  The research on layered van der Waals (vdW) magnets is rapidly progressing
owing to exciting fundamental science and potential applications. In bulk
crystal form, CrCl3 is a vdW antiferromagnet with in-plane ferromagnetic
ordering below 17 K, and CrI3 is a vdW ferromagnet below 61 K. Here, we report
on the electron spin resonance (ESR) properties of CrCl3 and CrI3 single
crystals upon photo-excitation in the visible range. We noticed remarkable
changes in the ESR spectra upon illumination. In the case of CrCl3, at 10 K,
the ESR signal is shifted from g = 1.492 (dark) to 1.661 (light), line width
increased from 376 to 506 Oe, and the signal intensity is reduced by 1.5 times.
Most interestingly, the observed change in the signal intensity is reversible
when the light is cycled on/off. We observed almost no change in the ESR
spectral parameters in the paramagnetic phase (>20 K) upon illumination. Upon
photo-excitation of CrI3, the ESR signal intensity is reduced by 1.9 times; the
g-value increased from 1.956 to 1.990; the linewidth increased from 1170 to
1260 Oe at 60 K. These findings are discussed by taking into account the skin
depth, the slow relaxation mechanism and the appearance of low-symmetry fields
at the photo-generated Cr2+ Jahn-Teller centers. Such an increase in the
g-value as a result of photo-generated Cr2+ ions is further supported by our
many-body wavefunction calculations. This work has the potential to extend to
monolayer vdWs magnets by combining ESR spectroscopy with optical excitation
and detection.
