Magnetization reversal in ultrathin ferromagnetic films with perpendicular anisotropy

This paper presents a detailed study of the magnetization reversal dynamics in ultrathin cobalt films (t(Co) = 6-12 Angstrom) sandwiched by gold (1 1 1) layers, their magnetic anisotropy being perpendicular to the film surface. The domain wall (DW) velocity and nucleation rate are determined from direct time-resolved domain structure imaging. Measurements as a function of the applied magnetic field and temperature enable us to estimate the magnetic parameters, such as activation volumes, coercive fields, Gilbert damping parameters, etc., controlling the magnetization reversal. Depending upon the applied magnetic field value three different regimes are evidenced and studied. Al low fields the DW pinning by structural inhomogeneities controls the thermally activated magnetization reversal dynamics. Above the propagation held value the dynamics is due to viscous DW motion. In higher fields a DW velocity breakdown is observed. The DW jaggedness and its change with the applied field and temperature is examined through the DW fractal dimension. A simple DW motion simulation taking into account an activation volume distribution explains qualitatively the data.