Depth-dependent magnetism of layered superconductors: Nb/Si

We have studied magnetic field penetration and vortex line formation in a [ Nb(100 Angstrom)/Si(15 Angstrom)]x20 multilayer by magnetization and polari zed neutron reflection. With the magnetic field applied parallel to the sur face, the magnetization revealed the presence of a kink above H-cl indicati ve of transitions between one row of fluxoids and two rows of fluxoids para llel to the surface. The spin dependence of neutron reflectivity below H-cl was consistent with a penetration depth of 1200 Angstrom, substantially la rger than that of bulk Nb. In the mixed phase (H-ext>H-et) the field was fo und to penetrate the surface, with a slope as found in the case of H-ext<H- cl. At H-ext>H-cl vortices form inside the multilayer, in addition to there being surface penetration. A modulation of the vortex fields was found wit h the periodicity of the Nb/Si bilayers as evidenced by the spin dependence of the reflectivity at the first Bragg peak of the multilayer. (C) 1998 Am erican Institute of Physics.