PHENOMENOLOGY OF NEUTRON SCATTERING IN HEISENBERG SYSTEMS

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. © 1968 The American Physical Society.