Antiferromagnetism in the face-centred cubic lattice by a spin-wave method

Source abstract

Abstract Formulae developed in previous papers by the author and D. ter Haar are applied to the problem of antiferromagnetism in a f.c.c. magnetic lattice with dominantly nearest-neighbour exchange. The stability of the type 1 and type 3 order is discussed with particular reference to the salts K2IrCl6 and (NH4)2IrCl6. For these salts it is shown that the ordering cannot be stabilized by nearest-neighbour anisotropy and must depend on the existence of more -remote -neighbour interactions. A method is devised for estimating the Curie temperature as a function of the ratio of next-nearest-neighbour exchange K to nearest-neighbour exchange J, and the influence of anisotropy is also considered. The agreement between theory and experiment for the above salts is good. The results are also applied to βMnS, MnS2 and MnTe2 and no conflict between experiment and theory is found although, for these salts, lack of experimental information does not enable so rigorous a test of the theory as is possible for the chloroiridates. A comparison is made between the present theory and the molecular-field theory for the f.c.c. lattice, and it is observed that the latter becomes progressively poorer as the ratio K/J is decreased from large values, until it breaks down completely for K/J < ~ 0-7. Finally, the experimental result K/J =:= 1.0 for MnO is shown to be in agreement with the spin-wave theory.