Perpendicular 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 ({alpha}) of perpendicularly magnetized W/CoFeB/MgO films with a high perpendicular magnetic anisotropy (PMA) and superb thermal stability. The {alpha} of these PMA films annealed at different temperatures is determined via an all-optical Time-Resolved Magneto-Optical Kerr Effect method. We find that {alpha} of these W/CoFeB/MgO PMA films decreases with increasing annealing temperature, reaches a minimum of {alpha} = 0.016 at an annealing temperature of 350 {deg}C, and then increases to 0.024 after post-annealing at 400 {deg}C. The minimum {alpha} observed at 350 {deg}C is rationalized by two competing effects as the annealing temperature becomes higher: the enhanced crystallization of CoFeB and dead-layer growth occurring at the two interfaces of the CoFeB layer. We further demonstrate that {alpha} of the 400 {deg}C-annealed W/CoFeB/MgO film is comparable to that of a reference Ta/CoFeB/MgO PMA film annealed at 300 {deg}C, justifying the enhanced thermal stability of the W-seeded CoFeB films.
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