A. Berenov

High critical current density and improved irreversibility field in bulk MgB2 made by a scaleable, nanoparticle addition route

Bulk samples of MgB2 were prepared with 5, 10, and 15 wt % Y2O3 nanoparticles, added using a simple solid-state reaction route. Transmission electron microscopy showed a fine nanostructure consisting of ∼3–5 nm YB4 nanoparticles embedded within MgB2 grains of ∼400 nm size. Compared to an undoped control sample, an improvement in the in-field critical current density JC was observed, most notably for 10% doping. At 4.2 K, the lower bound JC value was ∼2×105 A cm−2 at 2 T. At 20 K, the corresponding value was ∼8×104 A cm−2. Irreversibility fields were 11.5 T at 4.2 K and 5.5 T at 20 K.

Direct and indirect electrocaloric measurements on 〈001〉-PbMg1/3Nb2/3O3-30PbTiO3 single crystals

A direct electrocaloric effect (ECE) measurement system, based on a modified-differential scanning calorimeter (DSC), allowing the acquisition of both thermal (ECE, heat capacity) and electrical (P-E loops, leakage current) information simultaneously, was used to analyze 〈001〉-oriented PbMg1/3Nb2/3O3-30PbTiO3 single crystals. Different electric-field-induced phase transitions were identified on direct ECE measurements and confirmed by dielectric measurements. The strongest ECE (ΔTEC = 0.65 K) was measured for an applied electric field E = 10 kV/cm just above the temperature of depolarization. The direct ECE measurements were compared with indirect measurements obtained from dielectric polarization measurements versus electric field and temperature and a very good agreement was found. A region with negative ΔTEC was identified by both direct and indirect measurements. This phenomenon was attributed to the formation of a reversible field-induced phase transition towards a state with a different polar direction.

Oxygen tracer diffusion in undoped lanthanum manganites

Abstract The oxygen tracer diffusion was studied in cation stoichiometric (LaMnO 3± δ ) and A-site deficient (La 0.9 MnO 3± δ ) lanthanum manganites by the isotope exchange depth profiling (IEDP) technique using SIMS depth profiling. The measured oxygen tracer diffusion coefficients were in the range of 10 −12 to 10 −15 cm 2 s −1 and these values were comparable with the ones determined in Sr-doped lanthanum manganites. The activation energy of oxygen tracer diffusion was 240±23 kJ/mol in LaMnO 3± δ and 294±11 kJ/mol in La 0.9 MnO 3± δ . A-site deficiency of up to 10% does not affect oxygen tracer diffusion in these lanthanum manganites. Evidence for fast diffusion paths assigned to grain boundary diffusion was observed in the A-site deficient material.

Electrocaloric enhancement near the morphotropic phase boundary in lead-free NBT-KBT ceramics

The electrocaloric effects (ECEs) of the morphotropic phase boundary (MPB) composition 0.82(Na0.5Bi0.5)TiO3-0.18(K0.5Bi0.5)TiO3 (NBT-18KBT) are studied by direct measurements. The maximum ECE ΔTmax = 0.73 K is measured at 160 °C under 22 kV/cm. This corresponds to an ECE responsivity (ΔT/ΔE) of 0.33 × 10−6 K m/V, which is comparable with the best reported values for lead-free ceramics. A comparison between the direct and indirect ECE measurements shows significant discrepancies. The direct measurement of both positive and negative electrocaloric effect confirms the presence of numerous polar phases near the MPB of NBT-based materials and highlights their potential for solid-state cooling based on high field-induced entropy changes.

Growth of strongly biaxially aligned MgB2 thin films on sapphire by postannealing of amorphous precursors

MgB2 thin films were cold grown on sapphire substrates by pulsed laser deposition followed by postannealing in mixed, reducing gas, Mg rich, Zr gettered, environments (pO2∼10−24 atm) at 750 and 950 °C. The films had Tc in the range 29–34 K, Jc (20 K, H=0) in the range 3×104–3×105 A cm−2, and irreversibility fields H* at 20 K of 4–6.2 T. An inverse correlation was found between Tc and H*. The films had grain sizes of ∼0.1–1 μm and a strong biaxial alignment was observed in the 950 °C annealed film. (111) oriented MgO was also observed. Mg coating of films during crystallization appeared to improve film Tc.

