Relaxation function of a Heisenberg paramagnet at large wavevectors

An approach based on the Mori's theory to calculate the Kubo relaxation function is presented in order to explain the structure of neutron scattering cross section at large wavevectors for Heisenberg paramagnets. The limits of validity of the previous approximate theories are discussed. A low-frequency expansion for the third order memory function is performed: this corresponds to considering a more consistent long-time approximation for the second order memory function. The relaxation function has been computed for RbMnF₃ at T?T_N and T = 1.125T_N.