Exchange interactions in antiferromagnetic salts of iridium II. Magnetic susceptibility measurements

Measurements have been made of the magnetic susceptibilities of ammonium and potassium chloroiridates at temperatures between room temperature and 1°K. The results obtained at high temperatures have been analysed into a susceptibility following a Curie-Weiss law together with temperature-independent terms. The values of the isotropic exchange inter­actions between neighbouring iridium ions deduced from the Weiss constant agree well with those calculated from paramagnetic resonance measurements. At lower temperatures the magnetic susceptibilities become almost independent of temperature over a certain range and then fall sharply at the antiferromagnetic transtion points (2.16 °K for the ammonium salt, 3.08°K for the potassium salt). These results are discussed in term s of the exchange interaction between nearest- and next-nearest-neighbour iridium ions.