Polarized neutron reflectometry in high magnetic fields

A simple method is described to maintain the polarization of a neutron beam on its way through the large magnetic stray fields produced by a vertical field of a cryomagnet with a split-coil geometry. The two key issues are the proper shielding of the neutron spin flippers and an additional radial field component in order to guide the neutron spin through the region of the null point (i.e., point of reversal for the vertical field component). Calculations of the neutron’s spin rotation as well as polarized neutron reflectometry experiments on an $ErFe2∕DyFe2$ multilayer show the perfect performance of the used setup. The recently commissioned cryomagnet M5 with a maximum vertical field of up to $7.2T$ in asymmetric mode for polarized neutrons and $9T$ in symmetric mode for unpolarized neutrons was used on the C5 spectrometer in reflectometry mode, at the NRU reactor in Chalk River, Canada.