Ripple Hysteresis in Thin Magnetic Films

Hysteresis measurements and electron micrographs have shown that the rotation in films with uniaxial anisotropy in low fields is not reversible, in contradiction to the single domain theory. A film with Hc/HK=2.3, for instance, has a remanence of 0.04 Ms in an alternating field of the amplitude Ĥ=0.4 HK parallel to the hard axis. The remanence depends on Hc/HK, on the dispersion of the anisotropy, and on the amplitude and the direction of the applied field. This irreversibility is caused by the magnetization ripple. If the magnetization is rotated, the ripple small-angle walls rearrange themselves by wall motion normal to the new magnetization direction. This wall motion requires a certain excess of energy which results in hysteretic losses even if the applied field strength is lower than the Stoner-Wohlfarth critical field for irreversible rotations.