We present a comparative study of the angular dependent critical current density in YBa 2 Cu 3 O 7 films deposited on IBAD MgO and on single crystal MgO and SrTiO 3 substrates. We identify three angular regimes where pinning is dominated by different types of correlated and uncorrelated defects. We show that those regimes are present in all cases, indicating that the pinning mechanisms are the same, but their extension and characteristics are sample dependent, reflecting the quantitative differences in texture and defect density. In particular, the more defective nature of the films on IBAD turns into an advantage as it results in stronger vortex pinning, demonstrating that the critical current density of the films on single crystals is not an upper limit for the performance of the IBAD coated conductors.

The influence of lattice strain and Mg vacancies on the superconducting properties of MgB2 samples has been investigated. High quality samples with sharp superconducting transitions were synthesized. The variation in lattice strain and Mg vacancy concentrations were obtained by varying the synthesis conditions. It was found that high strain (~1%) and the presence of Mg vacancies (~ 5 %) resulted in lowering the Tc by only 2 K.Comment: 3 figures, HTML+GIF format to be published in AP

We report significantly enhanced critical current densities (J c ) and flux pinning forces (F p ) in applied magnetic fields for YBa 2 Cu 3 O 7 (YBCO) films with embedded Ba 2 YNbO 6 (BYNO) nanorods. The films were grown by pulsed laser deposition with a target consisting of YBa 2 Cu 3 O y with five molar per cent additions of BaNbO y and Y 2 O 3 . With this composition, deposited films were found to contain a high density of BYNO nanorods that frequently traversed the entire thickness of the film (up to 1 μm), depending upon the deposition conditions. Enhanced J c performance occurs primarily for applied field orientations near the c-axis of the YBCO, which is nominally along the growth direction of the BYNO nanorods. The threading nanorod density of one film of the present work was measured by plan-view transmission electron microscopy to be 710-850 nanorods μm −2 . For approximately 1 μm thick films, typical J c (75.6 K, sf) and J c (75.6 K, 1 T c) values were ∼4.5 MA cm −2 and 1.3-1.5 MA cm −2 , respectively. For a 0.5 μm thick film, J c (75.6 K, 1 T c) > 2 MA cm −2 was achieved, and values of F p in excess of 30 and 120 GN m −3 were achieved at 75.5 K and 65 K, respectively.

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This work studies the influence of microstructures and crystalline defects on the superconductivity of MgB 2 , with the objective to improve its flux pinning. A MgB 2 sample pellet that was hot isostatic pressed (HIPed) was found to have significantly increased critical current density (J c ) at high fields than its un-HIPed counterpart. The HIPed sample had a J c of 10000 A/cm 2 in 50000 Oe (5 T) at 5K. This was 20 times higher than that of the un-HIPed sample, and the same as the best J c reported by other research groups. Microstructures observed in scanning and transmission electron microscopy indicate that the HIP process eliminated porosity present in the MgB 2 pellet resulting in an improved intergrain connectivity. Such improvement in intergrain connectivity was believed to prevent the steep J c drop with magnetic field H that occurred in the un-HIPed MgB 2 pellet at H > 45000 Oe (4.5 T) and T = 5 K. The HIP process was also found to disperse the MgO that existed at the grain boundaries of the un-HIPed MgB 2 pellet and to generate more dislocations in the HIPed the pellets. These dispersed MgO particles and dislocations improved flux pinning also at H<45000 Oe. The HIPing process was also found to lower the resistivity at room temperature.74.70. Ad, 74.60.Ge, 74.62.Bf, 74.25.Fy 1

We have studied the temperature (T) and field (H) dependence of the magnetocaloric effect in a bulk amorphous Pd40Ni22.5Fe17.5P20 alloy. With decreasing T, and depending on H, the alloy exhibits superparamagnetic, field-induced ferromagnetic, and spin-glass behavior. The temperature-dependence of the magnetic entropy change, ΔS=S(H)−S(H=0), exhibits extreme values: a minimum at the superparamagnetic-to-ferromagnetic transition and a maximum at the ferromagnetic-to-spin-glass transition. At 80 K, and for H=50 kOe, the entropy change is −0.029 kB per Fe atom in the alloy. This value compares favorably with that measured in other crystalline and amorphous Fe-based alloys, but it is lower than that measured in some rare-earth elements.

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