AbstractThe host and impurity excitations in FeCO3 containing 3% Mn2+ 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 5D of the Fe2+ 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 TN cannot be explained by an existent theory of the g‐factor renormalization.