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

Abstract

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 part of the Ir-Ir exchange, J/k, is antiferromagnetic and of magnitude 11$\cdot $5 $\pm $ 1 degrees K and 7$\cdot $5 $\pm $ 1 degrees 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 compared 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).