Shift of the flux-pinning force curve in Nb3Sn thin films with very fine grain size

Nb₃Sn thin films were made by sputter deposition of Nb onto bronze substrates at various temperatures. Deposition at 660°C produced a Nb₃Sn reaction immediately, which resulted in films with 20-50 nm grains. The peak of F_p(H) for one of these films was at 0.35 to 0.38 H*, higher than the field where the standard flux-shear curve peaks, 0.2 H*. Deposition at 400°C produced Nb films that were later reacted to form Nb₃Sn films with 50-250 nm grains. These films also exhibited F_p(H) curves with peaks higher than 0.2 H*, but the shift was not as pronounced as for the rapidly formed films. The shift of the pinning force curve to higher fields is discussed in terms of the grain size difference and a recent flux-pinning model. All of the films exhibited an expanded Nb₃Sn lattice parameter and degraded superconducting properties, due to disorder and interstitial impurities