Diamagnetic Response of thin film Superconductors: Polarized Neutron Reflectivity (PNR) Experiments. S. Adenwalla1, E.E. Fullerton2, R. Osgood2 and G.P. Felcher2 1Dept. of Physics, Univ. of Nebraska-Lincoln, Lincoln NE 68588-0111, 2Material Science Division, Argonne National Lab, Argonne IL 60439
We report on PNR experiments that investigate the magnetic field profiles in a Type 2 superconducting thin film (Nb) in the mixed state and superconducting/magnetic multilayers (GdN/NbN). A superconductor in a magnetic field displays a diamagnetic response, expelling all flux except for a narrow region at the superconducting surface in which the field decays exponentially. This exponential decay and the decay length in the Meissner regime have been previously studied using PNR. As the field increases in a type II superconductor, flux can now enter the superconductor arranged in a regular array of flux lines (the Abrikosov lattice) each containing a single flux quantum. The symmetry of the lattice and the decay length are both dependent on the underlying symmetry of the crystal and the angle between the crystalline axes and the magnetic field. In addition, both the penetration depth and the flux lattice are dependent on the properties of the sample-the better the sample, the more symmetrical the flux lattice. Hence the measurements of the flux lattice provide useful information on the superconducting properties of the sample, in this case a thin film. PNR is extremely sensitive to small changes in the magnetic depth profile, but averages over the in-plane magnetic variations and is particularly well-suited to geometries in which the field (and hence the flux lines) are parallel to the sample surface (the geometry employed in all the experiments reported here). For the Nb thin film it was necessary to use a standing wave resonator technique. Flux lines are fairly weak scatterers-the standing wave resonator greatly enhanced the neutron flux in the Nb film. The amount of scattering was tracked as a function of external magnetic field and was found to be consistent with that caused by flux lines. For the magnetic/superconducting multilayer, the magnetic depth profiles were measured as functions of temperature, above and below the respective transition temperatures and compared with bulk magnetization measurements.