K. Heinemann

Superconducting, structural and magnetic properties of Y1Ba2(Cu100−z−Fez)0.03O7−δ

The superconducting, structural and magnetic properties of YiBa 2 ( Cu 100− z − Fe z ) 0.03 O 7−δ (O ⩽ x ⩽ 18) were determined by resistive and magnetic measurements as well as by X-ray diffraction experiments and thermogravimetrical analysis. Four Fe-concentration regimes can be distinguished: Below x = 2 T c remains constant, above 2.3 % Fe contents T c decreases linearly with x, first with 5.4 K jat% Fe and above x 10 with 9.0 Kiat% Fe. Above 15 at% Fe no superconductivity could be detected. This behaviour of T coincides with changes in the structure and the oxygen contents of the Y 1 Ba 2 ( Cu 100− x − Fe x ) 0.03 O 7−δ . Below 2.3 at% the orthorhombic perovskite structure is retained. At larger Fe concentration a tetragonal structure is attained with a = b /3 below x = 10 and with a = b = c /3 above x = 15. Between 10 and 15 at% Fe c /3 approaches the value of a = b . Magnetic measurements show an anomaly below T c , which indicates antiferromagnetic ordering. The magnetic ordering temperature increases with Fe concentration.

Effects of Re-doping on microstructure and superconductivity of HgBa2Ca2Cu3O8+δ

Abstract In order to study the solubility of Re in HgBa2Ca2Cu3O8+δ and the influence of Re-doping on microstructure and superconductivity, samples of Hg1−xRexBa2Ca2Cu3O8+δ with x=0.05, 0.10, 0.15, 0.20, 0.25 and 0.30, have been synthesized. Grain size and shape change with nominal Re content while the intra-grain microstructure which is dominated by stacking faults remains the same. Both, EDX and X-ray diffraction (XRD) measurements suggest a change of solubility of Re in HgBa2Ca2Cu3O8+δ at about x=0.10–0.15 towards lower solubility. For x≥0.15 a Re-rich secondary phase appears. The critical temperature, Tc, of the Re-doped samples is above 130 K and the magnetic hysteresis, ΔM, shows a maximum for x=0.15.

Microstructure of melt-textured YBa2Cu3O7-δ

Abstract The microstructure of stoichiometric Bridgman melt-textured YBa 2 Cu 3 O 7-δ was investigated by transmission electron microscopy and electron diffraction. Crystal defects like twins, dislocations, small second-phase inclusions and lattice distortions are observed. The density of the dislocations is extremely high (1.7 × 10 10 / cm 2 ) and the average diameter of the second-phase inclusions (BaCuO 2 ) of 35 nm is rather small. The density of the inclusions amounts to 6 × 10 19 / m 3 . The formation of the microstructural defects and their influence on flux pinning is discussed. It is supposed that the observed lattice distortions are possible defects which cause the fish-tail effect.

Microstructure and superconducting properties of Bridgman-melt-textured Y1Ba2Cu3O7-δ

Abstract The X-ray diffraction pattern (XRD) of Y 1 Ba 2 Cu 3 O 7−δ (YBCO), grown by a modified Bridgman method, exhibits a strong texture. The c -axis is found to be almost parallel to the temperature gradient. From measurements of the magnetization hysteresis loop and from ϱ( T, B ) measurements, the critical current density, j c , and the activation energy for thermally activated flux flow (TAFF), U O , were found to be about four times higher than in sintered bulk material. This is not only due to the strong texture which leads to better coupling between the grains, but also due to the presence of 10–60 nm fine precipitates with partially associated dislocations in the 123 matrix.

Critical current measurements in YBa2Cu3O7−δ/Sr,Fe samples

The critical currents in YBa 2 Cu 3 O 7−δ and Sr- (for Ba) and Fe- (for Cu) substituted samples were measured by 4-probe resistive measurements and magnetization experiments. The critical current density J c , is related..to the underlying microstructure, which differs for all three systems. The field dependence of J c , and the comparison of the V–I curves measured in the conventional and force-free configuration gives an indication on the effects which determine the critical currents in the superconducting state of sintered bulk samples.

