Irreversibility of the magnetisation process in ultrathin Pt/Co/Pt films

Magnetic aftereffect processes and field-induced domain structures are investigated in Pt/Co(h)/Pt ultrathin ferromagnetic films with perpendicular anisotropy (h <1.6 nm) deposited on Si/Si3N4 substrates. Irreversibility in the dynamics of the magnetisation reversal is studied for different Co thickness h, starting from the demagnetised domain state. This behaviour can be well interpreted from dynamic domain observations, implying "Swiss cheese" domain structures consisting of magnetically non-reversed entities into large reversed domains. The irreversibility is much more pronounced near the reorientation critical thickness.