Domain-wall dynamics, pinning, and nucleation in ultrathin epitaxial Fe films

We have studied the dynamics of magnetic switching in in-plane magnetized ultrathin epitaxial Fe films by time-resolved magneto-optical magnetometry and microscopy. Experimental evidence has been found for two domain-wall pinning mechanisms: macropins and micropins. Macropins are extrinsic defects that are spatially distributed on the 10-\ensuremath{\mu}m length scale; micropins are intrinsic defects that are spatially distributed on a much shorter length scale. Once free from macropins, domain walls even in ultrathin films show line tension effects. We report a strong dependence on the magnetization direction of the relative importance of domain nucleation processes compared with domain-wall propagation processes. In-plane magnetized films do not readily nucleate reverse domains, whereas out-of-plane magnetized films of identical morphology do. This can be understood as being due to the different demagnetizing fields and domain-wall pinning energies.