Highly aligned, spin polarized thin films of Sr2FeMoO6 by a chemical vapor process

Highly oriented films of Sr2FeMoO6 were fabricated by ultrasonic spray pyrolysis. A combined organic/inorganic solution was developed and the films were deposited on single-crystal LaAlO3 between 600 °C and 900 °C followed by postannealing at 850 °C or 1200 °C in Ar/5% H2. Optimum films showed a metallic resistivity behavior with less than a 0.25% magnetoresistive effect at 50 K, 1 T, indicative of highly quality intra- and intergranular material. At 4.2 K, the lower bound saturation magnetization, Ms was ∼2.5 μB/f.u. and the transport spin polarization was 60±3%.

Enhancement of critical current density of (Pb,Sn)-doped Bi-2212 superconductors at high temperature

Abstract The magnetic properties of ceramic samples of Bi 2−2 x (PbSn) x Sr 2 CaCu 2 O y ( x =0,0.1,0.2,0.3 and 0.4) superconductors have been investigated using a vibrating sample magnetometer from 20 to 50 K and up to 8 T. Phase analysis shows that most of the (Pb,Sn) dopant enters the Bi-2212 lattice and occupies Bi sites. The current-carrying scale-length analysis has been determined from the reverse leg of the magnetization hysteresis loops and compared to the actual dimensions of the sample. Magnetization hysteresis loops of the (PbSn)-doped sample exhibit strong pinning behaviour in the higher temperature range. The effect of (Pb,Sn) doping on intragranular critical current J c was found to be enhanced by one order of magnitude at 50 K. We discuss the experimental results within a model of (PbSn) cluster pinning in blocking layers. Doping by (PbSn) clusters into the Bi–O layers provides effective pinning centres, as well as a large reduction of the resistivity anisotropy.

Study of thermal oxidation of NiO buffers on Ni-based tapes for superconductor substrates

Abstract NiO on Ni-based textured substrates is a potential buffer layer system for growth of superconductor films for conductor applications. In this work, the high temperature oxidation of NiO has been studied under different oxygen partial pressures. Pure Ni, Ni–10%Cr and Cr-coated Ni have been studied. Pure Ni when oxidised at 1250°C for 3 h in P O 2 ∼1 atm has shown excellent pure cube texture. Ni–10%Cr oxidised under the same condition has shown the same texture, however the tape becomes very brittle. The conditions for reproducible growth of highly cube textured NiO are determined.

Enhanced intergrain magnetoresistance in bulk Sr2FeMoO6 through controlled processing

Enhanced low field magnetoresistance (LFMR) values have been obtained in bulk Sr2FeMoO6 through controlled processing. MR values, R(H)/R(0) of ∼60% [(R(H)−R(H=0)/R(H=0)=40%] were achieved at 50 K, in 1 T, and R(H)/R(0)∼8% [(R(H)−R(H=0)/R(H=0)=92%] at 290 K, in 1 T. For a ∼1 μm starting particle size, the optimum sintering temperature was 1300 °C, 100 °C higher than normally used. The optimum sintering time in Ar/H2 was only 5 h, much shorter than normally used. The magnitude of the LFMR appears to depend strongly on grain boundary oxygen content, controlled through optimization of sintering temperature and time.

Structure of the superconducting gap in MgB2 from point-contact spectroscopy

We have studied the structure of the superconducting gap in MgB2 thin films by means of point-contact spectroscopy using a gold tip. The films were produced by depositing pure boron on a sapphire substrate, using e-beam evaporation, followed by reaction with magnesium vapour. The films have a Tc of 38.6 ? 0.3 K and resistivity of about 20 ?? cm at 40 K. The point-contact spectra prove directly the existence of a multi-valued order parameter in MgB2, with two distinct values of the gap, ?1 = 2.3 ? 0.3 meV and ?2 = 6.2 ? 0.7 meV at 4.2 K. Analysis of the spectra in terms of the Blonder?Tinkham?Klapwijk model reveals that both gaps close simultaneously at the Tc of the film. Possible mechanisms that can explain the intrinsic coexistence of two values of the gap are discussed.