The influence of plastic deformation on the peak effect in a type II superconductor

Abstract

It is shown that the peak effect, a peak in the curve of critical current [J_c ] 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 F_p to lower reduced field h, broadens it and increases it in height without markedly increasing F_p on the high-h side of the peak. The broadening of the peak in F _p(h) with deformation gradually transforms the peak in J_c (h) into a plateau, and finally J_c(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 F_p (h) in materials which exhibit the peak effect is caused by the same mechanism that causes the peak in F_p (h) at h × 0.3 to 0.5 in commercial superconductors.