The effect of laboratory magnetic fields on neutron reflection

Modest magnetic field gradients in the region of a reflecting sample surface (either magnetic or nonmagnetic) are sufficient to significantly perturb neutron reflectivity. Polarized neutron experiments on a 850 Å-thick film of deuterated polystyrene on silicon showed a significant spin dependence of the reflectivity when the material was surrounded by magnetic circuitry that provided a transverse gradient (ΔB ≃ 0.004 MA m^-1 when integrated over the neutron path. The main effect of a field gradient is to modify the component of the neutron momentum perpendicular to the surface, k_z0, to k^±_z = (k²_z0 ± c)^1/2. The sign indicates the neutron spin orientation relative to the applied field and c is proportional to ΔB. The effect may be of some use: the insertion of a known gradient around a nonmagnetic sample permits the set of measurements necessary to obtain the reflectivity of the sample not only in amplitude but also in phase.