Magnetization-reversal processes in an ultrathin Co/Au film

Magnetization-reversal processes in a ferromagnetic cobalt film structure (Au/Co/Au), with perpendicular anisotropy, were investigated by magneto-optical magnetometry and microscopy. In the considered ultrathin Co film, the magnetization reversal between the two Ising-spin equilibrium states is dominated by the domain-wall motion mechanism. We focused our studies on processes initiated from a given demagnetized state. Starting from a magnetically saturated state generated under a large field ${\mathrm{H}}_{\mathrm{S}}$, applied perpendicular to the film, this demagnetized state is created through magnetic aftereffects in a field ${\mathrm{H}}_{\mathrm{d}}$ antiparallel but smaller than ${\mathrm{H}}_{\mathrm{S}}$ and applied during a selected time. Direct (${\mathrm{R}}^{\mathrm{D}}$) and indirect (${\mathrm{R}}^{\mathrm{I}}$) magnetization processes are then studied from this state for application of the field parallel and antiparallel to ${\mathrm{H}}_{\mathrm{d}}$, respectively. The dynamics of the magnetization reversal is much faster for the ${\mathrm{R}}^{\mathrm{I}}$ process since it is initiated from a quasihomogeneous 'Swiss cheese' domain state with small nonreversed regions. The magnetic accommodation phenomenon is studied, and a domain-shape memory effect evidenced. A theoretical analysis of the dynamics of magnetization processes is proposed, starting from the model of a patchy inhomogeneous media with a realistic distribution of local coercivities. The pertinent parameters for calculations are deduced from our experimental data using appropriate analytical expressions of the magnetic relaxation time and domain-wall velocity under a field. Computer simulations using these parameters reproduce well the time evolution of the magnetic domain pattern and different magnetization curves both for ${\mathrm{R}}^{\mathrm{D}}$ and ${\mathrm{R}}^{\mathrm{I}}$ magnetization processes.