Far-Infrared Exchange Resonance in Ytterbium Iron Garnet

An exchange resonance has been observed in a pressed powder sample of ytterbium iron garnet at 14.1±0.2 ${\mathrm{cm}}^{-1\mathrm{}}$ at 2°K. The temperature dependences of the resonance frequency and absorption strength were measured from 2°K to 66°K. The resonant frequency at all temperatures is in good agreement with the two-sublattice theory when the appropriate spectroscopic $g$ value ($3g_J$) is used and when the anisotropy energy is taken into account at low temperatures. The two-sublattice model also reproduces the measured absorption strength to within a factor of two, and its temperature dependence even more precisely. Two other magnetic absorptions with temperature-independent frequencies were observed at 23.4±0.3 ${\mathrm{cm}}^{-1\mathrm{}}$ and 26.4±0.3 ${\mathrm{cm}}^{-1\mathrm{}}$. At 30°K, an 11-koe field on a single crystal was found to vary the frequency and intensity of these lines, which arise from the exchange splittings of the ytterbium ground-state doublet. The contribution via dispersion theory of the infrared absorption lines to the static susceptibility at 2°K is in good accord with the dc value measured by Pauthenet. This agreement suggests that all major contributions to the static susceptibility have been measured.