Enhancement of the critical current by grain size refinement in Ta-cosputtered NbN thin films

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Abstract
NbN thin films cosputtered with Ta show approximately an order of magnitude enhancement in critical current density over a wide range of applied magnetic fields, while the critical fields and transition temperatures are essentially unchanged by the Ta addition. Both thin-film x-ray diffraction and cross-sectional transmission electron microscopy analyses show a highly textured structure with a second phase (also highly textured) present in these films at the grain boundaries. The Ta acts to refine the grain size of the NbN, increasing the effective pinning strength and hence the critical current density. Comparison of the global pinning force to current models of flux pinning supports individual pin breaking as the limiting mechanism for critical current in NbN films.