffraction study.At higher temperatures, and more particularly near T_N, a line broadening is observed, and the spectra have to be fitted by a hyperfine field distribution. The broadening comes from the presence of about 7% “abnormal” Fe-sites observed in the paramagnetic spectra, and whose origin is discussed.The Néel temperature was determined to be 112 ± 1 K for the powdered sample and 106 ± l K for the single crystal.     

Effect of Gd addition on the activation energies of Bi-2223 superconductor

We have investigated the effect of addition of Gd in Bi_1.8Pb_0.35Sr_1.9Ca_2.1Cu₃Gd_xO_y superconductor with x=0, 0.1, 0.2, 0.3, 0.4 and 0.5. The samples were prepared using the standard solid-state reaction method. The activation energies, irreversibility fields (H_irr), upper critical fields (H_c2) and coherence lengths at 0 K (ξ(0)) were calculated from the resistivity versus temperature (R-T) curves under DC magnetic fields up to 7 T. The superconducting transition temperature, T_c, and activation energy, U₀, were found to decrease with increase in Gd concentration and with increase in applied magnetic field. The offset transition temperature of the pure (Gd00) sample without applied magnetic field is 108 K, whereas for Gd05 sample, the offset transition temperature drops to 5 K with 7 T applied magnetic field. The activation energy of the Gd00 sample without applied magnetic field is 34,980 K, and for Gd05 sample with 7 T applied field it is 98 K. H_irr and H_c2 values also decrease with increase in Gd addition. The possible reasons for the observed degradation in microstructural and superconducting properties due to Gd addition were discussed.     

Low-temperature thermal conductivity of terbium-gallium garnet

Thermal conductivity of paramagnetic Tb₃Ga₅O₁₂ (TbGG) terbium-gallium garnet single crystals is investigated at temperatures from 0.4 to 300 K in magnetic fields up to 3.25 T. A minimum is observed in the temperature dependence κ(T) of thermal conductivity at T _min = 0.52 K. This and other singularities on the κ(T) dependence are associated with scattering of phonons from terbium ions. The thermal conductivity at T = 5.1 K strongly depends on the magnetic field direction relative to the crystallographic axes of the crystal. Experimental data are considered using the Debye theory of thermal conductivity taking into account resonance scattering of phonons from Tb³⁺ ions. Analysis of the temperature and field dependences of the thermal conductivity indicates the existence of a strong spin-phonon interaction in TbGG. The low-temperature behavior of the thermal conductivity (field and angular dependences) is mainly determined by resonance scattering of phonons at the first quasi-doublet of the electron spectrum of Tb³⁺ ion.     

Effect of pressure and magnetic field on bilayer La1.25Sr1.75Mn2O7 single crystal

We report the temperature dependence of susceptibility for various pressures, magnetic fields and constant magnetic field of 5 T with various pressures on La_2−2xSr_1+2xMn₂O₇ single crystal to understand the effectiveness of pressure and magnetic field in altering the magnetic properties. We find that the Curie temperature, T_c, increases under pressure (dT_c/dP=10.9 K/GPa) and it indicates the enhancement of ferromagnetic phase under pressure up to 2 GPa. The magnetic field dependence of T_c is about 26 K for 3 T. The combined effect of pressure and constant magnetic field (5 T) shows dT_c/dP=11.3 K/GPa and the peak structure is suppressed and broadened. The application of magnetic field of 5 T realizes 3D spin ordered state below T_c at atmospheric pressure. Both peak structure in χ_c and 3D spin ordered state are suppressed, and changes to 2D-like spin ordered state by increase of pressure. These results reveal that the pressure and the magnetic field are more competitive in altering the magnetic properties of bilayer manganite La_1.25Sr_1.75Mn₂O₇ single crystal.     

