Extracting buried twists with polarized neutron reflectometry

Polarized neutron reflectometry can extract the depth-dependent magnetization of thin-film magnets more precisely than other techniques, which are limited to a measurement of the average moment or the moment at the surface. Measuring the reflectivity first with neutrons glancing off the front surface and again with neutrons glancing off the back surface yields eight spin cross sections instead of the usual four. Differences in the front and back reflectivities indicate the presence of magnetic twists in the sample. We have applied this method, as well as the more conventional front-surface-only reflectometry, to study a soft ferromagnet exchange-coupled to a hard ferromagnet in fields from 5 to 50 mT.