Host and Impurity Excitations in Antiferromagnetic and Paramagnetic FeCO 3 in the Far Infrared

The host and impurity excitations in FeCO 3 containing 3% Mn 2+ are studied in the range from 12.5 to 500 cm −1 . The host excitations are observed both in the antiferromagnetic and the paramagnetic temperature range and are well described for a wide temperature range with a single ion Hamiltonian, including the interactions between all sublevels coming from the ground term 5 D of the Fe 2+ ion: the temperature dependent trigonal field, the spin–orbit interaction, and the Ising molecular field. For the first time in an Ising antiferromagnet a pair mode (host excitation plus impurity excitation) characterized by the absence of magnon‐magnon interaction is observed. The g ‐factors of the observed lines are measured. A sharp drop of the g ‐factors near T N cannot be explained by an existent theory of the g ‐factor renormalization.