Development of spin-contrast-variation neutron reflectometry for the structural analysis of multilayer films

The spin-contrast-variation neutron reflectometry technique was developed for the structural analysis of multilayer films. Polarized-neutron reflectivity curves of film samples vary as a function of their proton polarization (P_H). The P_H-dependent reflectivity curves of a polystyrene monolayer film were precisely reproduced using a common set of structural parameters and the P_H-dependent neutron scattering length density of polystyrene. This result ensures that these curves are not deformed by inhomogeneous P_H but can be used for structural analysis. Unpolarized reflectivity curves of poly(styrene-block-isoprene) films were reproduced using a flat free-surface model but P_H-dependent polarized reflectivity curves were not. The global fit of the P_H-dependent multiple reflectivity curves revealed that holes with a depth corresponding to one period of the periodic lamellae of microphase-separated polystyrene and polyisoprene domains were produced on the surface of the films, which agrees with the microscopic results. In this manner, the spin-contrast-variation neutron reflectometry technique determines the structure of multiple surfaces and interfaces in a film sample while excluding the incorrect structure that accidentally accounts for a single unpolarized reflectivity curve only.