MEASUREMENTS OF MAGNETIC SCREENING LENGTHS IN SUPERCONDUCTING NB THIN-FILMS BY POLARIZED NEUTRON REFLECTOMETRY

polarized neutron reflectivity measurements have been performed on two polycrystalline niobium films grown on silicon substrates. The sample s were characterized with x-ray diffraction and reflection, electrical resistivity, and unpolarized neutron reflection measurements. For the film of 310 nm thickness, polarized neutron reflectivity measurements were carried out on both the Si side as well as the vacuum side, and we found that substantially higher quality data could be obtained from the Si side due to the enhanced contrast between the weak diamagnetic scattering and the nuclear scattering from the films. A large number of interference fringes from the waves reflected from the front and ba ck surfaces of the film could be observed, attesting to the high quali ty and flatness of the sample. The vacuum-Nb interface had a surface r oughness of sigma similar to 3.4 nm, while the Nb-Si interface was nea rly atomically smooth. We also carried out an experiment on a 300-nm-t hick film of YBa2Cu3O7, but the roughness was so severe that no interf erence fringes could be observed, and reliable measurements of lambda could not be obtained. The magnetic screening length for the Nb films was measured to be lambda = 110 +/- 2 nm for the sample with an electr on mean free path l = 10 nm, and lambda = 55 +/- 2 nm for the sample w ith l = 35 nm. Taking into account the effects of crystalline defects and impurities, we obtain the intrinsic London penetration depth in su perconducting Nb to be lambda(L) = 43 +/- 8 nm at T = 4.5 K. This resu lt is in good agreement with that of Felcher et al.