Pin potential effect on vortex pinning in YBa₂Cu₃O_7-δ films containing nanorods: Pin size effect and mixed pinning

The pin size effect and mixed pinning of nanorods and matrix defects are discussed for YBa₂Cu₃O_7-δ films containing nanorods. BaSnO₃ nanorods with a diameter of 11 nm and BaHfO₃ nanorods with a diameter of 7 nm were prepared, and critical current density (J_c) and resistivity were measured in the films. When the coherence length was larger than the nanorod size at high temperatures near the critical temperature, the trapping angle and activation energy of the vortex flow depended on the nanorod diameter. At a moderate temperature of 65−77 K, the pin size effect on J_c disappeared since the coherence length became smaller than the nanorod size. At a low temperature of 20 K, the contribution from matrix pinning became comparable to that of nanorods in a high magnetic field due to the small coherence length. Thus, the temperature-dependent coherence length caused the pin potential situation to vary significantly, namely, the pin size effect and mixed pinning, which strongly affected vortex pinning in YBa₂Cu₃O_7-δ containing nanorods.