  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.
