Moessbauer investigation of the magnetic behaviour of hexahalides of Ir(IV) at very low temperatures

The low temperature magnetic behaviour of K2IrF6, Li2IrCl6.xH2O, Rb2IrCl6 and Cs2IrCl6 has been studied in a 3He/4He dilution refrigerator with the 73 keV resonance of 193Ir. Longitudinally magnetic fields up to 56 kOe could be applied. Around 100 mK all compounds show hyperfine spectra indicating the existence of a magnetically ordered state. The hyperfine fields at the Ir nuclei below 100 mK are similar to those found in other hexahalides of Ir(IV)(5d5). They are interpreted as the sum of orbital and contact contributions of opposite sign. Cs2IrCl6 and K2IrF6 show an increase of the magnetic hyperfine splitting with external field. In K2IrF6 this increase amounts to more than 50% of the zero field hyperfine splitting. (Auth.)