Nitrogen-vacancy (NV) centers in diamond and newly discovered Boron-vacancy (BV) centers in h-BN also known as color-centers provide a non-invasive and sensitive method to probe weak magnetic field fluctuations generated by magnons. Enabling the detection of magnetic dynamics at frequencies exceeding the color-center ESR frequency (typically a few GHz) is essential in order to probe the dynamics of many ferri- and antiferromagnetic materials. The recent discovery of optical detection of magnetic resonance occurring above the NV center ESR frequency reveals a route to the exploration of the antiferromagnetic spin dynamics [1,2]. The scattering of two high-frequency magnons, or two-magnon scattering, generates electromagnetic noise at their difference frequency which can match the color-center frequency enabling the detection of higher-frequency dynamics. Here, we show the optical detection of antiferromagnetic resonance in a bulk CrCl₃ crystal. At 9 K, the optical branch resonates at 9 GHz, three times larger than the NV center ESR frequency. The temperature evolution of the resonance mode confirms that it originates from antiferromagnetic dynamics. Such dynamics can also be excited using spin-orbit torque (SOT) generated by current in an adjacent layer. The low in-plane symmetry of WTe₂ generates an out-of-plane SOT component excites antiferromagnetic dynamics measured using NV/BV centers in diamond/hBN.

[1] Nature Comm 11, 5229 (2020)

[2] Sci. Adv. 8, eabg8562 (2022)