Superconductivity coexisting with ferromagnetism in mixed-valence Mn doped La1.85−(4/3)xSr0.15+(4/3)xCu1−xMnxO4

The effect of Mn substitution for Cu in mixed-valence Mn doped La_{1.85−(4/3)x}Sr_0.15+(4/3)xCu_1−xMn_xO₄ (x=0.06) has been investigated by electric resistivity, magnetization and electron spin resonance experiments. Coexistence of superconductivity and ferromagnetism was observed.     

A mechanical spectrum study of bilayer cuprate La1.82Sr0.18CaCu2O6+δ

The mechanical spectrum of bilayer cuprate La_1.82Sr_0.18CaCu₂O_6+δ (La2126) was measured using the vibrating reed method from 30 K to room temperature at kilohertz frequency. A clear modulus softening was observed at 223 K accompanied by a sharp internal friction peak, which evidences a phase transition. Another broad internal friction peak around 250 K was observed, accompanied by a large modulus change. This contrasts with the broad internal friction peak of Ca doped Y Ba₂Cu₃O_7−δ (Y123) for which no clear modulus change was observed. The main contribution to the broad internal friction peak of La2126 is expected to be from the glass transition of the interstitial oxygen atoms.     

Enhanced superconducting properties of Eu2O3-doped MgB2

Bulk polycrystalline samples of Eu₂O₃-doped MgB₂ have been synthesized by a standard solid state reaction route and their structural and superconducting properties have been investigated. As a function of Eu₂O₃ content we have found a significant increase in the critical current density (J_c) and the irreversibility field (H_irr) in the magnetic field range 0–6 T. The XRD results reveal the presence of MgO and EuB₆ secondary phases along with the main hexagonal phase of MgB₂. The strain values and the lattice distortions have been found to increase almost linearly with the nominal Eu₂O₃ content. The observed significant improvement in J_c(H) and H_irr in the Eu₂O₃-doped MgB₂ samples, thus is mainly attributed to the lattice distortions introduced by Eu₂O₃ doping.     

Critical current density and flux pinning in La2−xPrxCa2x Ba2Cu4+2xOz (x=0.1-0.5) superconductors

Polycrystalline La_2−xPr_xCa_2xBa₂Cu_4+2xO_z (LPCaBCO) compounds with x=0.1-0.5 were synthesized by solid-state reaction method and studied by room temperature X-ray diffraction, dc resistivity, dc magnetization and iodometry. The superconducting transition temperatures in these tetragonal triple perovskite compounds increases from 32 to 62 K (T_c^onset values) with increasing dopant concentration. The mixing of rare earth La³⁺ and Pr^3+/4+ ions at rare earth site (La³⁺) along with substitution of divalent Ca²⁺ results in the shrinkage of unit cell volume. The contraction of unit cell volume due to larger ion being substituted by smaller ions, gives rise to creation of pinning centres in the unit cell leading to increase in critical current density and flux pinning.     

Magnetic studies of phase separation in La2CuO4+° single crystals

The phase seperation effect in La₂CuO_4.015 single crystal has been studied by magnetic susceptibility and EPR experiments. The experiments confirm the percolative phase separation model based on the concept of magnetic polarons formed by hole doping. It has been shown that the superconducting subphase has specific spatial structure which is responsible for unconventional low temperature behavior of susceptibility. The EPR signal as well as the paramagnetic contribution to the lowtemperature susceptibility have been shown to arise from isolated Cu²⁺ ions located inside the phase domain walls.     

Magnetocaloric effect in DyCu2

The magnetocaloric effect of the rare-earth intermetallic compound DyCu₂ is explored through magnetization measurements. DyCu₂ is paramagnetic at the room temperature but becomes antiferromagnetic below 27 K (Neel temperature). Strong temperature and field dependence of magnetization in DyCu₂ at and around the Neel temperature lead to a large magnetocaloric effect. An appreciable magnetocaloric effect persists well above the Neel temperature probably because of the presence of short-range ferromagnetic correlations in the paramagnetic state of DyCu₂. This along with the absence of magnetic hysteresis lead to a large effective refrigerant capacity of 194 J/kg below 44 K, which makes the material important as a potential magnetic refrigerant for the cryogenic liquefier cycles.     

First-principles study on the electronic structure and magnetic properties of metallic antiferromagnet C7H3S2N2

First-principles full potential linearized augmented plane wave (FPLAPW) calculations have been performed to study the electronic structure and the magnetic properties of 3-Cyanobenzo-1,3,2-dithiazolyl,C₇H₃S₂N₂. The density of states (DOS), the total energy of the cell, and the spontaneous magnetic moment of C₇H₃S₂N₂ were all calculated. The calculations reveal that the low-temperature phase of the compound C₇H₃S₂N₂ has a stable metal-antiferromagnetic ground state, and there exists an antiferromagnetically coupled interactions between the dithiazolyl radical(1), which is in good agreement with experiment.     

