Paramagnetic reentrance of the ac-screening: Evidence of vortex avalanches in Pb thin films

TL;DR

This study investigates vortex avalanches in Pb thin films with engineered pinning arrays, revealing temperature-dependent magnetic responses, vortex instabilities, and a paramagnetic reentrance in ac-screening linked to vortex dynamics.

Contribution

It provides new insights into vortex avalanche behavior in patterned superconducting films and demonstrates the influence of nanoengineered pinning on vortex stability and magnetic response.

Findings

Vortex avalanches cause a paramagnetic reentrance in ac-screening.

Matching effects fade with decreasing temperature, leading to vortex avalanches.

Patterned films show larger vortex instability regions than unpatterned ones.

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 Tc 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 x'(T). Within the flux jump regime two subregimes can be identified. Close to the boundary where vortex avalanches develope, the field separation between consecutive jumps follow 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.