Identification of pinning centres in high T_c superconducting thin films by AC susceptibility

We compare vortex pinning properties of high temperature superconductors (HTS) thin films and correlate these to microstructure to identify pinning centres. We have grown YBa₂Cu₃O_7-δ (YBCO), YBCO/PrBa₂Cu_2.8Ga_0.2O _7-δ (YBCO/PBCGO), NdBa₂Cu₃O _7-δ (NBCO) thin films and superlattices on (100) SrTiO ₃. We show that pinning by intersections of twin boundaries usually dominates except for strained superlattices and for the NBCO thin films presenting only one [1, 1, 0] twin variant (the [1 1̄ 0] being completely eliminated), thereby drastically reducing the density of twin boundary intersections (TBI). As expected, AC susceptibility measurements show low pinning properties of these samples. Using High Resolution AC susceptibility, we have discriminated several mechanisms contributing to vortex pinning in a given sample. Indeed, when we eliminated TBI pinning in NBCO thin films, several peaks were observed in _χ(T) curve, each one presumably associated with a specific pinning mechanism.