Optical Detection of Antiferromagnetic Resonance in van der Waals Materials*

Measurements of antiferromagnetic dynamics in thin samples is challenging due to a small number of spins which have no static magnetization. One of the ideal candidates for measuring the weak dipole field fluctuations from antiferromagnetic dynamics are single-spin color centers such as the NV center in diamond. Here we present optical detection of antiferromagnetic resonance in the van der Waals material CrSBr by the method of NV relaxometry in a bulk sample. Strongly driven uniform mode antiferromagnetic resonance populates finite wavevector magnon states via magnon scattering processes. Gapless magnetic field fluctuations produced by two-magnon processes are amplified by increasing the magnon population to levels detectable by an ensemble of NV centers on the surface. Field fluctuations at the NV electron spin resonance frequencies relax the spin polarization resulting in a decrease of photoluminescence. These measurements allow estimation of the strength of field fluctuations emanating from antiferromagnetic spinwaves and pave the way for measurements of antiferromagnetic dynamics in ultrathin materials.