Spin-polarized neutron reflectivity: a probe of vortices in thin-film superconductors

It is demonstrated that the specular reflectivity of spin-polarized neutrons can be used to study vortices in a thin-film superconductor. Experiments were performed on a 6000 Å thick c-axis film of YBa₂Cu₃O_7-x with the magnetic field applied parallel to the surface. A magnetic hysteresis loop was observed for the spinpolarized reflection and, from these data, the average density of vortices was extracted. A model is presented which relates the specular reflectivity to the one-dimensional spatial distribution of vortices in the direction perpendicular to the surface. Unlike other techniques, neutron reflectivity observes vortices in a geometry where they are parallel to the interface.