Modern ferromagnetism theory is based on the Landau-Lifshitz-Gilbert equation that governs the dynamics of magnetic moment in a ferromagnet. As a deep-rooted notion, the Gilbert damping parameter must be positive because it describes the (energy and angular momentum) dissipation. In this paper, we report a negative Gilbert damping via the magneto-optical interaction of three orthogonal circularly-polarized laser beams with a submicron magnet placed in an optical cavity. We show that the off-resonant coupling between the driving laser and cavity photon in the far-blue detuning can induce a magnetic torque exactly of the Gilbert type with negative damping coefficient. A hyperbolic-tangent function ansatz is found to well describe the time-resolved spin switching as the intrinsic dissipation is overcome. Feasible experiments and materials are discussed to test our theory.

• Received 3 August 2021
• Revised 20 January 2022
• Accepted 7 February 2022

DOI:https://doi.org/10.1103/PhysRevB.105.064418