Magnetic excitations of mixed CoF₂/ZnF₂ crystals

The magnetic excitations in mixed crystals of Co_1-cZn_cF₂ with c=0.14+or-0.02, 0.31+or-0.03 and 0.77+or-0.10 have been measured using neutron inelastic scattering techniques. The results for the first two crystals show that the excitations decrease in frequency with increasing zinc concentration and also have a finite lifetime. For the crystal containing the largest concentration of ZnF₂ well defined magnetic excitations were not observed. These results are very similar to the predictions of the virtual lattice model in which each exchange interaction of CoF₂ is reduced by the concentration of CoF₂ in the mixed crystal. Reasonable agreement is also found with the results of calculations based on the single-site CPA approximation but the results do not clearly show that any one of the particular variants of this model gives a markedly better result than any of the other variants.