Theory of spin waves in disordered antiferromagnets. I. Application to (Mn, Co)F2and K(Mn, Co)F3

The coherent potential approximation has been extended to provide a theory of the magnetic excitations in substitutionally disordered quasi-binary antiferromagnetic alloys. It is shown how the self consistent equations for a two sublattice antiferromagnet may be reduced to a scalar form with the aid of the single site approximation and time reversal symmetry. The theory is applied to the Mn 1-c Co c F 2 and KMn 1-c Co c F 3 systems and numerical results obtained for comparison with recent neutron scattering experiments. The theory correctly predicts the two-peaked spectral function observed experimentally at each wavevector, and gives a good description of the dispersion relations of the two bands, and of the widths, relative intensities and lineshapes of the excitations.