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 samples 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 back surfaces of the film could be observed, attesting to the high quality and flatness of the sample. The vacuum-Nb interface had a surface roughness of σ∼3.4 nm, while the Nb-Si interface was nearly atomically smooth. We also carried out an experiment on a 300 nm-thick film of YBa₂Cu₃O₇, but the roughness was so severe that no interference fringes could be observed, and reliable measurements of λ could not be obtained. The magnetic screening length for the Nb films was measured to be λ=110±2 nm for the sample with an electron mean free path l=10 nm, and λ=55±2 nm for the sample with l=35 nm. Taking into account the effects of crystalline defects and impurities, we obtain the intrinsic London penetration depth in superconducting Nb to be λ_L=43±8 nm at T=4.5 K. This result is in good agreement with that of Felcher et al.