Spin dynamics and critical fluctuations of a site-random, two-dimensional antiferromagnet:Rb2Mn0.5Ni0.5F4

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Abstract
In Rb2Mn5 Ni 5F4 the Mn and Ni ions are randomly distributed on a plane square lattice. The exchange energy between neighboring spins is of the Heisenberg form JS↘⋅S↘ with J=7.4 K, 25.7 K and 89 K for pairs of Mn−Mn, Mn−Ni, and Ni−Ni spins. The mean anisotropy field for the Mn and Ni spins is about 0.7% of the exchange field. The spin dynamics studied by inelastic neutron scattering has been described elsewhere (Phys. Rev. B12, 4963, (1975)) and only the results are summarized here. The dynamic susceptibility χ″ (q,ω) exhibitis two well‐defined peaks in frequency ω for a fixed wave‐vector q. The dispersion relations of the two modes shown in fig. 1 are well accounted for in a simple 4‐sublattice model (J. Phys. C, 8, L328 (1975)), and the width of the zone boundary peaks corresponds to the quintet of Ising frequencies of a spin with different configurations of neighboring spins. The results are in good agreement with χ″ (q,ω) as calculated by the CPA method (J. Phys. C, 8, 1889 (1975)) and also with Monte Carlo calculations (Phys. Rev. B12, 4980 (1975)), the only discrepancy being on the intensity of the lower branch for ζ<0.2, cf fig. 10 p 4971 and fig. 12 p 4998 in Phys. Rev. B12 (1975).

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