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 reflecti vity spectra of FeF2 in the vicinity of the antiferromagnetic resonanc e frequency. The magnetic field is vertical and the plane of incidence horizontal. Calculations are presented and illustrated numerically fo r an arbitrary angle theta between the crystal c axis and the magnetic field H0. The main point of principle is that the equilibrium spin co nfiguration and therefore the magnetic susceptibility depend on theta. The reflectivity R is nonreciprocal, i.e., R(- H0) not-equal R(H0). F or 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 ref lections are predicted. We discuss the instrumental developments requi red for the high resolution that is necessary for this spectroscopy. E xperimental results for theta = 45-degrees are presented (theta = 0-de gree 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 f or with the same set of parameters as were used previously to fit the theta = 0-degree and 90-degrees results.