| When a ferromagnet/antiferromagnet (FM/AF) bilayer is cooled below the Neel temperature TN of the AF in a magnetic field, exchange bias (EB) phenomenon arises. Although EB was discovered nearly 50 years ago, a general understanding is still lacking regarding the competing interactions and length scales involved and how they give rise to a rich variety of magnetization reversal behavior. In this thesis, I will address these questions by studying the cooling field dependence, asymmetric magnetization reversal, and spontaneous reversal in epitaxial FeF2/polycrystalline FM bilayers. Two types of transitions from negative to positive EB with increasing cooling fields were found: for in-plane twinned FeF2, a continuous transition was found, while coexistence of EB of both signs was observed for untwinned FeF2. This is attributed to the relevance between the AF "domain" size and the FM domain wall width and confirmed by micromagnetic simulation. Nanostructuring the FM significantly decreases the onset cooling field for positive EB with decreasing dot sizes when the FM dot size is comparable with the AF "domain" size (∼500nm). The high quality epitaxial exchange bias system also constitutes a model system for studying magnetization reversal asymmetry. By vector magnetometry and simulation, we found that the FM reverses through parallel domain walls, which results in highly asymmetric hysteresis loops. Also, the FM near the FM/AF interface exhibits a more asymmetric reversal than that farther away from the interface. These results unambiguously show the existence of a FM parallel domain wall and its importance in asymmetric magnetization reversal. Moreover, we found another surprising phenomenon that when positively exchange biased, the FM can spontaneously reverse its magnetization in a constant field when cooled below TN due to the strong interfacial coupling. When heating up in the same field, the FM magnetization reverses at T > TN, giving rise to a tunable thermal hysteresis. Discovery of this phenomenon suggests a revised energy competition mechanism for positive EB, which includes parallel FM and AF domain wall energy. By studying these different but strongly related magnetization reversal behaviors, we demonstrated the central role of competing length scales and interactions in heterostructured magnetic systems. |
