Exchange interactions in antiferromagnetic salts of iridium. I. Paramagnetic resonance experiments

Paramagnetic resonance methods have been used to investigate Ir-Ir exchange interactions in K 2 IrCl 6 and (NH 4 ) 2 IrCl 6 . Measurements are described of the resonance spectrum from nearest-neighbour pairs of Ir ions in semi-dilute mixed crystals where Pt is substituted for Ir. The results show that the isotropic p art of the Ir-Ir exchange, J / k , is antiferromagnetic and of magnitude 11.5 ± 1°K and 7.5 ± 1°K for the potassium and ammonium salts, respectively. There is also found to be an anisotropic part with rhombic symmetry and with magnitude of order 1 cm -1 . No lines attributable to next-nearest-neighbour pairs were found, and it is suggested that this interaction is small. The results are com pared briefly with the magnetic susceptibility experiments of Cooke et al . (part II) who find that the concentrated salts go antiferromagnetic in the liquid-helium temperature range, and also with the theoretical analysis of the superexchange mechanism given by Judd (part III).