Abstract
The magnetic scattering of neutrons from Heisenberg systems is discussed within the rigorous framework of spectral representations for the longitudinal and transverse spectral weight functions, low-order moments of these functions, and their sum rules. The spectral weight functions are expressed as functionals of longitudinal and transverse diffusivities. This exact framework leads naturally to a two-parameter phenomenological description of the neutron-scattering cross section in Heisenberg ferromagnets. It is shown that replacing the diffusivities by two-parameter Gaussian functions, which satisfy the first two sum rules, gives the essential features for the neutron-scattering cross section in ferromagnets.