Effects of anisotropy and domain structures in exchange biased thin films.

摘要

I studied (Ni or Fe)/FeF2 and Co/FeMn exchange biased systems with interesting behaviors that depend on their structural details and are not described by the original exchange bias theory by Meiklejohn and Bean. I showed that in Ni/epitaxial FeF2 thin films, the sample divided into two regions of oppositely biased domains with increasing cooling field. The positively biased region grew at the expense of the negatively biased one. These results were attributed to a large antiferromagnet (AF) domains compared to ferromagnet (FM) domains, showing the importance of the length scales of the domain structures. Next, I showed that in exchange biased films containing epitaxial, twinned or polycrystalline FeF2 AF layers, the exchange field direction depended on the crystallinity of the AF layer and the angle of the cooling field. The cooling field selected a symmetry direction, defined by the AF spin axes, as the exchange field direction. This showed that the cooling field and the AF anisotropy jointly determine the exchange bias direction. I also studied the magnetization reversal mechanisms of Fe/epitaxial FeF2 with vector magnetometry and the first order reversal curve (FORC) technique. The magnetization reversal was predominantly by rotations when the applied field made an angle with the AF spin axis. FORC showed that the rotations were highly irreversible when the angle was small and became reversible at larger angles. A modified Stoner-Wohlfarth model reproduced the overall trend of the irreversibility evolution. The remaining discrepancies between the modeled and measured irreversibility were attributed to the formation of local incomplete domain walls during the reversal of the FM. Lastly, I studied the effects of rotating the applied field with respect to the cooling field direction on Co/FeMn and Ni/FeF2 thin films. For Co/FeMn, the exchange field no longer pointed along the cooling field direction when the applied field was rotated. The angular change in the direction of the exchange field was hysteresic and more apparent in thinner FeMn layers. In Ni/FeF2, there was no change in the direction of the exchange field. This rotational hysteresis (or lack thereof) was due to the anisotropy strength in the AF layer.