Optical Absorption of CoO and MnO above and below the Néel Temperature

The optical absorption of single-crystal CoO and MnO both above and below the N\'eel temperature is reported and interpreted. The absorption spectrum in each case is similar to that found in dilute paramagnetic salts of the same ions. Tanabe and Sugano's strong-field theory was used to interpret the data. In CoO the tetragonal distortion of the cubic lattice present in the antiferromagnetic phase was experimentally observed. An estimate of the strength of the tetragonal field is given. It is also found that the temperature dependence of the absorption coefficient in CoO is in accord with a second order electric dipole-phonon transition mechanism.