Dynamics of the magnetization reversal in Au/Co/Au micrometer-size dot arrays

In high-quality Au/Co/Au ultrathin films with high perpendicular magnetic anisotropy, magnetization reversal occurs through easy domain-wall (DW) propagation following rare nucleation events, located at major structural defects. By patterning arrays of dots in such films, we block the DW propagation, and thus sample the intrinsic distribution of nucleation sites, improving precision as the dot diameter decreases. In a Au/Co (1 nm)/Au film we have fabricated large area arrays of round dots, with diameters of 1 and 2 mu m, leaving aside an unpatterned area as a witness of the magnetization reversal in a "continuous film" having undergone all patterning steps. Polar magneto-optical (MO) Kerr effect was used in both global and imaging experiments to accurately measure the hysteresis loops and aftereffect phenomena. We show that, despite limited damage induced by patterning, the expected behavior is indeed observed. A statistical mode! was developed, assuming an intrinsic distribution of nucleation sites in the initially continuous film, a uniform nucleation volume V-n, and a linear dependence of the nucleation energy barrier E-n=2M(s)V(n)(H-n-H) on both applied field H and nucleation field H-n at a given site (M-s is the saturation magnetization). Comparison between experiments and theory shows an excellent overall agreement, and allows one to obtain an approximate view of the distribution of nucleation fields. We could extract two fundamental lengths, the nucleation length xi(n), (related to V-n), and the mean distance I between nucleation sites. xi(n) was found to be equal to 26+/-1 nm, in good agreement with previous determinations on similar films, l, equal to about 430 nm, could be viewed as a measure of the typical distance between major structural defects in the "continuous film." Our method is indeed a means to characterize nanometer scale magnetic events (reversal of a nucleation volume), using micrometer scale resolution experiments (MO imaging). This is an example of how microfabrication can help us to understand magnetization reversal in a continuous ultrathin film.