Low Gilbert Damping Constant in Perpendicularly Magnetized W/CoFeB/MgO Films with High Thermal Stability

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 (T-ann) 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 T-ann, reaches a minimum of alpha=0.015 at T-ann=350 degrees C, and then increases to 0.020 after post-annealing at 400 degrees C. The minimum alpha observed at 350 degrees 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 alpha of the 400 degrees C-annealed W/CoFeB/MgO film is comparable to that of a reference Ta/CoFeB/MgO PMA film annealed at 300 degrees C, justifying the enhanced thermal stability of the W-seeded CoFeB films.