Theory of infrared absorption by pairs of cobalt ions in MgF₂

The microscopic mechanism responsible for the electric dipole two-magnon absorption in rutile antiferromagnets remains in some dispute. Three mechanisms have been suggested which appear to make contributions of comparable orders-of-magnitude for some materials. As a contribution to the resolution of this problem we have studied the case of two next-nearest-neighbor Co ²⁺ ions in a nonmagnetic rutile lattice. The results suggest some further experimental studies of the polarization properties of the observed pair absorption spectrum.