Surface pinning and Lorentz force dependence of transport critical current in NbTa thin films and heterostructures

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Abstract
The critical current densities of NbTa and Ta/NbTa/Ta thin films have been measured as a function of the orientation of an applied magnetic field with respect to both the current and the film surface. It is shown that appropriate control of the deposition conditions and the film composition allow the surface pinning and Lorentz force dependence to be reliably and reproducibly altered. Comparison of the results with those reported elsewhere for high Tc films shows broadly similar behavior in the two systems. In particular, we show that surface pinning results in a (sin φ)−1/2 dependence of Jc(φ) for B perpendicular to current, similar to the dependence attributed to intrinsic pinning in high Tc systems. The results also confirm that a lack of Lorentz force dependence of transport critical current is a characteristic of a two dimensional pinning system.