Magnetic-field-induced phase transition in Tb0.95Bi0.05MnO3+δ multiferroic

The effect of a magnetic field on the dielectric properties of Tb_0.95Bi_0.05MnO_3+δ single crystals has been analyzed. It has been shown that the state of the crystal for temperatures of 5–440 K is inhomogeneous and restricted domains of polar and spin correlations are exhibited in it. A phase transition in which the inhomogeneous state of the crystal changes substantially has been observed at a temperature of T ? 180 and 225 K (in the absence and presence of a magnetic field, respectively). The high-temperature phase contains large dielectric domains with a high dielectric constant (ε ～ 10⁵) and thin conducting layers at the boundaries of these domains. The magnetic field significantly affects the state of both low-and high-temperature phases of the crystal, shifting the temperature of the phase transition between them, and induces an additional phase transition at T ? 441 K.     

Studying the magnetic properties of CoSi single crystals

The magnetic properties of CoSi single crystals have been measured in a range of temperatures T = 5.5–450 K and magnetic field strengths H ≤ 11 kOe. A comparison of the results for crystals grown in various laboratories allowed the temperature dependence of magnetic susceptibility χ(T) = M(T)/H to be determined for a hypothetical “ideal” (free of magnetic impurities and defects) CoSi crystal. The susceptibility of this ideal crystal in the entire temperature range exhibits a diamagnetic character. The χ(T) value significantly increases in absolute value with decreasing temperature and exhibits saturation at the lowest temperatures studied. For real CoSi crystals of four types, paramagnetic contributions to the susceptibility have been evaluated and nonlinear (with respect to the field) contributions to the magnetization have been separated and taken into account in the calculations of χ(T).     

Low temperature specific heat of bi-layered manganites La2−2xSr1+2xMn2O7 (x=0.3 and 0.5)

The specific heat (C) of bi-layered manganites La_2−2xSr_1+2xMn₂O₇ (x=0.3 and 0.5) is investigated for the ground state of low temperature excitations. A T^3/2 dependent term in the low temperature specific heat (LTSH) is identified at zero magnetic field and suppressed by magnetic fields for x=0.3 sample, which is consistent with a ferromagnetic metallic ground state. For x=0.5 sample, a T² term is observed and is consistent with a two-dimensional (2D) antiferromagnetic insulator. However, it is almost independent of magnetic field within the range of measured temperature (0.6-10 K) and magnetic field (6 T).     

Antiferromagnetic transition and electrical conductivity in α-Gd2S3

Magnetic susceptibility and electrical resistivity of α-Gd₂S₃ with an orthorhombic structure (space group: Pnma) have been measured for powder and single-crystal samples. While the magnetic susceptibility of powder sample exhibits a broad peak having a maximum at 4.2 K, the susceptibility for a single crystal with an applied magnetic field along the b-axis demonstrates a sharp drop below 10 K. Nevertheless, the susceptibility with the field perpendicular to the b-axis keeps increasing with decreasing temperature even below 10 K. The electrical resistivity ρ for the powder sample of 4.2×10³ Ω cm around room temperature increases with decreasing temperature and shows a slight discontinuity at about 65 K. In both regions above and below 65 K, is proportional to T^−1/4 with respective coefficients, which is associated with Mott variable-range hopping conductivity. The resistivity of a single crystal along the b-axis is considerably smaller than the value for the powder sample as 0.35 Ω cm at room temperature, and its temperature dependence is fairly weak. While cooling, the resistivity first decreases down to 240 K and then keeps the value independent of the temperature down to 140 K, and subsequently rises gently below 140 K.     

Effect of regimes of cooling in a magnetic field on the magnetization of a La0.9Sr0.1MnO3 single crystal

Magnetization measurements were performed on a lanthanum manganite La_0.9Sr_0.1MnO₃ single crystal in the temperature interval 4.2–300 K and magnetic field interval 50 Oe-55 kOe in two sample cooling regimes: 1) cooling down to 4.2 K in a high (55 kOe) magnetic field, and 2) cooling in a “zero” field. It is shown that the temperature dependences of the magnetization M(T) are substantially different in these regimes. Pronounced anomalies of M(T) were observed at temperatures T*=103 K and T _c =145 K. The first anomaly is attributed to a structural transition, while the second one corresponds to a ferromagnet-paramagnet phase transition. The magnetization of a La_0.9Sr_0.1MnO₃ single crystal in the cooling regimes studied shows typical “spin-glass” behavior. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 39–43 (10 July 1998)