C.A.M. van der Klein

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.

Peak effect in superconducting niobium, induced by neutron irradiation

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.

MAGNETIC PERMEABILITY OF TYPE-II SUPERCONDUCTORS.

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.

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

Abstract Magnetic properties and resistive transitions were investigated on five samples of niobium which, after plastic deformation, were heat treated at different temperatures up to 1600°C. Electron micrographs show the decreasing dislocation density with increasing temperature of the heat treatment. The magnetization curves show increasing reversibility with increasing temperature. The susceptibility curves of samples heat treated up to 1200°C can be interpreted by means of a model based on surface effects only. Susceptibility curves of samples heat treated at higher temperatures, however, show that here bulk effects also play an essential role. Here the χ″(χ′) plots are asymmetric. Also the resistive transitions can be interpreted by different models.

The effect of neutron irradiation damage on the magnetic behavior of superconducting niobium in alternating fields

The effect of irradiation with fast neutrons (at reactor ambient temperatures up to doses of 1.5×1020 n/cm2) on the magnetic behavior in stationary fields of superconducting niobium samples has been studied. Surface effects have been removed by oxidation of the samples. Irradiation leads to an increase ofHc2, which disappears after annealing the samples at 400°C. Also, the irreversibility of the magnetization increases and a peak effect is induced in the magnetization curves. These effects, due to an increased bulk pinning, are rather small, except in the samples irradiated with the highest dose. Electron microscope pictures showed that this is due to the fact that only for the highest dose do the clusters of interstitials which are introduced in the sample by the irradiation form extended defects, with sizes larger than the coherence length ξ. A discussion is given of te surface barrier before and after irradiation, of theHc2 shift due to the irradiation, and of the increased irreversibility of the magnetization. It turns out that nearHc2 the irradiation damage has to be treated as line defects, rather than as point defects, whereas at lower fields neither the critical state formula of Labusch for point pinning nor that of Good and Kramer for line pinning describes the experimental results exactly, although the differences do not seem to be very dramatic. We conclude that in the lower field region a combination of point and line pinning occurs. The peak effect has been discussed in a foregoing paper.

Flux Pinning in Type II Superconductors

Experiments were carried out on the effect of flux pinning in type II superconductors. Several pinning mechanisms were investigated: surface pinning, bulk pinning due to grain boundaries, dislocations (caused by cold-working), and clusters of point defects (caused by neutron irradiation). The quantities determined in the experiments were the dc magnetization (or average flux density), the ac permeability components µ′ and µ″, and the heat conductivity.