Theory of Magnetic Excitations in FeCl2 and CoO

A theory has been developed whereby single-ion effects in a magnet may be treated exactly providing an RPA is made to the exchange interaction. The method is applied to FeCl2 and CoO, both of which have unquenched orbital angular momentum, a factor which might complicate conventional approaches. For FeCl2 the available data on static properties and the AFMR can be explained. In addition to the AFMR, there are many lines predicted, but experiments are lacking. Measurements on CoO can also be accounted for, but there is a serious lack of interpretable data on absorptions to test the theory.