Fig. 2. Antiferromagnetic resonance in Cu-EA single crystals with a strong magnon–magnon coupling induced by the DMI.

a Schematic illustrations of the parallel and transverse pumping geometries for selectively exciting the optical and acoustic modes when the external magnetic field is applied parallel and perpendicular to the microwave RF field, ${\mathbf{h}}_{\mathbf{rf}}$. b Schematic illustrations of the broken symmetry (${\mathbf{H}}_{\mathbf{ext}}\parallel \mathbf{D}$) and symmetric (${\mathbf{H}}_{\mathbf{ext}}\perp \mathbf{D}$) pumping configurations when the external magnetic field is applied parallel and perpendicular to the DMI vector of the Cu-EA single crystal. c–f antiferromagnetic resonance spectra collected under the four possible experimental geometries as described in a and b. Insets in c–f show the resonance spectra under an adjusted color scale to elucidate the presence of dark modes at high fields.