Pinning Mechanism Studies on the Nb/Al2O3 Superconducting Multilayer

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Abstract
The transport critical current J c of the Nb/Al2O3 superconducting multilayer films has been measured at 4.2 K and 1.5 K under both the parallel and the perpendicular magnetic fields to the film plane. From the obtained J c values, the pinning force density F p in the parallel (F p//) and the perpendicular (F p⊥) directions is estimated. F p⊥ enhancement is drastic in comparison with F p// but F p// itself is also remarkably enhanced by the multilayer structure. The pinning mechanisms are discussed in reference to the pinning function curve F p//,⊥ (h). It has been found that for F p⊥ of multilayers showing quasi-two-dimensional superconductivity the temperature scaling law breaks down. F p// of all multilayers roughly obeys the scaling law and shows some characteristics of usual core pinning.