Magnetic Circular Dichroism of Sharp Optical Transitions in Antiferromagnetic FeF2

Using magnetic circular dichroic (MCD) modulation techniques, we have reinvestigated the spectrum of antiferromagnetic Fe${\mathrm{F}}_2$, concentrating on pure electronic transitions and their associated magnetic structure. We show in this paper that because vibronically induced bands do not exhibit circular dichroism, MCD spectra complement absorption spectra in aiding our understanding of magnetic effects. In particular, we present the results of studies of the magnetic field and temperature dependence of the MCD spectra in two absorption regions of Fe${\mathrm{F}}_2$, i. e., the 21 500- and 25 900-${\mathrm{cm}}^{-1\mathrm{}}$ regions, and report the effective $g$ factors of all excitations, simple and compounded, in these two regions. In the 21 500-${\mathrm{cm}}^{-1\mathrm{}}$ region we report the identification of a two-magnon sideband and its thermal and magnetic properties; in the other, we observe an additional purely electronic transition which is obscured by broad-band absorption and appears only through its MCD properties. Several observations are made on the additional information obtainable by consideration of the polarity of the MCD signal in antiferromagnets.