Vortex activation energy in the AC magnetic response of superconducting YBa₂Cu₃O₇thin films with complex pinning structures

Abstract

The vortex activation energy UAC in the AC magnetic response of superconductors exhibits a logarithmic variation with the screening current density J (regardless of the pinning structure details), and takes surprisingly high values in the vicinity of the DC irreversibility line, especially at low external DC magnetic fields, as often reported. This is essentially different from the behaviour of the vortex-creep activation energy at long relaxation time scales in DC magnetic measurements, and is not completely understood. We investigated the DC relaxation and the AC response for YBa2Cu3O7 films containing nanorods and nanoparticles, with the DC and AC fields oriented perpendicular to the film surface. It is shown that the large UAC values in the vicinity of the DC irreversibility line, where the critical-state-related AC signal occurs, are generated by a non-diffusive vortex motion during the AC cycle, with the mean vortex hopping length longer than the average distance between the pinning centres. In these conditions, the smearing of the vortex pinning potential by thermally induced vortex fluctuations is weak, and UAC mainly results from the strong influence of the pinning-enhanced viscous drag on the vortex hopping process. The logarithmic UAC(J) dependence is consistent with a high UAC.

Vortex dynamics driven by AC magnetic field in YBCO thin films with complex pinning structures

“…As might be seen for and a plastic creep process similar to nonuniform single vortex creep dominates de flux dynamics [17,18]. In this scenario, it is known that for a magnetic relaxation curve at given H dc and T, U * (J)=U c (J c0 /J) p [10], so the creep exponent p can be obtained from the linear fit of U * versus 1/J in log-log scale which in our case provides p~−1 (see the inset to figure 3). For a better understanding of our data, specifically, the (H, T)-range relative to the IL (the line in H-T diagram that separates the vortex liquid region from the region where the vortices are pinned), we proceeded to build the IL from the plot of the onset of third harmonics m T H , 3 dc ¢ ( )extrapolated at f=0 for h ac =0.5 Oe.…”

“…To obtain information about the flux dynamics and pinning potential we used an AC susceptibility technique which will be described below. It is known that the in-phase component of the AC magnetic moment m′ is proportional to the screening current density J, whereas the out-of-phase component m″ is a measure of dissipation and has a maximum at the peak temperature T p where the flux front reaches the sample centre [10]. For a thin disk in a perpendicular field, the critical current density J c , at T p where the sample is fully penetrated, is given by [12]…”

“…In this case, the slope n=U c /T provides the characteristic pinning energy U c =940.5 K. As it will be shown below, this value is close to the value obtained from the slope of J(t) dependence in log-log scale. For comparison, in the case of the classic vortex creep which occurs during the relaxation of the DC magnetic moment m, the effective vortex activation energy obeys a different law [1,10]:…”

“…Finally, following the manner described in [10], with U eff (J, 0 K) obtained from ln( f ) versus 1/T p plot, the value of the effective pinning energy at T=T p ( f max ) can be estimated using the relation U eff (J, T)=U eff (J, 0 K)(1 − T/T c ).…”

“…In a previous work [10] we have investigated the AC magnetic response of the same YBa 2 Cu 3 O 7 (YBCO) film and we have found high U eff values even the data were collected in close proximity to the DC irreversibility line (IL).…”