Field-Dependent Antiferromagnetic Resonance Modes in CoCl2

Results have been obtained for the magnetic field dependence of the far‐infrared transmission of single‐crystal antiferromagnetic ${\mathrm{CoCl}}_2$ at 4.2°K. The energy range was $\text{ħω\hspace{0.17em}>\hspace{0.17em}10\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ and fields up to 26 kOe were applied perpendicular to the hexagonal axis. The two uniform‐mode antiferromagnetic resonances, ${\omega }_1$ and ${\omega }_2$ , are nondegenerate in zero field with ${\mathrm{ħ\omega }}_1\text{\hspace{0.17em}=\hspace{0.17em}19.0±0.2\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ . The normalized energy‐field behavior follows simple curves: for ${\omega }_1$ , a circle of radius 32 kOe centered at (0, 0) ; for ${\omega }_2$ , a straight line crossing ${\omega }_1$ at 23 kOe and appearing to pass through the (0, 0) point. These results, coupled with high‐field magnetization measurements, permit calculation of the isotropic and anisotropic coefficients of exchange (intra‐ and interlayer) and the $g$ factors.