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

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 K 2 IrCl 6 and (NH 4 ) 2 IrCl 6 . 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, MnS 2 and MnTe 2 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.