Magnetic field-induced instabilities and irreversible evolution in modulated ferromagnetic phases of cobalt films

Ferromagnetic films of HCP cobalt with a thickness above 50 nm possess an out-of-plane magnetization. Magnetic force microscopy (MFM) is used to investigate the modulated domains of alternatively up and down magnetization. By applying a perpendicular magnetic field, the low-energy stripe configuration undergoes a zigzag distortion. As is revealed by MFM, a further increase of the maximum applied field produces a pinching of the stripes, which break up into magnetic bubbles, We show to which extent the formation of two types of topologically different bubbles can explain the particular shape of hysteresis curves of these cobalt films. (C) 2002 Published by Elsevier Science B.V.