Paramagnetic reentrance ofacscreening: Evidence of vortex avalanches inPbthin films

Abstract

We have studied the influence of a square array of pinning centers on the dynamics of vortex avalanches in Pb thin films by means of ac-and dc-magnetization measurements. Close to the superconducting transition T c , the commensurability between the vortex lattice and the pinning array leads to the well known local increments of the critical current. As temperature T decreases, matching features progressively fade out and eventually disappear. Further down in temperature, vortex avalanches develop and dominate the magnetic response. These avalanches manifest themselves as jumps in the dc magnetization and produce a lower ac shielding, giving rise to a paramagnetic reentrance in the ac screening Ј͑T͒. Within the flux-jump regime, two subregimes can be identified. Close to the boundary where vortex avalanches develop, the field separation between consecutive jumps follows the periodicity of the pinning array and a field-and temperature-dependent screening is observed. In this regime, the response also depends on frequency f in agreement with theoretical models for magnetothermal instabilities. At low enough temperatures and fields, the screening saturates to a constant value independent of T, H, and f, where jumps are randomly distributed. We have also found that vortex instabilities occupy a larger portion of the H-T diagram in patterned samples than in films without nanoengineered pinning sites. Finally, we discuss the possible origin of the vortex avalanches and compare our results with previous experimental and theoretical studies.

Vortex interaction enhanced saturation number and caging effect in a superconducting film with a honeycomb array of nanoscale holes

“…Experimentally, various techniques such as ac susceptibility measurements 7 and scanning Hall-probe microscopy 13 have been applied to determine the saturation number in a superconducting film containing an array of holes and the values were found to be consistent with those predicted for an isolated pinning defect. On the other hand, scanning tunneling microscopy (STM) 22 and Bitter decoration 23 imagings show an increase of the vorticity of the multi-quanta vortices in the hole after interstitial vortices were observed, revealing the effect of vortex-vortex interaction on the saturation number 16,19 .…”

“…18 for the extension of their work) established that the saturation number is given by n si ∼ = R/2ξ(T ) with ξ(T ) the temperature dependent coherence length. However, this expression can underestimate the saturation number for an array of holes where the interaction between vortices needs to be considered 7,16 . Indeed, careful analysis within the Ginzburg-Landau (GL) theory shows that 19 for an array of dense pinning centers the saturation number for a defect with a radius of the order of ξ becomes n sa ∼ [R/ξ(T )] 2 due to vortex-vortex interactions.…”

Dendritic flux penetration in Pb films with a periodic array of antidots

“…12,13 In the former case, the vortex avalanche regime manifests itself as a jumpy response of the magnetization, whereas in ac-susceptibility measurements the signature of the transition between the different flux penetration regimes is a local paramagnetic reentrance in the ac screening. In Fig.…”

“…A model system to investigate this effect can be realized by tailoring a periodic pinning array in superconducting samples. Vortex avalanches have been previously detected [10][11][12][13] in thin films with periodic pinning arrays. Global magnetization measurements [11][12][13] in samples with arrays of antidots have shown that the region in the H-T phase diagram dominated by flux jumps is more extended as compared to the case of plain films.…”

“…Vortex avalanches have been previously detected [10][11][12][13] in thin films with periodic pinning arrays. Global magnetization measurements [11][12][13] in samples with arrays of antidots have shown that the region in the H-T phase diagram dominated by flux jumps is more extended as compared to the case of plain films. Besides that, commensurability of flux jumps with the matching field of the pinning lattice and invariance of the magnetization and flux-jump size distribution at low temperatures and fields were observed.…”