NbZr multilayers. I. Structure and superconductivity

Multilayers of NbZr have been made by the process of magnetron sputtering. The samples were prepared with modulation wavelengths $\Lambda$ ranging from 200 Å down to 4 Å. The structural properties have been studied using x-ray scattering. The composition wave constructed from the x-ray scattering shows that the as-deposited square wave is not square but rather the Zr component has partially diffused into the Nb component. The Zr is believed to substitute on Nb sites and expand the lattice constant of the Nb layer. The compositional amplitude increases above $\Lambda =31\mathrm{}$ Å and decreases below $\Lambda =31\mathrm{}$ Å. A composition modulation was detected by x-ray scattering as low as $\Lambda =13\mathrm{}$ Å. Above $\Lambda \ge 31$ Å the Nb is bcc with the [110] along the growth direction and the Zr is hcp with the [002] along the growth direction. The $T_c$ in this region decreases linearly as $\Lambda$ increases. In the region $\Lambda \le 31$ Å the Zr undergoes a structural phase transition to form a bcc lattice that is coherent with the Nb lattice. Once the multilayer becomes coherent there is a sharp increase in the superconducting transition temperature with decreasing $\Lambda$. This increase in $T_c$ approaches 9.7 K which is the value for a 50 at.% solid solution of NbZr. A trilayer model is applied in the Cooper limit to explain the critical temperatures. This model implies a superconducting coherence length of approximately 100 Å.