Fe<SUP>57</SUP> Mossbauer effect in nickel oxide

The Fe57 mossbauer effect in antiferromagnetic NiO, used as a source, has been studied over a wide range of temperatures below as well as above its Neel temperature (523&#176;K). The after effects of decay produce only ferrous and ferric states, which have been identified by their characteristic hyperfine fields and isomer shifts. These observations are similar to those reported on CoO. The temperature dependence of the hyperfine field at the nuclei of Fe2+ and Fe3+ ions, although almost identical, deviates from the sublattice magnetization of NiO as well as from the Brillouin function for the Ni2+ state (S=1). The spectrum at liquid-nitrogen temperature indicated the presence of an electric field gradient at the ferrous nucleus. The intensity of the ferrous peak is highly temperature-dependent, decreasing with an increase in temperature finally vanishing above about 466&#176;K. This dependence has been qualitatively explained in terms of the electron-capture mechanism. It has been concluded that the temperature dependence of the capture cross section of the ferric ion is mainly responsible for the observed behavior