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Abstract
It is shown that the peak effect, a peak in the curve of critical current [Jc ] versus magnetic field [H], is due to the existence of a narrow peak in the pinning force density F p just below Hc2. The J c of a Nb-Ta alloy, which was progressively deformed by rolling to greater plastic strain, has been measured as a function of H. It is observed that plastic deformation shifts the peak in Fp to lower reduced field h, broadens it and increases it in height without markedly increasing Fp on the high-h side of the peak. The broadening of the peak in F p(h) with deformation gradually transforms the peak in Jc (h) into a plateau, and finally Jc(h) becomes a monotonically decreasing function of h. The results are in quantitative agreement with the predictions of a recent model of flux pinning in hard superconductors. It appears certain that the peak in Fp (h) in materials which exhibit the peak effect is caused by the same mechanism that causes the peak in Fp (h) at h × 0.3 to 0.5 in commercial superconductors.
