Relaxation measurements in the regime of the second magnetization peak in Nb films

Abstract

We report on magnetic measurements as a function of field, temperature and time (relaxation) in superconducting Nb films of critical temperature T c = 9.25 K. The magnetic measurements as a function of field exhibited a “second magnetization peak” (SMP) which in general is accompanied by thermomagnetic instabilities (TMIs). The lines H smp( T) where the SMP occurs and the H fj( T) where the first flux jump in the virgin magnetization curves is observed, end at a characteristic point ( T 0, H 0) ≈ (7.2 K, 80 Oe). Relaxation measurements showed that for T < T 0 ≈ 7.2 K the activation energy U 0 and the normalized relaxation rate S exhibit non-monotonic behavior as a function either of temperature or field. The extrema observed in U 0 and S coincide with the points H on( T) or H smp( T) of the SMP. In the regime T > T 0 ≈ 7.2 K both U 0 and S present a conventional monotonic behavior. These results indicate that the SMP behavior observed in our Nb films is promoted by the anomalous relaxation of the magnetization.