Light scattering from the mixed antiferromagnet KNixMn1-xF3

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Abstract
Light scattering from the magnetic excitations in the mixed antiferromagnet KMn1-xNixF3 has been studied experimentally. Measurements have been made on four samples with X=0.84, 0.73, 0.54 and 0.15. At low temperatures the spectra show three two-magnon bands corresponding to the creation of excitations on neighbouring pairs of Ni ions, neighbouring Ni and Mn ions and neighbouring Mn ions. The frequencies of these bands are in excellent agreement with the predictions of an average Ising cluster model, provided that allowance is made for the change in the exchange constants with lattice parameter. The resulting exchange constants are then in agreement with those deduced from other measurements. The intensity of the Mn-Mn band is much weaker than that of the Ni-Mn band than would be expected on the basis of simple theory. It is suggested that this arises because the incident light is nearly resonance with some electric transitions of the Ni ions so that the scattering occurs very largely in the neighbourhood of the Ni ions. The temperature dependence of the magnetic bands was also measured.