The peak effect, summation problem, and magnetic history in a superconducting Nb-38 at.% Hf alloy

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Abstract
Fluxoid pinning has been re-examined in the model system Nb-38 at.% Hf which contains small coherent or semi-coherent normal precipitates as the major pinning centres. Since the growth of the precipitates has been found to be a coarsening reaction, the pinning strength of the system could be continuously increased by increasing the precipitate size without accompanying changes in the fundamental superconductivity parameters T c and HC2. Peaks in volume pinning force, F p, developed on ageing. The temperature dependence of these peaks in F p indicated that the pinning mechanism was not a simple matching of fluxoid lattice and precipitate spacing. The scaling laws were not observed. Large magnetic history effects were observed, the occurrence of which appears to be related to the relative ‘softness’ of the fluxoid lattice. The results of the superconducting and microstructural data are compared with summation models. Plots of the specific pinning force, Q, versus the elementary interaction force, f p, indicate that the local statistical summation model provides a good fit to the data although the criterion of low pin density is not fulfilled and the pinning threshold criterion is not met. The direct summation model is clearly not appropriate in this system.

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