OBLIQUE-INCIDENCE FAR-INFRARED REFLECTIVITY STUDY OF THE UNIAXIAL ANTIFERROMAGNET FEF2

Theoretical and experimental results are presented for high-resolution frequency scans of 45-degrees oblique-incidence far-infrared reflectivity spectra of FeF2 in the vicinity of the antiferromagnetic resonance frequency. The magnetic field is vertical and the plane of incidence horizontal. Calculations are presented and illustrated numerically for an arbitrary angle theta between the crystal c axis and the magnetic field H0. The main point of principle is that the equilibrium spin configuration and therefore the magnetic susceptibility depend on theta. The reflectivity R is nonreciprocal, i.e., R(- H0) not-equal R(H0). For theta = 0-degree magnetic features are seen only in s polarization (E field vertical), but otherwise they are seen in both polarizations. Except for theta = 0-degree and 90-degrees weak s to p and p to s reflections are predicted. We discuss the instrumental developments required for the high resolution that is necessary for this spectroscopy. Experimental results for theta = 45-degrees are presented (theta = 0-degree and 90-degrees are in a previous publication). The whole range of spectra, including the s to p and p to s reflections, are accounted for with the same set of parameters as were used previously to fit the theta = 0-degree and 90-degrees results.