Spin-and-valley-tronics explores the spin and valley degree of freedom for power-efficient and high-speed information storage and processing. A critical challenge in advancing spin-and-valley-tronic devices toward quantum operation lies in achieving coherent control over the spin and valley dynamics. A gigantic magnetic field is generally required to initiate fast coherent spin and valley precession to beat the subpicosecond valley decoherence in transition metal dichalcogenides. The magnetic proximity effect (MPE) can enhance the effective valley and spin magnetic moment, which has been explored extensively for engineering the magneto-optical properties of magnetic heterostructure. However, its influence on ultrafast coherent spin and valley dynamics remains unexplored. Herein, we investigate the MPE in ${\mathrm{WSe}}_2/\mathrm{CrSBr}$ heterostructures, which feature resonantly aligned band structures that promote strong charge transfer (CT) with a noncollinear spin configuration. The valley Zeeman splitting and emission helicity of ${\mathrm{WSe}}_2/\mathrm{CrSBr}$ are found substantially enhanced. We show that the enhanced exchange coupling together with noncollinear CT spin state drives subpicosecond coherent spin and valley precession, giving rise to anomalous magneto-optical properties. The anomalous MPE also leads to the observation of switchable exchange bias on the 2D ferromagnet with light helicity. Our work sheds light on the intriguing coherent spin and valley dynamics at the magnetic van der Waals interface and paves the way for ultrafast encoding and processing of coherent spin and valley information.

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• Magnetic coupling
• Magnetism
• Magneto-optical effect
• Spin dynamics
• Valleytronics
• 2-dimensional systems
• Photoluminescence