Oblique-incidence far-infrared reflectivity study of the uniaxial antiferromagnetFeF2

Theoretical and experimental results are presented for high-resolution frequency scans of 45° oblique-incidence far-infrared reflectivity spectra of ${\mathrm{FeF}}_2$ 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 θ between the crystal c axis and the magnetic field ${\mathit{H}}_0$. The main point of principle is that the equilibrium spin configuration and therefore the magnetic susceptibility depend on θ. The reflectivity R is nonreciprocal, i.e., R(-${\mathit{H}}_0$)≠R(${\mathit{H}}_0$). For θ=0° magnetic features are seen only in s polarization (E field vertical), but otherwise they are seen in both polarizations. Except for θ=0° and 90° 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 θ=45° are presented (θ=0° and 90° 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 θ=0° and 90° results.