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 Tc = 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 (To, Ho) ≈ (7.2 K,80 Oe). Relaxation measurements showed that for T < To ≈ 7.2 K the activation energy Uo and the normalized relaxation rate S exhibit non-monotonic behavior as a function either of temperature or field. The extrema observed in Uo and S coincide with the points Hon(T ) or H′′ smp ′′ (T ) of the "SMP". In the regime T > To ≈ 7.2 K both Uo 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.

Degradation of the remanent ferromagnetic state under the action of ferroelectric relaxation processes in Co/(1−x)PMN-xPT/Co hybrids: Possible implications on cryogenic and room-temperature applications

“…An avalanche event is observed when thermal activation motivates overcritical conditions and drives abrupt movement of bundles of flux lines that ultimately appears in the form of magnetization jumps. [27][28][29][30][31][32] As we show here, Figures 4(a)-4(c), in our case the observed MIs were exclusively motivated by the externally applied electric field, E ex thus we coin the term electric fieldinduced MIs (EMIs). Specifically, from Figure 4(a) it is evident that EMIs do not exist for E ex ¼ 0 kV/cm since only a gradual increase of m rem (T) was observed upon decreasing the temperature.…”

“…Referring to the flux-lines avalanche effect, 27 it is associated with the interplay of three energy scales, the interaction energy between flux lines, the pinning energy of flux lines with quench disorder, and finally the thermal disorder. [27][28][29][30][31][32] It is well known that during the entrance of flux lines in a superconductor they selfassemble in a critical state 33,34 so that the current reaches a critical value, at the cutting edge just before movement of flux lines takes place. An avalanche event is observed when thermal activation motivates overcritical conditions and drives abrupt movement of bundles of flux lines that ultimately appears in the form of magnetization jumps.…”

Origin of magnetic flux-jumps in Nb films subject to mechanical vibrations and corresponding magnetic perturbations

“…The observation of flux avalanche activity at higher magnetic fields enables a more detailed investigation of corresponding dependencies on B a and/or measurement parameters. The measurements were performed at T = 2.5-3 K since the range of magnetic fields over which flux avalanches occur rapidly decreases at higher temperatures [14,16,17].…”

“…In our case, they correspond to the onsets of the critical state of the superconductor as discussed in Refs. [14][15][16][17]35].…”

“…Although, generally, it is believed that the flux avalanches observed in Nb films during magnetization measurements have a TMI nature [14][15][16][17], a precise mechanism of these flux avalanches is debatable. At the same time, magneto-optical imaging (MOI) reveals dendrite flux avalanches in superconducting Nb thin films.…”