Application of Spin Wave Theory to Three Magnetic Salts

Numerical calculations, based on spin wave theory, have been carried out in connection with the magnetic properties of two simple ferromagnetic salts, EuO and EuS, and the antiferromagnetic compound MnF 2 . The resulting predictions regarding magnetization and specific heat have been compared with experiment over as wide a temperature range as possible. The calculations envisage the possibility that more than one set of exchange interactions may be involved and in the case of MnF 2 allowance is made for a temperature dependent anisotropy field. Account is taken of two-magnon dynamical interactions and renormalized spin wave energies are used in evaluating the magnon populations. The effects of kinematical interactions between spin waves are, however, ignored. It is found that the theoretical estimates of magnetization appear to fit experiment over a temperature range extending to about three-quarters of the Curie or Néel temperature of the system concerned. Specific heat calculations, on the other hand, seem to depart from the experimentally observed values at a much lower temperature, of the order of one-third the critical temperature. The calculations on EuO and EuS confirm the general model for the exchange constants of these salts proposed by McGuire, Argyle, Shafer and Smart and by Gorter and his co-workers.