Commensurability and strong vortex pinning in nanopatterned Nb films

We present a study combining magnetization measurements and direct magneto-optical flux imaging on Nb films, which contain periodic arrays of nanoholes and have been grown on anodized aluminum oxide (AAO) substrates. The magnetization measurements reveal pronounced matching effects in the critical current over the entire temperature range from 4.2 K to Tc. The flux images do not reveal signatures of a domain (terraced) state. Instead, the flux profiles obtained from the images are consistent with the development of a conventional critical state with enhanced vortex pinning. Even though the critical current density in the patterned samples is largely enhanced as compared to the unpatterned reference samples, the self-fields corresponding to the critical state are substantially smaller than the first matching field, H₁. For this reason, a domain state does not appear, and strong pinning arises in a state that is almost commensurate.