Gigahertz frequency antiferromagnetic resonance and strong magnon-magnon coupling in the layered crystal CrCl₃

Antiferromagnetic spintronics is an emerging field with the potential to realize logic and memory devices with high speed and bit density. Compared with well-studied ferromagnetic materials, the understanding on antiferromagnetic dynamics remains very limited, partly due to the ultrahigh instrinsic frequency, which often requires specialized terahertz techniques beyond the reach of facile device integration. Here we report broadband microwave magnetic resonance spectroscopy of the layered antiferromagnet CrCl₃. We observe a rich structure of resonances arising from quasi-two-dimensional antiferromagnetic dynamics. Due to the weak interlayer magnetic coupling in this material, we are able to observe both optical and acoustic branches of antiferromagnetic resonance in GHz frequency range. By breaking the rotational symmetry of the crystal, we further show that strong magnon-magnon coupling with large tunable gaps in orders of GHz can be induced between the two resonant modes. Our results therefore provide a versatile system for microwave control of antiferromagnetic dynamics.