Magnetic excitations in two-dimensional random antiferromagnets

Calculations are made of the magnetic excitations in the random systems Rb 2 NiF 4 /Rb 2 MnF 4 and K 2 MnF 4 /K 2 ZnF 4 using various approximations based upon the single-site coherent potential approximation. For the former system, three different ways were used to treat the transverse part of the exchange interaction; the most successful was to reduce the disorder in the transverse exchange interactions by appropriate scaling of the single-site Green functions. The calculations then gave good agreement with recent measurements. In the calculations for the latter system, two approximations have been used for treating the transverse part of the interactions and two ways of keeping the magnetic excitations off the non-magnetic zinc ions. The results predict considerable structure which is absent in theories which average the single-site energies.