N. Harish Kumar

Enhanced critical current density due to flux pinning from lattice defects in pulsed laser ablated Y1-xDyxBa2Cu3O7-δ thin films

The effect of lattice defects at the unit cell level created by the stress field when two rare earths with different ionic radii are mixed at the rare earth site, on the flux pinning and critical current density in Y1-xDyxBa2Cu3O7-δ thin films prepared by the pulsed laser deposition technique, was investigated using SQUID magnetometry at different temperatures for x = 0 to 1 in steps of 0.2. From the isothermal magnetic hysteresis recorded up to 5.5 T at 5, 35 and 77 K, the critical current density, pinning force density and irreversibility fields are estimated and their variation as a function of x analysed. An inverse relation between current density and irreversibility field is observed at all the temperatures except for the film having a 20% concentration of Dy in Y which also exhibits the highest critical current density among the films investigated, suggesting this to be the optimum composition for effective flux pinning from stress field induced lattice defects.

Infiltration-growth processing of superconductor

Melt processing of (Sm-123) is done by infiltrating liquid phases into an Sm-211 preform and allowing the growth of Sm-123 to take place. The melt-textured samples are found to be free of macrodefects, and the microstructure contains uniformly distributed fine-sized Sm-211 particles. The behaviour of the critical current density of the samples in high fields is reported here.

Dc magnetic properties of Mn doped YBa2Cu3O7-δ superconductor

Dc magnetization measurements were carried out on YBa2(Cu1-xMnx)3O7-δ samples with x taking values 0, 0.01, 0.015, 0.02, 0.025, 0.035 and 0.05, YBa2(Cu0.975Fe0.025)3O7-δ, YBa2(Cu0.975Ni0.025)3O7-δ and YBa2(Cu0.975Zn0.025)3O7-δ samples up to a maximum field of 5 T at temperatures 5, 50 and 77 K. The effect of Mn on the zero field critical current density, Jcg(0), estimated from the remnant magnetization, in general, is a monotonic suppression up to a concentration of 2.5% Mn and shows a saturation beyond that concentration. The field and temperature dependences of 2.5% Mn doped YBa2Cu3O7-δ samples were better than those of the Ni, Fe and Zn at the same doping level. Anomalous increases in Jcg(0), the indicative lower critical field, H*c1g, and the bulk flux pinning force, Fp, observed in the 2.0% Mn doped sample, at 50 and 77 K, have been attributed to the matching of the Abrikosov vortex density generated over these temperatures and the defect induced number of pinning sites due to the Mn doping.

The hydriding behaviour of U(Fe1−xNix)Al system (0≤x≤0.75) and magnetic studies on U(Fe1−xNix)AlH0.8

Abstract The hydriding behaviour of the solid-solution series of compounds, U(Fe 1− x Ni x )Al, has been investigated. In the parent pseudoternary compounds, as Ni is gradually substituted for Fe, the magnetic correlations grow. As a result, the magnetic properties change from exchange enhanced Pauli-paramagnetic and spin fluctuating ( x =0), via weakly magnetic with no long range magnetic ordering down to 4 K (0 x ≤0.3) to ferromagnetic (0.35≤ x ≤0.75) and eventually to antiferromagnetic for x ≥0.9. The present hydriding report covers compositions up to the ferromagnetic regime. It is found that hydrogen absorption does not take place for x below ≅0.7. The lowest nickel containing composition, which forms a well defined hydride phase, viz., U(Fe 0.3 Ni 0.7 )Al, absorbs y =0.8 H atoms per formula unit. This matches exactly with the lower hydride phase of pure UNiAl. No other hydride phase is formed either above or below y =0.8. The magnetization studies show that U(Fe 0.3 Ni 0.7 )AlH 0.8 has much higher values of the magnetic ordering temperature ( T C ), paramagnetic Curie temperature ( θ P ) and the uranium magnetic moment, relative to the unhydrided composition. These findings are in consonance with the large increase in the 57 Fe Mossbauer isomer-shift value, on hydriding. It is suggested that the electron charge transfer from H to the Fe 3d band (inferred from the Mossbauer studies) weakens the 5f–3d hybridization, thus enhancing the ferromagnetic correlations. The observed large increase in the a -axis cell parameter also implies a reduction in U–U hybridization, further justifying the observed increase in T C , θ P and U moment values.

Origin of peak effect in the magnetization hysteresis loop of melt-processed REBa2Cu3Oy superconductors

Melt-processed Sm-123 samples with an excess Sm2BaCuOy (Sm-211) phase is studied by transmission electron microscope (TEM) and superconducting quantum interference device magnetometer, to explain the origin of the peak effect exhibited in the magnetization hysteresis loops of light RE-123 samples in intermediate fields at 77 K. TEM images of the sample showing peak effect reveal dilute concentrations of areas with ortho-II structure distributed on a nanometer scale within the twin regions of the sample. The ortho-II structure is an oxygen deficient 123 with a lower Tc than the bulk of the material and will be a source of flux pinning by turning normal in high fields, thus contributing to the peak effect. With an increase in Sm-211 concentration, the peak effect disappears from the hysteresis loop, as does the ortho-II structure in the TEM images.

X-ray absorption studies of RRhAl (R ¼ La and Ce) compounds

Abstract The compound CeRhAl is neither magnetically ordered nor superconducting down to 2 K and LaRhAl is superconducting at 2.4 K . We report X-ray absorption spectra at the L 3 (2 p →5 d ) and M 5,4 (3 d →4 f ) edges of Ce and La in the above ternary intermetallic compounds. Based on this spectroscopic investigation, it is found that Ce shows mixed valent nature and La is in trivalent state.

Effect of oxygen-induced disorder on magnetic flux dynamics and critical current in a NdBa2Cu3O7-δ single crystal

The magnetic flux dynamics in a large (4.3×3.4×2.8 mm3) single crystal NdBa2Cu3O7-δ sample was investigated under fully-oxygenated and oxygen-deficient conditions using vibrating sample magnetometry (VSM) and ac flux profile measurements in the B∥c-axis configuration. Using a split coil VSM the anisotropy (Γ) was determined. From the equilibrium magnetization above the irreversibility field (Birr), the London penetration depth (λ) was calculated. While a near total absence of the fishtail effect and more or less a constant J (15 000 A cm-2) in the field range of 1-10 T at 77 K was exhibited by the fully-oxygenated sample, a pronounced fishtail effect with an increase in current with a peak value of 32 000 A cm-2 was observed after oxygen reduction. However, the irreversibility field (Birr), was reduced from 13.1 to 7 T at 77 K after oxygen reduction. A vortex melting transition is indicated by the fully-oxygenated sample, with a marked deviation from this behaviour for temperatures below 86 K. A dynamic phase shift was observed through the superconducting transition which varies with field and exhibits a peak with a maximum around the irreversibility field.

Effect of Sm2BaCuO5 addition on the properties of melt processed SmBa2Cu3Oy

Melt processing of SmBa 2 Cu 3 O y (Sm-123) is done in argon atmosphere by the solid liquid melt growth (SLMG) route, with increasing concentration of insulating Sm 2 BaCuO 5 (Sm-211) content. The effects of Sm-211 on the microstructure and on the behaviour of current density (J c ) in high magnetic fields, are discussed. It is observed that as the Sm-211 concentration is increased, the morphology of the trapped 211s change from acicular having a high aspect ratio to nearly spherical. The J c increases for low additions of 211, but for high concentrations, the J c drops drastically. The above data is explained on the basis of pinning from the 211 particles and the matrix.