Long columns and vortex mobility: a case study in NBCO films

NdBa2Cu3O7-δ (NBCO) thin films (2000 A) grown by excimer KrF laser were irradiated with Au+13 (200 MeV) ions at doses equivalent to 'matching field' ranging from 1 T to 4 T. Atomic force microscope (AFM) and scanning electron microscope (SEM) studies show significant topological changes consequent to heavy ion irradiation. Magnetization measurements were done with a SQUID magnetometer at different ambient temperatures. There is almost a four-fold increase in Jc, at the optimum ion dose, indicating a robust vortice pinning. The emergence of a diffused peak at the 'matching field' value in the optimum flux data is a salient feature. We explain the morphological changes in the framework of existing models. We also suggest that the overriding peak in Jc is the signature of a process similar to the self-organized criticality of a sand pile - a common occurrence in nature. (author)