  We describe a new sensitive method for the investigation of weakly magnetic
films placed inside a tri-layer planar waveguide. Polarized neutrons tunnel
into the waveguide through the surface, channel along the layers and are
emitted from the end face as a narrow and slightly divergent microbeam.
Polarization analysis permits to detect very small magnetization in the order
of a few 10 Gauss. The magnetic film containing the rare-earth element Tb was
investigated using both fixed wavelength and time-of-flight polarized neutron
reflectometers. The experimental results are presented and discussed.
