Large scale magnetic domain wall fluctuations in ultrathin cobalt films

Controlling anisotropy through ion bombardment is a convenient method for manipulating domain walls in perpendicularly magnetized films. In ultrathin (\textless 1nm) cobalt deposited on platinum, exposure to 50eV argon ions reduces the perpendicular magnetic anisotropy until the magnetization lies in plane.~ Just before this in-plane transition, the domain wall energy and pinning strength are reduced such that zero-field Barkhausen-like domain wall jumps become observable at zero field and room temperature. The domain wall jumps are large enough (\textgreater 100nm) to be measured optically. In this work we use magneto-optic Kerr effect to measure how these fluctuations depend on the film thickness and applied magnetic field. Furthermore, we observe magnetostatic correlations between fluctuations in nearby domain walls.