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The spin-wave dispersion relations for Rb 2 CrCl 4 , which orders ferromagnetically at 63K have been measured at 4.5K. The spin-wave energies confirm the two-dimensional nature of the interactions; writing H 12 =-2J 12 S 1 .S 2 they can be fitted by a single exchange parameter J 1 =0.66+or-0.01 meV (7.7+or-0.1K) representing the interaction between nearest-neighbour in-plane ions. Rb 2 CrCl 4 should therefore typify a two-dimensional, S=2, easy-plane ferromagnet.
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Hutchings et al. (1976) studied this question.
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