The infrared-active antiferromagnetic resonance modes in ${\mathrm{Gd}}_2$${\mathrm{CuO}}_4$ have been measured by far-infrared transmission through single-crystal and polycrystalline samples at fields from 0 to 9 T and temperatures from 1.4 to 35 K. The observed spectra are compared to the predictions of three different spin-wave models for ${\mathrm{Gd}}_2$${\mathrm{CuO}}_4$. A model including four Gd sublattices without coupling to the Cu spins cannot produce a mode at high enough frequency to match the data, while a two Gd and two Cu sublattice model fails to predict the correct temperature dependence. It is found that to explain the temperature dependence of the observed spectrum, it is necessary to use a model with four Gd and two Cu sublattices, in which a triangular-to-antiparallel transition in the Gd spins occurs near the Gd Néel temperature of 6.5 K. In this model, the Cu spins combine to produce an effective field of about 0.13 T at the Gd sites, which polarizes the Gd spins into ‘‘captive sublattices’’ at higher temperature. The observed mode strengths and those predicted by the model are both found to be in good agreement with the known dc susceptibility.

• Received 8 September 1992

DOI:https://doi.org/10.1103/PhysRevB.47.5300