AbstractPerpendicular magnetic materials with low damping constant and high thermal stability have great potential for realizing high-density, non-volatile, and low-power consumption spintronic devices, which can sustain operation reliability for high processing temperatures. In this work, we study the Gilbert damping constant (α) of perpendicularly magnetized W/CoFeB/MgO films with a high perpendicular magnetic anisotropy (PMA) and superb thermal stability. The α of these PMA films annealed at different temperatures (T_ann) is determined via an all-optical Time-Resolved Magneto-Optical Kerr Effect method. We find that α of these W/CoFeB/MgO PMA films decreases with increasing T_ann, reaches a minimum of α = 0.015 at T_ann = 350 °C, and then increases to 0.020 after post-annealing at 400 °C. The minimum α observed at 350 °C is rationalized by two competing effects as T_ann becomes higher: the enhanced crystallization of CoFeB and dead-layer growth occurring at the two interfaces of the CoFeB layer. We further demonstrate that α of the 400 °C-annealed W/CoFeB/MgO film is comparable to that of a reference Ta/CoFeB/MgO PMA film annealed at 300 °C, justifying the enhanced thermal stability of the W-seeded CoFeB films.