The temperature dependence of the Ni2+ ion absorption-transition maxima has been studied in the magnetic host materials of KNi1-xMgxF3, RbFeF3, RbCoF3, RbMnF3, MnF2, and KMnF3. The behavior of the absorption transitions in these materials differs from the behavior due to purely thermal effects observed in the nonmagnetic hosts. Additional shifts, due to the exchange interactions responsible for the magnetic properties, are superimposed on the thermal shifts. In the pure-nickel salts, the departures from purely thermal behavior occur in the vicinity of the ordering temperature of the crystals. In KNil-xMgxF3 crystals, the departures are observed in materials of sufficient Ni2+ concentration, below the Néel temperature of the pure material. In magnetic materials where the Ni2+ ion is an impurity, the magnetically induced shifts in the nickel absorption bands are observed near the ordering temperature of the host crystal.
W. W. HOLLOWAY, JR. and M. KESTIGIAN, "Temperature Dependence of the Ni2+ Ion Absorption Transition in Magnetic Host Materials," J. Opt. Soc. Am. 62, 539-541 (1972)