Total Intensities of Some Crystal Field Transitions in MnO and MnS Related to the Antiferromagnetism

In recent years there have been reports of enhanced absorption intensities for crystal-field spectra of transition-metal ions in crystals where appreciable magnetic exchange coupling exists between ions. We report here some oscillator strength determinations for the two lowest-energy transitions of the Mn2+ ion (6A1q → a4T1g and a4T2g) in the antiferromagnets MnO and α-MnS. These broad bands are anomalously intense (f≃10−3) for such spin- and parity-forbidden transitions. The data show an increase of oscillator strength with temperature below the Néel temperature which can be approximately related to the sublattice magnetization. Above TN, there is a leveling off of the oscillator strengths. The large intensities of these transitions and their relation to the magnetism suggest that the violation of ``spin-forbiddenness'' is associated with the antiferromagnetic coupling in these broad, phonon-assisted transitions as well as in the much narrower and weaker magnon sidebands.