MAGNETIC-INDUCTION PROFILE IN A TYPE-I SUPERCONDUCTOR BY POLARIZED-NEUTRON REFLECTOMETRY

We have used the technique of polarized-neutron reflectometry to study the magnetic-induction profile just beneath the surface of a 1-mum-thick film of lead with the magnetic field applied parallel to the surface. The sample was maintained at a temperature of 1.5 K throughout the experiment, and the applied field H was varied from below H(c), the bulk critical field, up to the critical field for surface superconductivity, H(c3). From the measurements with the lead film in the bulk superconducting phase (H < H(c)) we found that the spin-dependent reflectivity profiles are consistent with an exponential decay of magnetic induction with a penetration depth lambda=39+/-1 nm. At higher applied fields (H(c)<H<H(c3)) we obtained information on the diamagnetism in the surface superconducting layer. We discuss the sensitivity of the measurements to the form of the magnetic-induction profile, in particular to nonlocal effects, and show that a good description of the surface diamagnetism, over the whole range of applied fields, can be achieved with the local Ginzburg-Landau theory if the Ginzburg-Landau parameter kappa is allowed to vary with the applied field.