Mixed-state properties of superconducting Nb/Ni superlattices

Abstract

Superlattices of ( Nb 10 nm / Ni t) 8 (1⩽ t⩽5 nm) have been grown on Si(1 0 0) by magnetron sputtering. Samples show textured growth of Nb(1 1 0) and Ni(1 1 1), with negligible interdiffusion and interface roughness below 0.3 nm. The critical superconducting temperature ( T cs) of these superlattices decreases as the Ni thickness ( t Ni) increases. Ferromagnetic order in Ni layers is suppressed for t Ni<2.2 nm. We have explored the behavior of these superlattices in the mixed state by measuring the upper critical fields. Critical fields in perpendicular ( H c2⊥) and parallel ( H c2∥) configurations show moderate superlattice-induced anisotropy values, which increase as t Ni increases. From H c2( T) measurements, we have found that superconducting superlattices behave like a three-dimensional system. This dimensional behavior is discussed in terms of coupling of Nb through Ni layers.