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We report here a far-infrared spectroscopy experiment in which we observe absorption lines due to pairs and triads of magnetic ions. A crystal of Mg${\mathrm{F}}_{2}$ containing ${\mathrm{Co}}^{2+}$ ions substituted for ${\mathrm{Mg}}^{2+}$ is investigated and is found to show spectra due to next-nearest-neighbor ${\mathrm{Co}}^{2+}$ pairs, and also spectra due to groups of three ${\mathrm{Co}}^{2+}$ ions containing two next-nearest-neighbor bonds. The isotropic component of the pair exchange interaction is determined to be 9.8 ${\mathrm{cm}}^{\ensuremath{-}1}$, and the anisotropic components are also evaluated. Identification and evaluation are carried out with the information obtained from the Zeeman effect. The exchange and Zeeman Hamiltonian matrices for pairs and triads are developed using symmetry considerations, and it is shown that magnetic dipole transitions are allowed and are of the correct strength to account for the observed spectra. It is also shown that the next-nearest-neighbor exchange constant found from the spinwave dispersion curve for Co${\mathrm{F}}_{2}$ is in close agreement with the spectroscopic value found here.

The EPR spectra of Gd3+, Yb3+, and Er3+ in diamagnetic garnet hosts have been measured as uniaxial pressures were applied to the crystal, and the parameters ∂gkl/∂eij and ∂Dkl/∂eij evaluated. A theory connecting these parameters with the magnetoelastic energy of the rare‐earth iron garnets has been developed, and the single‐ion contribution to the magnetostriction constants λ100 and λ111 of GdIG, YbIG, and ErIG calculated. In most cases the predicted single‐ion magnetostriction constants are of the same sign and within a factor of 2 of the observed magnetostriction constants at low temperatures. We conclude that single‐ion effects play a dominant role in the magnetoelastic behavior of the rare‐earth iron garnets.