Ultrafast control of spin orientations without relying on highly nonequilibrium electron distributions offers a new pathway for femtosecond spintronics. It is shown that excitation of specific vibronic modes can drive a substantial demagnetization of a local moment relative to the magnetization axis, while preserving its total spin quantum number. Using a fully quantum many-body treatment of a local $S=2$ ($d^6$) many-body spin coupled to a driven Jahn-Teller mode with quantum dissipation, we find that the projected spin can be strongly reduced via a Jahn-Teller-induced $xy\uparrow \to yz/zx\downarrow$ transition, which occurs without additional Coulomb exchange cost. The excited state subsequently relaxes back to its original electronic configuration with a rotated spin direction, enabling spin reorientation without generating a large net angular momentum in the electronic subsystem.

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• Jahn-Teller effect
• Magnetism
• Ultrafast demagnetization
• Vibronic effects
• Strongly correlated systems