Electronic and magnetic properties of57Fe in NiCl2, NiBr2, NiI2and CoI2from Mossbauer spectroscopy

Results from an investigation by Mossbauer spectroscopy of approximately 1 at.% 57 Fe in NiCl 2 , NiBr 2 , NiI 2 and CoI 2 are presented. In NiCl 2 and NiBr 2 the results are satisfactorily analysed by a crystal-field model, and new information concerning the magnetic spin arrangements has been obtained. At all temperatures below T N for NiCl 2 + 57 Fe, and at temperatures between T N and 23K for NiBr 2 + 57 Fe, the spins are along a hexagonal (210) direction. Below 23K the results from Mossbauer spectra of NiBr 2 + 57 Fe are consistent with a different magnetic structure. Optical spectra taken at several temperatures suggest that iron impurities modify the NiBr 2 helical arrangement. Mossbauer spectra of NiI 2 + 57 Fe show extreme line broadening; possible explanations are discussed. Lastly, in CoI 2 + 57 Fe the results are consistent with a magnetic structure in which the spins lie in the basal plane.