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The influence of the dipolar forces on the static susceptibilities of Heisenberg ferromagnets

The longitudinal susceptibility of the Heisenberg ferromagnet, with short-range interactions only, is known to diverge below Tc at H=0 as 1/k and at k=0 as H-12/. This divergence has never been observed in neutron scattering experiments. The authors compute the longitudinal and transverse susceptibilities including dipolar effects at low temperatures and their variation with k and H. They use linear spin-wave theory and find that for k2+H<<m, where m is the ratio of the dipolar strength to the exchange strength, the dipolar susceptibility is half that of the exchange-only susceptibility for both longitudinal and transverse cases. For k2+H>>m, the two susceptibilities are asymptotically identical. Thus they conclude that it is the applied magnetic field that destroys the divergence. Numerical results are presented for the susceptibilities as a function of k and H.