Search for Noncollinear Spin Density in Hexagonal Cobalt

The interpretation of the magnetic form factors for the $3d$ metals has been based on the assumption that the magnetic moment is everwhere collinear, so that a scalar density function may be used. It has been suggested that if the spin density is noncollinear, the form-factor measurements should be reinterpreted. We have conducted experiments, by means of the new technique of neutron-polarization analysis, designed to detect a noncollinear spin component in hexagonal cobalt, but have seen no evidence for scattering from such a component. Upper limits on the magnitude of such scattering have been established for the (002) and (110) reflections. Further, it is shown that the polarized-beam technique used in the form-factor measurements is not very sensitive to a small noncollinear spin component. Ignoring the presence of noncollinear spin scattering, of magnitude given by the upper limit of the polarization analysis experiment, produces an error in the Co form factor smaller than the experimental error due to other effects. For Fe and Ni, it is shown that the form-factor experiment is much less sensitive to a noncollinear spin density than is the case for Co. We conclude that the form-factor measurements need not be reinterpreted, provided it is understood that they apply to the component of the total spin density which is parallel to the average spin density.