D. de Klerk

Calculations on the reversible and irreversible magnetic behavior of low-kappa type-II superconductors

A simple algebraic expression for the reversible magnetization curve in low-κ type-II superconductors is proposed. This model has been checked on the experimental results of very pure Nb and V, as measured by several authors. A quite good agreement is found forT/Tc>0.4; for lower temperatures the agreement becomes worse, but this model still seems better than any other model known thus far from the literature. Using this relation together with a critical-state model it is possible to calculate the irreversible magnetization curves in increasing and decreasing fields betweenHc1 andHHcH2. The results are compared with the experimental data we obtained on a rectangular Nb sample. It turns out that Goedemoeds pinning model gives a good description. Also, a method is described to obtain the flux density gradient as a function ofB directly from magnetization measurements. This provides a more sensitive method to compare the experiments with critical-state models.

A pulsed magnetic field of half a million oersteds during 17.5 milliseconds

Abstract A pulsed magnetic field of 517 kOe with a duration of 17.5 milliseconds was obtained by discharging a condenser bank of 0.03 F, 3.5 kV through an air core coil with an inner diameter of 2 cm. The coil was wound from rectangular annealed copper wire and surrounded by a stainless-steel cylinder with a wall thickness of 2.5 cm. The accuracy of the field measurement was about one per cent.

Thermal conductivity of niobium in the purely superconducting and normal states

AbstractThe thermal conductivity λ of four niobium samples has been measured between 1 and 10 K, both in the superconducting and normal states. The specimens differed in their crystal defect structures due to annealing at different temperatures (dislocations, grain boundaries) and, in one case, to subsequent fast neutron irradiation (dislocation loops). A procedure has been developed with which the electron and phonon contributions to the thermal conductivity can be separated with an accuracy not hitherto obtainable. All the samples proved to have the same energy gap at 0K:δ(0)=(1.95±0.02)kTc. The phonon conductivity in the superconducting stateλps has been compared with the formula of Bardeen, Rickayzen, and Tewordt extended for scattering mechanisms other than phonon-electron interaction. For the unirradiated samples at

MAGNETIC PERMEABILITY OF TYPE-II SUPERCONDUCTORS.

{\text{T}} \lesssim 0.15T_{\text{c}}

The influence of the physical lattice imperfections on a type II superconductor

, λps is proportional toT2, showing that dislocations are mainly responsible for the phonon scattering. The results are qualitatively in agreement with the theory of Klemens, giving a rough indication that the grain boundaries may be considered as arrays of line dislocations. Dislocation loops introduced by the neutron irradiation turn out to behave like clusters of point defects. A second consequence of the irradiation is an enhancement of the original dislocation scattering term.

The magnetic diagram of state of manganese formate dihydrate

Abstract Xeutron irradiation at reactor ambient temperature gives rise to a peak in the magnetization curve of superconducting niobium near the upper critical field HC 1. It is correlated to a maximum in the critical current density Jc and it is referred to as the ‘peak effect’. The magnitude depends strongly on the irradiation dose. The peak effect can be explained qualitatively by a theory proposed by Pippard but quantitatively the agreement is not very good. A modification of Pippards theory in which the pinning is assumed to be caused by point defects does not give any improvement, but a modification introduced by Campbell and Evetts, based on pinning by line defects, leads to results in very close agreement with our experimental data.

Phase transitions in an antiferromagnet with a hidden canting

A number of simple models are discussed for the magnetic permeability behavior of type-II superconductors. It turns out that the London-Bean model extended with a surface barrier as discussed by Ullmaier and by Wipf, though it gives a satisfactory description of the alternating current losses, fails as soon as the real part of the permeability is also taken into account. It proves to be possible, however, to give a different extension to the London-Bean model consisting of a surface layer without pinning. This model is in satisfactory agreement with the experimental data.

Influence of oxygen diffusion profiles on the surface barrier of superconducting niobium

Abstract The magnetic susceptibility of single crystals of CoBr2·6H2O was measured between 1.2 and 4.3 K in magnetic fields up to 110 kOe. Two different crystallographic directions have been investigated. Phase boundaries separating the antiferromagnetic, spin-flop and paramagnetic phases were found. With the help of the experimental results we could construct the full diagram of state.