Effect of Mn-site vacancies on the magnetic entropy change and the Curie temperature of La0.67Ca0.33Mn1−xO3 perovskite

Single-phase polycrystalline samples of La_0.67Ca_0.33Mn_1−xO₃ (x=0.00, 0.02, 0.04, 0.06) have been prepared using the sol-gel method. The structure, magnetocaloric properties and the Curie temperature of the samples with different Mn vacancy concentrations have been investigated. The experimental results show that vacancy doping at the Mn-sites has a significant influence on the magnetic properties of La_0.67Ca_0.33Mn_1−xO₃. The Curie temperature decreases monotonically with increasing the Mn-site vacancy concentration x. A remarkable enhancement of the magnetic entropy change has been obtained in the La_0.67Ca_0.33Mn_0.98O₃ sample. The entropy change reaches |ΔS_M|=3.10 J kg⁻¹ K⁻¹ at its Curie temperature (264 K) under an applied magnetic field H=10 kOe, which is almost the same value as that of pure Gd.     

Superconductivity and magnetism of La2Cu1−xZnxO4+δ chemically oxidized by NaClO

The structural, superconducting and magnetic properties of La₂Cu_1−xZn_xO_4+δ (0≤x≤0.1) chemically oxidized by NaClO at room temperature were studied. All the samples before and after oxidation are single phase with orthorhombic structure, as indicated by their powder X-ray diffraction analysis. The iodometric titration results indicate that Zn-substituted La₂Cu_1−xZn_xO₄ is more favorable for the insertion of the excess oxygen, as compared to the Zn-free La₂CuO₄. The T_c suppression rate resulting from Zn substitution in La₂Cu_1−xZn_xO_4+δ is −12.4 K/%. The effective magnetic moment induced by the non-magnetic Zn ion is the order of one Bohr magneton, which decreases with increasing the Zn concentrations in the range examined. The latter two results are qualitatively well consistent with those obtained in La_2−xSr_xCu_1−yZn_yO₄ with the Sr optimal doping. This reveals that the non-magnetic Zn ions play the same role in both of the La₂Cu_1−xZn_xO_4+δ with the excess oxygen content of about 0.1 and the La_2−xSr_xCu_1−yZn_yO₄ with the Sr optimal doping.     

Magnetic properties of R2Fe17−xGax (R=Y and Gd) compounds

The magnetic properties of Y₂Fe_17−xGa_x for 3≤x≤7 and Gd₂Fe_17−xGa_x for 5≤x≤7 have been investigated using ⁵⁷Fe Mössbauer spectroscopy. These compounds have the rhombohedral Th₂Zn₁₇ structure. X-ray diffraction analysis of aligned powders shows that the easy direction of magnetization is parallel to the c-axis in Y₂Fe₁₀Ga₇ and Gd₂Fe₁₀Ga₇ and is perpendicular to the c-axis in Y₂Fe₁₄Ga₃, Y₂Fe₁₂Ga₅, Gd₂Fe₁₂Ga₅ and Gd₂Fe₁₁Ga₆. Mössbauer studies indicate that those samples are ordered ferromagnetically. The ⁵⁷Fe hyperfine field decreases with increasing Ga content. This decrease results from the decreased magnetic exchange interactions resulting from Ga substitution. The average isomer shift, δ, for R₂Fe_17−xGa_x (R=Y and Gd) at room temperature is positive and the magnitude of δ increases with increasing Ga content.     

Magnetic and photomagnetic studies in Nd0.7Sr0.3CoO3

We report magnetic and photomagnetic studies on polycrystalline Nd_0.7Sr_0.3CoO₃. Strong irreversibility in field cooled and zero field cooled data is observed. The hysteresis plot exhibits a very high coercivity at low temperatures. On photon irradiation, a decrease in the coercivity and an irreversible magnetization enhancement are observed. The analysis of all these data suggest that microscopic phase separation of competing ferromagnetism and antiferromagnetism and/or light-induced spin conversion processes among low, intermediate and high spin states of cobalt ions are responsible for all the properties.     

Magnetostriction and magnetic anisotropy of (Sm,Ce)Fe2 compounds

The structural, magnetic and magnetostrictive properties of Sm_1−xCe_xFe₂ (0≤x≤1) Laves compounds have been investigated. The magnetostriction coefficient λ₁₁₁ and the anisotropy of the Sm_1−xCe_xFe₂ compounds decrease with increasing Ce content. The decreasing of anisotropy results in an enhancement of the anisotropic magnetostriction at low magnetic fields for the compounds with a small amount of Ce substituted for Sm.     

AC susceptibility and intergranular critical current density study in pure and Ag doped (La1−xYx)2Ba2CaCu5Oz superconductors

The temperature and AC field amplitude variations of AC susceptibility have been measured on pure and 5 wt% Ag doped (La_1−xY_x)₂Ba₂CaCu₅O_z superconductors. The AC susceptibility as a function of field have been analyzed using Kim's critical state model. The temperature dependence of intergranular critical current density and the effective volume fractions of the grains have been estimated. The Ag doped samples show relatively large critical current density due to the improved intergranular coupling. The exponent of temperature variation of critical current density suggests that the weak links form superconductor-normal metal-superconductor (SNS) type of junctions for all the samples.     

