Antiferromagnetic Resonance in CoCl2 and FeCl2

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.