The magnetic properties of three exchange coupled iridium ions

Abstract

The magnetic properties of three exchange coupled Ir4+ ions (triads) in K2Ir,PtCl6 and (NH4)2Ir,PtCl6 with Ir:Pt #1:10 have been investigated by electron spin resonance techniques. All possible triads containing Ir4+ ions which are first or second nearest neighbours to each other in the face-centred cubic lattice are considered. The analysis gives the following values for the first and second neighbour isotropic exchange interactions: K₂Ir,PtCl₆ J₁/k = 10.5 ± 2 °K, J₂/k = 0.55 ±0.04 °K (NH₄)₂Ir,PtCl₆ J₁/k = 6.5 ± 1 °K, J₂/k = 0.39 ± 0.04 °K. In addition, the sign and magnitude of the first neighbour anisotropic interaction has been measured and the second neighbour anisotropic interaction is shown to be extremely small. The results are used in the spin wave theory developed for this problem by Lines (1963) to predict the antiferromagnetic properties of the concentrated salts. The agreement with experiment is found to be satisfactory. In particular, the unusually large θ/T_N ratio of about 10 is satisfactorily explained and it is predicted that the magnetic ordering is of the third type.