MAGNON-OPTIC PHONON INTERACTION IN ANTIFERROMAGNETIC COF2

Far infrared absorption experiments in the antiferromagnet CoF2 reveal a sharp, transverse magnetic-dipole absorption in the vicinity of 250 cm-1. The resonance is interpreted as the excitation of an E g optic phonon. Normally inactive, this phonon may derive intensity from the strong spin-phonon coupling. The more interesting experimental results show that the linewidth narrows by more than a factor of 2 as the temperature is raised from 4.2°K to TN. The intensity drops abruptly at the Néel point but does not vanish at temperatures as high as 4.5T N. A theory of the temperature dependence of the transfer of magnetic dipole intensity to the Eg phonon has been derived. One contribution monitors the infinite-range longitudinal-spin correlation where as a second contribution is insensitive to ground-state correlations. Comparison between experiment and theory indicates that in CoF2 the correlation sensitive contribution is absent. The temperature variation of the intensity is caused solely by the change in magnetic excitation energies and the thermal population of these states. © 1969 The American Institute of Physics.