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

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 Tsub(c) and Hsub(c2). Peaks in volume pinning force, Fsub(p), developed on ageing. The temperature dependence of these peaks in Fsub(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. It is shown that the direct summation model is not appropriate in this system. (Author)