Intragranular flux pinning mechanism in Fe and Ni doped YBa2Cu3O7−δ

Abstract The intragranular pinning properties of polycrystalline YBa 2 (Cu 1−x M x ) 3 O 7−gd with M=Fe,Ni and x=0.05 are determined by magnetization measurements. The volume pinning force is interpreted in terms of flux-line shear, i.e. F p is related to the on-set of dissipative flux-flow within weakly pinning channels in the superconductor. A quantitative analysis of our data yields a channel width comparable to the flux-line spacing a o . This indicates that single rows of flux-lines seem to move once the Lorentz force exceeds the flow stress of the flux-line lattice. Experimental evidence is given that the pinning of the flux lines outside the channels cannot be caused by twin boundaries.

Critical current density enhancement by phase decomposition of YBa2Cu4O8 into YBa2Cu3O7−σ and CuO

Abstract Bulk YBa 2 Cu 4 O 8 (Y-124) is prepared from YBa 2 Cu 3 O 7−σ (Y-123) and CuO by a powder-metallurgical method. The superconducting features of the Y-124, in particular critical current densities and activation energies, are measured resistively using a four-probe technique and magnetically using a Faraday magnetometer. In a second step the Y-124 is decomposed at high temperatures. The intragranular critical current density is measured at different annealing times, t A , in order to determine and discuss the characteristics of the j c ( t A ) curves.

Interaction of magnetic ions and superconductivity in YBa2Cu3O7−δ

Abstract The superconducting and magnetic properties of TBa 2 (Cu 1−x M x )O 7−δ with M=Fe,Ni and T=Y,Gd were studied. Ni and Fe cause a linear decrease of Tc with x, but for small concentrations of Fe, x 2 Cu 3 O 7−δ .

Microstructure of PtO2-doped melt-textured Y1Ba2Cu3O7-δ

Abstract Melt-textured Y 1 Ba 2 Cu 3 O 7-δ was processed by the VGF (vertical gradient freeze)-method. The precursor had an excess of 25mol% Y 2 BaCuO 5 (211) and 1.8mol% PtO 2 . j c of these samples is enhanced compared to melt-textured Y 1 Ba 2 Cu 3 O 7-δ with excess 211 but without PtO 2 doping. In order to understand this enhancement in j c , a detailed microstructural study was performed by transmission electron microscopy. The twin spacings in these samples are reduced to 50–100nm compared to twin spacings of 200–300nm in melt-textured Y 1 Ba 2 Cu 3 O 7-δ without PtO 2 doping. The PtO 2 doping leads to a reduced size (≤ 1μm) and an enhanced density (2×10 18 /m 3 ) of the 211 particles. Most of the 211/123 interfaces are almost clean and free of crystal defects. Only a few of them were surrounded by defects like stacking faults. Therefore, we conclude that the 123/211 interface is the dominant pinning center in the PtO 2 samples. The Cu required for the stacking fault formation near the 211 particles originates from the 211 inclusions as confirmed by EDX.

Phase formation and superconducting properties of NbAl thin films

Abstract Amorphous and crystalline thin Nb Al films were prepared in an UHV-sputter system from single metal targets. Films of different concentrations were deposited on sapphire substrates at room temperature. The structure of the films was detected by X-ray and scanning electron microscopy (SEM), including energy dispersive X-ray analysis (EDX). Measurements of the transition temperature Tc and critical transport current measurements in magnetic fields up to 15.5 T characterize the superconducting properties. The films were heat treated (220°C; 1 h) and the resulting effect on structure and superconducting properties was recorded. For the heat treated films a reduction of Tc and Bc2 as well as the volume pinning force Fp were observed. The field dependence of Fp, however, was similar for all films: Fp ∼ b(1−b)2.