Going beyond the dipole approximation to improve the refinement of magnetic structures by neutron diffraction

We have made an analysis of neutron-diffraction data for a number of rare-earth compounds to assess the use of the dipole approximation in magnetic structure refinements. We present two examples. In the case of CePd(2)Si(2), our analysis confirms the published magnetic structure, but we find that use of the exact magnetic form factor of Ce(3+) gives a significantly improved description of the data. For NdBa(2)Cu(3)O(6+x), however, we find that incorrect conclusions have previously been drawn about the magnetic structure through the use of the dipole approximation. We have extended the magnetic modeling suite MCPHASE to enable a straightforward calculation of the higher-order anisotropic terms in the form factor of rare-earth-based systems.