F.M. Sauerzopf

On the calculation of Jc from magnetization measurements on superconductors

Abstract A simple but reliable procedure to evaluate J c ( B ) from magnetization measurements has been developed. It applies to the fully penetrated state and to situations where the flux density gradient has reversed completely. In an approach consistent with the Bean model, a uniform current density is assumed to flow throughout the entire sample being correlated to the corresponding average magnetic induction in the sample. In combination with the anisotropic Bean model, we present evaluations of J c ( B ) for a typical single crystalline high-temperature superconductor.

Magnetization of Bi2Sr2CaCu2O8 single crystals: Field-induced diamagnetism above Tc, anisotropic irreversibility lines, and reversible mixed state properties

Abstract We report on magnetization measurements made on Bi 2 Sr 2 CaCu 2 O 8 single crystals with an 8T-SQUID magnetometer. The results confirm the existence of a broad field range with completely reversible characteristics for all temperatures above ∼ 15 K and for both field orientations. This is used to determine the penetration depths ( λ ab (0)=860 nm, λ c (0)=390 nm) as well as the thermodynamic critical field ( μ 0 H c (0)=231 mT). Detailed experiments near T c showed a large rounding of the transitions, which increased with increasing field and led to the observation of field-induced diamagnetism above T c . Attempts to evaluate H c2 in the sense of a mean field theory fail. The irreversibility lines are found to be anisotropic and occur at higher fields and temperatures for H ∥ a , b .

Radiation effects on lead-containing perovskite ceramics

Abstract Optical spectra and dielectric parameters are used to study PLZT X/65/35 (X=4.5−11 at.% La) and PSN ceramics subject to gamma- electron-, and neutron irradiation. In neutron-irradiated ceramics stable defects, maintained at annealing up to 700 °C are manifested by a gradual shift of extinction edge near the fundamental absorption to longer wavelengths and by reduced polarization values. The thermal dependence of the extinction edge in reference samples of PLZT and PSN has been determined to be ca. 0.10 nm/deg (0.9 meV/deg) and 0.125 nm/deg, respectively in the range 20-500°C. In PLZT 6.5-8/65/35 ceramics a defect enforced phase transformation from the initial rhombohedral structure to “quasicubic” with essential broadening of the phase transition takes place.

Magnetization of neutron irradiated YBa2Cu3O7-δ single crystals☆

Abstract YBCO single crystals with very sharp transitions at T c were subjected to reactor neutron irradiation at ambient temperature. The magnetization curves, which were measured in a SQUID magnetometer in fields up to 8 T, show increasing irreversibility and decreasing anisotropy with neutron fluence. Bean-model evaluations of critical current densities are presented and discussed in terms of induced flux pinning centers.

Magnetization of low-κ superconductors I the phase transition at Hc1

Abstract Experimental and theoretical investigations of the magnetization in low-κ superconductors are reported. From experiments on a series of Ta, Nb, V and LaAl 2 superconductors doped with varying amounts of impurity atoms, the phase boundaries between type-I superconductivity, type-II superconductivity with a first order phase transition at the lower critical field H c1 and “normal” type-II superconductivity could be established experimentally and compared with theory. The analysis in terms of weak-coupling isotropic, strong-coupling isotropic and strong-coupling anisotropic theories demonstrates clearly that the location of the phase boundaries is determined primarily by anisotropy effects with only contributions of coupling corrections. The magnitude of the discontinuity at H c1 , on the other hand, is affected by both coupling and anisotropy effects in nearly the same way. In all cases, the validity of the Ginzburg-Landau theory for temperatures near T c is confirmed with high accuracy.

Anisotropic shift of the irreversibility line by neutron irradiation

The irreversibility line of high-Tc superconductors is shifted considerably by irradiating the material with fast neutrons. The anisotropic and non-monotonous shift is qualitatively explained by a simple model based on an interaction between three pinning mechanisms, the intrinsic pinning by the ab-planes, the weak pinning by the pre-irradiation defect structure, and strong pinning by neutron induced defect cascades. A correlation between the cascade density and the position of the irreversibility line is observed.

Fast neutron irradiation effects on magnetization relaxation in YBCO single crystals

A high-quality YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} single crystal has been investigated by torque magnetometry prior to and following fast neutron irradiation to a fluence of 2{times}10{sup 21} m{sup {minus}2} (E > 0.1 MeV). In addition to large enhancements of the critical current densities, which have been observed in similar form previously by Sauerzopf et al, we find a dramatic change in the relaxation behavior following irradiation. At low temperatures ({le} 50 k) the relaxation rates are lowered by factors up to 4 in the irradiated state in a magnetic field of 1.5 T. At higher temperatures, on the other hand, they are enhanced compared to the unirradiated state. Both before and after irradiation, the magnetization relaxation follows a logarithmic time dependence, which we ascribe to thermally activated flux motion.

Radiation damage of PLZT and PSN ceramics

Abstract Selected optical (absorption coefficient), dielectric (polarization) and electrooptical (quadratic electrooptical coefficient) parameters of PLZT X/65/35 and PSN ceramics exposed to various radiation environments (γ-rays, electrons, neutrons) have been studied. The observed property changes depend on the type of radiation, the dose and, in the case of the dielectric properties, on the state of sample polarization. Special features observed upon annealing the irradiated samples are discussed.

Flux pinning enhancement by twin boundaries

The authors have measured the magnetization of single crystal YBa2Cu3O7- delta with all the twin planes orientated in only one direction. By rotating the magnetic field in the (ab)-plane, they find significant enhancements of the magnetization (up to a factor of 4.6), when the field is directed parallel to the twin boundaries. No angular dependence of the magnetization in the (ab)-plane could be detected at temperatures exceeding approximately=40 K.

Critical currents in neutron irradiated YBCO and BiSCCO single crystals

Neutron irradiation of high-Tc single crystals in the fluence range up to 2*1022 m2 (E>0.1 MeV) has been found to lead to the following property changes: 1. The critical temperature decreases only moderately at a rate of approximately 4 K per 1022 neutrons per m2; 2. The critical current densities Jc as deduced from magnetization measurements on the basis of an extended Bean model increase significantly; 3. These Jc-enhancements are larger at higher temperatures; 4. The anisotropy of Jc decreases significantly with neutron fluence; and 5. The irreversibility line in BiSCCO-2212 is shifted to much higher fields and temperatures.