THE EFFECT OF LABORATORY MAGNETIC-FIELDS ON NEUTRON REFLECTION

Modest magnetic field gradients in the region of a reflecting sample s urface (either magnetic or nonmagnetic) are sufficient to significantl y perturb neutron reflectivity. Polarized neutron experiments on a 850 Angstrom-thick film of deuterated polystyrene on silicon showed a sig nificant spin dependence of the reflectivity when the material was sur rounded by magnetic circuitry that provided a transverse gradient (Del ta B similar or equal to 0.004 MA m(-1) when integrated over the neutr on path. The main effect of a field gradient is to modify the componen t of the neutron momentum perpendicular to the surface, k(z0), to k(z) (+/-) = (k(z0)(2) +/- c)(1/2). The sign indicates the neutron spin ori entation relative to the applied field and c is proportional to Delta B. The effect may be of some use: the insertion of a known gradient ar ound a nonmagnetic sample permits the set of measurements necessary to obtain the reflectivity of the sample not only in amplitude but also in phase.