Two-dimensional magnetic materials exhibit strong magneto-optic effects and high tunability by electrostatic gating, making them very attractive for new magneto-photonic devices. Here, we characterize the magneto-optic Kerr effect (MOKE) spectrum of thin ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ from 1.13 to 2.67 eV, and demonstrate electrostatic control over its magnetic and magneto-optic properties. The MOKE spectrum exhibits a strong feature around 1.43 eV, which we attribute to a magnetic-exchange-split excitonic state in ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$, in agreement with ab-initio calculations. The gate dependence of the MOKE signals shows that the magneto-optical efficiency—rather than the saturation magnetization—is affected by electrostatic gating. We demonstrate a modulation of the magneto-optical strength by over 1 mdeg, with some wavelengths showing a modulation of 65% of the total magneto-optical signals, opening the door for efficient electrical control over light polarization through two-dimensional magnets. Our findings bring forward the fundamental understanding of magneto-optic processes in two-dimensional magnets and are highly relevant for the engineering of devices that exploit excitonic resonances for electrically tunable magneto-photonic devices.

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• Excitons
• Magneto-optical Kerr effect
• Magneto-optics
• Spintronics
• 2-dimensional systems