Critical current density characteristics in YBCO films on MgO substrates by pulsed laser deposition

It is known that both low angle grain boundaries and linear dislocation defects act as strong flux pinning centers in YBa₂Cu₃O_7-x (YBCO) thin films achieving high critical current density J_c. In this experiment, in order to effectively introduce such functional pinning centers, YBCO thin films were fabricated on (100) MgO substrates with a large lattice mismatch of 8.8% by PLD method with varying the substrate temperatures. By AFM studies of the surface morphology after a short-time etching by a 1% Br/ethanol solution, both submicron-sized large etch pits and very small dotted etch pits are clearly observed. It is speculated that small etch pits correspond to edge or screw dislocations and large pits are thick plate-like crystal defects like high angle grains or a-axis oriented domains. It was observed that J_c was independent of B up to characteristic field B^*. According to the single vortex pinning, the effective pinning densities N_eff and the average of the elementary pinning force f_pav were estimated from B^*. N_eff approximately coincide with the etch pit densities, and f_pav is the same order as f_p estimated using normal conductive pin model. Thus, it is almost certain that both linear defects and thick plates act as effective pins up to B^*. On the other hand, in higher fields, the bulk pinning force density varies as B^0.5, consistent with flux pinning model where surface area of the thick plates is dominant pinning centers.