Raman spectroscopy of RuSr2(Eu1.5Ce0.5)Cu2O10 magneto-superconductor

Raman spectroscopy studies are reported for the RuSr₂Eu_1.5Ce_0.5Cu₂O₁₀ (Ru-1222) compound at various temperatures of 300, 250, 200 and 90 K. Three distinct vibrational bands: the first at 110, 140, and 160 cm⁻¹, the second at 295 and 347 cm⁻¹, and third one at 651 cm⁻¹ are seen in Raman spectra of the compound at room temperature. These bands are attached to the Cu atoms’ c-direction, the Ru atoms’ ab-plane stretching and Ru atoms’ c-direction anti-stretching modes. Below 200 K, an extra vibrational mode is also seen at 260 cm⁻¹. Also, with a decrease in temperature, though the Cu vibrational modes remain intact, the Ru atoms’ ab-plane stretching (295 cm⁻¹) and c-direction anti-stretching (651 cm⁻¹) modes shift gradually to higher wave number positions. The frequencies of modes at 260 and 651 cm⁻¹ showed anomalous softening and line-width broadening below 100 K that corroborates well with the spin ordering seen in susceptibility studies. The studied compound is a ferromagnetic superconductor with magnetic ordering of the Ru spins at 200 K and superconductivity below 30 K. A magnetic and electrical transport characterization of the compound is also presented briefly.     

Magnetic and structural studies of the double perovskites Ba2REMoO6

A magnetic, electronic and structural study of the double perovskites Ba₂REMoO₆ (RE=Sm, Eu, Gd, Dy) has been performed. All materials crystallise in the cubic symmetry space group and the cell volume decreases as RE varies from Sm to Dy in accordance with Vegard's law. An antiferromagnetic transition is observed below T_N=130 and 112 K for RE=Sm and Eu, respectively. The Néel temperatures of these ordered rare earth molybdenum double perovskites are much higher than previously observed in double perovskites containing Eu or Sm and a 4d or 5d transition metal arranged in an ordered rock salt configuration. The high Néel temperatures arise due to a strong superexchange magnetic interaction via the Mo-O-RE-O-Mo pathway. All of the phases are electronically insulating and there is no evidence of magnetoresistance at any temperature.     

High pressure investigation on double perovskite Ba2MgWO6

The results of high-pressure angle dispersive X-ray diffraction measurements up to 34.3 GPa on the double perovskite Ba₂MgWO₆ are presented. The ambient rock salt phase (SG: Fm-3m) is found to be stable up to the highest pressure of the present measurements. The third order Birch-Murnaghan equation of state when fitted to pressure-volume data, yielded a zero pressure bulk modulus (B₀),and its first and second pressure derivatives as 137.0(81) GPa, and 3.9(5) and −0.03 GPa⁻¹, respectively.     

Enhancement of the upper critical field of MgB2 by carbon-doping

We have measured the temperature dependence of the upper critical field, H_c2(T), of carbon-doped MgB₂. H_c2(T) does not follow the well-known Werthamer-Helfand-Hohenberg (WHH) result for a one-gap dirty superconductor but can be described well by the result of a recent theoretical calculation for a two-gap dirty superconductor. H_c2(0) of the carbon-doped material is determined to be between 29 and 38 T, substantially higher than that of pure MgB₂ (15-23 T).     

Angular dependence of vortex matter phase transition in MgB2 single crystal

The vortex matter phase transitions and intrinsic pinning effect were investigated in an MgB₂ single crystal using the torque magnetometry. For the field directions apart from the ab plane, we succeed in the observation of the vortex lattice melting transitions, which are transformed from the order-disorder transitions at low temperatures. Both transition fields with field directions can be describe by the GL effective mass model. For the field direction along the ab plane, these transitions become unobservable. Instead, the sudden increase in the hysteresis of magnetization curve occurs, indicating the existence of the intrinsic pinning coming from the layer structure.     

The Hall-Lorenz number in the La1.855Sr0.145CuO4 single crystal

The temperature dependence of the Hall-Lorenz number () for the optimally doped La_1.855Sr_0.145CuO₄ (LSCO) has been obtained from the experimentally determined transverse and longitudinal transport coefficients. A comparison between L_xy(T) dependence found for LSCO and L_xy(T) reported previously for copper indicates that the Hall-Lorenz number in LSCO follows standard metallic behavior from room temperature down to . Below this temperature the L_xy coefficient deviates from regular metallic dependence in a way characteristic of an electronic system with lowered density of electronic states at the Fermi level. We present results of calculations provided in terms of the Boltzmann equation for a two-dimensional model of the electronic structure with a d-symmetrical pseudogap. A temperature T_max, where a maximum in the L_xy(T) dependence appears, turns out to be dependent on the width of the supposed pseudogap . The best agreement between the model and the experimental data was obtained for , which corresponds well with values reported previously by other groups.