Physical property characterization of bulk MgB<Subscript>2</Subscript> superconductor

We report synthesis, structure/micro-structure, resistivity under magnetic field [ρ(T)H], Raman spectra, thermoelectric power S(T), thermal conductivity κ(T), and magnetization of ambient pressure argon annealed polycrystalline bulk samples of MgB₂, processed under identical conditions. The compound crystallizes in hexagonal structure with space group P6/mmm. Transmission electron microscopy (TEM) reveals electron micrographs showing various types of defect features along with the presence of 3–4 nm thick amorphous layers forming the grain boundaries of otherwise crystalline MgB₂. Raman spectra of the compound at room temperature exhibited characteristic phonon peak at 600 cm^-1. Superconductivity is observed at 37.2 K by magnetic susceptibility χ(T), resistivity ρ(T), thermoelectric power S(T), and thermal conductivity κ(T) measurements. The power law fitting of ρ(T) give rise to Debye temperature (Θ_D) at 1400 K which is found consistent with the theoretical fitting of S(T), exhibiting Θ _D of 1410 K and carrier density of 3.81 × 10²⁸/m³. Thermal conductivity κ(T) shows a jump at 38 K, i.e., at T_c, which was missing in some earlier reports. Critical current density (J_c) of up to 10⁵ A/cm² in 1–2 T (Tesla) fields at temperatures (T) of up to 10 K is seen from magnetization measurements. The irreversibility field, defined as the field related to merging of M(H) loops is found to be 78, 68 and 42 kOe at 4, 10 and 20 K respectively. The superconducting performance parameters viz. irreversibility field (H_irr) and critical current density J_c(H) of the studied MgB₂ are improved profoundly with addition of nano-SiC and nano-diamond. The physical property parameters measured for polycrystalline MgB₂ are compared with earlier reports and a consolidated insight of various physical properties is presented.     

Synthesis and structural details of MSr2RECu2O7+δ(M-1212;0⩽δ<1) compounds with M = Al,Nb, Fe,Ru, Ga & Co and RE = Eu,Y

Synthesis and structural details of MSr₂RECu₂O_z (M-1212) compounds with M = Ga, Nb, Fe, Al and Co) and RE = Eu, Y are reported. Reitveld refinement of X-ray diffraction (XRD) patterns shows that all compounds are crystallized in single phase. Nb-, Fe- and Al-1212 possess tetragonal P4/mmm space group structure while the Ga-1212 and Co-1212 are crystallized in orthorhombic Ima2 space group. The change of space group from P4/mmm to Ima2 indicating towards the doubling of unit cell. The buckling angle [Cu(2)–O(2)–Cu(2) angle] shows that most of the studied samples are heavily under doped and hence they could not exhibit superconductivity. Thermogravimetric (TGA) analysis shows the M-1212 compounds to be more stable than widely studied 90 K superconductor Cu-1212 (RE-123).     

Magnetic and magneto-transport properties of double perovskite Ba2−xSrxFeMoO6 system

The structural magnetic and magneto-transport properties of double perovskite system Ba_2−xSr_xFeMoO₆ (0?x?1.0) prepared in bulk polycrystalline form are reported in this paper. X-ray diffraction analysis showed that samples are single phase and the lattice constants decreases with increase in the Sr content. The degree of Fe-Mo ordering has been found decreasing in the series with an increase in the Sr content. Parent compound Ba₂FeMoO₆ exhibits saturation magnetic moment value of 3.54 μ_B/f.u. at 85 K in a magnetic field of 6000 Oe. Temperature dependence of resistivity shows metallic behavior for all the samples. The magneto-resistance (MR) of the compound with x=0.4 is higher than that of the other samples. At room temperature this system shows a saturation magnetization value of 1.73 μ_B/f.u. and MR value of 7.08% (1 T). The observed variations in the structural and magnetic properties are attributed to the change of chemical pressure due to the substitution of Sr in place of Ba. The effect of antisite disorder (ASD) defects on magneto-transport properties is studied in more detail.     

Superconductivity with transition temperature up to 80 K for TbSr2Cu2.7Mo0.3O7+δ

A Tb-123 phase with the composition, TbSr₂Cu_2.7Mo_0.3O_7+δ, has been synthesized in single-phase form by the solid-state reaction route. Its phase purity has been confirmed from neutron powder diffraction studies. The as-synthesized Tb-123 sample does not show superconductivity down to 5 K. On the other hand, an unusually high antiferromagnetic ordering temperature (T_N) of around 7 K is seen for the Tb moments. After 120-atm-O₂ post-annealing, bulk superconductivity is achieved in this compound with a superconducting transition temperature (T_c) of about 30 K, without any significant effect on T_N. To achieve higher oxygen content and higher T_c, the as-synthesized sample was subjected to high-pressure oxygenation, carried out in a closed cell, at 5 GPa and 400 °C in the presence of AgO as an excess-oxygen source. This sample exhibited superconductivity onset at around 80 K with a Meissner fraction larger than 10% at 5 K. Our observation of superconductivity at 80 K is the highest T_c to-date for the Tb-123 type compounds.     

