Magnetic Excitations in Antiferromagnetic CoF2. I. Spin-Optical-Phonon Interaction

The interaction of the low-lying magnons and excitons with the $E_g$ optical phonon in antiferromagnetic Co${\mathrm{F}}_2$ is directly observed by measuring the magnetic-dipole intensity transferred from magnetic excitations to the otherwise optically inactive $E_g$ lattice mode. The anomalous behavior of the frequency, linewidth, and magnetic-dipole intensity of this phonon have been measured as the temperature is raised from 4.2 °K through the Néel point (37.7 °K) to ∼ 4.5 $T_N$ (180 °K). The frequency drops continuously with a break in slope at the Néel point, while the linewidth narrows by more than a factor of 3 when the temperature passes through the Néel point. A theory of the temperature dependence of the transferred intensity is derived which distinguishes two contributions. The first is proportional to the sublattice magnetization and vanishes in the paramagnetic state; the second is proportional to the square of the Boltzmann factor for the exciton states and vanishes only at arbitrarily high temperature. By fitting the experimental temperature dependence of the intensity to the theory, the local spin-lattice-interaction parameters for the $E_g$ distortions can be determined.