Magnetization dynamics in perpendicular magnetic anisotropy CoFeB/MgO system

Abstract

We study the ultrafast magnetization dynamics of perpendicularly magnetized MgO/CoFeB/Ta and Ta/CoFeB/MgO stack structures which gains wide attention from a viewpoint of perpendicular magnetic tunnel junctions. An all-optical time-resolved magneto-optical Kerr effect measurement reveals that effective Gilbert damping α stays unchanged at ~0.02 in high external field regardless of pump fluence, but it declines drastically with the increase of pump fluence in weak external field. This can be explained by the enlarged apparent relaxation time due to slow remagnetization. Genuine damping tends to attract the precessing magnetization vector towards the effective equilibrium axis, while the tip of the reduced magnetization vector after pump pulse heating recovers growing away from the effective axis. These two competing contributions determine the apparent relaxation time. In a weak field regime, slow recovery of the magnetization vector results in the increased relaxation time and low effective Gilbert damping. We believe that low Gilbert damping found in CoFeB/MgO structure will be expected to reduce the critical current for current-induced magnetization switching.