Critical fields of multilayered films of Al and Ge

Films consisting of multiple alternating layers of Al and Ge were fabricated with variable layer spacing, and the superconducting properties measured as a function of the layer spacing. The critical temperatures followed the theory of Garland as formulated by Petitt and Silcox with $\beta =0.26\mathrm{}$ and $k=20\mathrm{}$ Å. The critical fields parallel to the layers varied as ${\mathrm{}(1-t)\mathrm{}}^{\frac{1}{2}}$ with extrapolated zero-temperature values similar to a single film with the thickness of one Al layer. The critical fields perpendicular to the layers showed a marked positive curvature with respect to temperature at high reduced temperatures. These findings are discussed. A theory by Gray and Schuller can be used to predict quite accurately the grain size in the layers from the degree of positive curvature, but this theory does not account for the lack of positive curvature in parallel fields.