DYNAMICAL CRITICAL BEHAVIOUR OF HEISENBERG FERROMAGNETS FOR T Tc

From an 0 (E)-calculation (E = 6 d) of the dynamical spin correlation function in a Heisenberg system for T 2 T, an expression for that function applicable in dimension d= 3 is deduced. It comprises the expected limiting behaviour in different sectors of the critical region. It has been shown that the MaiMazenko model of a Heisenberg ferromagnet [I], evaluated within an &-expansion, E = 6 d [2, 31, accounts very well for certain features found by neutron scattering in Fe, Ni and EuO [P6] at T,. To investigate the applicability of the model also for T > T,, one has to calculate the spin correlation function for arbitrary values of q, w, and the correlation length J. Since the calculation involves an expansion around d= 6 [7], the result must be processed further in order to apply in d= 3 [8]. The equation of motion for the order parameter S in a Heisenberg system with effective Hamiltonian