Flux pinning by thin chromium layers

The flux pinning strength and upper critical field have been measured in multilayered evaporated films composed of thick layers of polycrystalline Pb-18% Bi alloy periodically interleaved with much thinner layers of chromium. The flux pinning strength of a single Cr layer with the applied magnetic field parallel to the layers is estimated to be at least 180 N/m² at a reduced field ofh=H/H_c2=0.7, and is independent of Cr layer thickness. The parallel flux pinning strength drops sharply, however, if the layer spacingd₁ is reduced to less than 60 nm. The upper critical field with the applied field normal to the layers is slightly elevated for larged₁, but is strongly depressed ford₁<100 nm; the parallel upper critical field is independent ofd₁.This work was supported financially by the U.S. Air Force Office of Scientific Research through grants AFSOR-77-3107 and F44620-74-C-0019. This work benefited from the use of the facilities of the Materials Science Center at Cornell, which is funded by the U.S. National Science Foundation.