Hole doping into Co-12s2 copper oxides with s fluorite-structured layers between CuO2 planes

In this work, the first three members (s=1, 2, 3) of the Co-12s2 homologous series of multi-layered copper oxides are gradually doped with holes through high-pressure oxygenation (HPO). The phases differ from each other only by thickness of the fluorite-structured layer block, (Ce,Y,Ca)-[O₂-(Ce,Y)]_s−1, between two identical CuO₂ planes. High-resolution transmission-electron microscopy (HRTEM) and electron diffraction (ED) analyses together with both synchrotron X-ray and neutron powder diffraction data, reveal that as a consequence of HPO the charge-reservoir CoO₄-tetrahedra chains get broken and the lattice symmetry of the Co-12s2 phases changes from orthorhombic to tetragonal. Oxygen contents are analyzed for the samples with wet-chemical and thermogravimetric techniques. The valence state of copper in the CuO₂ plane is determined from Cu L-edge X-ray absorption near-edge structure (XANES) spectra to be compared with the values estimated through bond-valence-sum (BVS) calculations from the crystal structure data. The positive charge induced by oxygen loading (or aliovalent Ca^II-for-Y^III substitution in CoSr₂YCu₂O_7+δ) is found not to be completely accommodated in the CuO₂ planes but be rather effectively trapped at the charge-reservoir Co atoms. Superconductivity appears in the Co-1212 (CoSr₂YCu₂O_7+δ) samples with the copper valence of 2.13 or higher, whereas in the Co-1222 (CoSr₂(Ce_0.25Y_0.75)₂Cu₂O_9+δ) and Co-1232 (CoSr₂(Ce_0.67Y_0.33)₃Cu₂O_11+δ) samples Cu valence does not increase high enough to induce superconductivity.     

Direct evidence for charge ordering and electronic phase separation in BixSr1−xMnO3 at room temperature

We present a study of charge ordering and electronic phase separation (EPS) phenomenon in Bi_xSr_1−xMnO₃, for an exhaustive range of x (0.25x0.75), by STM/STS at room temperature (RT) and specific heat measurements at high temperatures (350–650 K). Atomically resolved STM images of the samples, in real space, show the presence of stripe-like charge-ordered (CO) phase coexisting with charge-disordered (CD) phase. The STM images further reveal that the fraction of CO phase increases with an increase in x. The conductance spectra of these phases measured at nano level by STS are discussed. The transition to CO phase above RT is corroborated by specific heat measurements in all samples, giving a T_CO(x) phase diagram for this system.     

Room temperature magnetic entropy change and magnetoresistance in La0.70(Ca0.30−xSrx)MnO3:Ag 10% (x=0.0−0.10)

The magnetic and magnetocaloric properties of polycrystalline La_0.70(Ca_0.30−xSr_x)MnO₃:Ag 10% manganite have been investigated. All compositions are crystallized in single phase orthorhombic Pbnm space group. Both, the insulator–metal transition temperature (T^IM) and Curie temperature (T_c) are observed at 298 K for x=0.10 composition. Though both T^IM and T_c are nearly unchanged with Ag addition, the MR is increased. The MR at 300 K is found to be as large as 31% with magnetic field change of 1 T, whereas it reaches up to 49% at magnetic field of 3 T for the La_0.70Ca_0.20Sr_0.10MnO₃:Ag_0.10 sample. The maximum entropy change (ΔS_Mmax) at near its T_c (300.5 K) is 7.6 J kg⁻¹ K⁻¹ upon the magnetic field change of 5 T. The La_0.70Ca_0.20Sr_0.10MnO₃:Ag_0.10 sample having good MR (31%^1 T, 49%^3 T) and reasonable change in magnetic entropy (7.6 J kg⁻¹.K⁻¹, 5 T) at 300 K can be a potential magnetic refrigerant material at ambient temperatures.     

Search for room temperature high-TCR manganite/silver composites

We have synthesized La_0.7Ba_0.3MnO₃ (LBMO):wt%Ag_x and La_0.7Sr_0.3MnO₃ (LSMO):wt%Ag_x composites with x=0.0, 0.1, 0.2, 0.3 and 0.4 at different sintering temperatures of 1300 and 1400 °C by solid-state reaction route. In our previous work [Awana et al. Solid State Commun. 140 (2006) 410] on La_2/3Ca_1/3MnO₃ (LCMO):Ag_x (x=0.0, 0.1, 0.2, 0.3, 0.4) composites, we reported about 12% temperature coefficient of resistance (TCR) for Ag_0.4 samples near metal–insulator (MI) transition, which was explained on the basis of better grains connectivity. A sharp MI transition results in high TCR. Such high-TCR material can be used as a bolometer or infrared detector at room temperature. In the present work, it is shown that the TCR is improved with Ag addition for both LBMO:wt%Ag_x and LSMO:wt%Ag_x composites. In addition to improvement in TCR, high MR is also achieved in the vicinity of room temperature.