Antiferromagnetic Resonance in CoCl2 and FeCl2

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Abstract
The far‐infrared transmission has been measured for anhydrous CoCl2 and FeCl2 which are highly anisotropic hexagonal‐layer antiferromagnets at low temperatures. The zero‐field, 4.2°K, antiferromagnetic resonance is observed to occur at 19.2±0.4 cm−1 (CoCl2) and 16.3±0.3 cm−1 (FeCl2). The temperature dependence of each has been studied and compared to that of the sublattice magnetization relative to the existing theory and to the presumed origin of the anisotropy. The 0°K position of the CoCl2 resonance is satisfactorily calculated, using newly measured magnetization parameters. The uniaxial anisotropy energy deduced for FeCl2 is approximately equal to that of FeF2. The FeCl2 absorption has been observed in external magnetic fields to 17 kOe along [0001]. The line splits into two branches, but a third absorption (16.7 cm−1) appears at the metamagnetic transition field corresponding to ferromagnetic resonance in the strong anisotropy field. As the transition is completed, the antiferromagnetic absorptions vanish.