Asymmetric magnetization reversal processes in exchange-biased microstructures

Over the last decades, the exchange bias (EB) effect has captured the attention of the research community striving for high-density logic memory devices and sensitive sensor technologies based on bi-, multilayered, or patterned magnetic thin film systems. Macroscopically, the EB manifests in a horizontal shift of the ferromagnetic remagnetization loop and an enhancement of the coercivity, both connected to the induced unidirectional anisotropy (UDA) at the exchange-coupled ferromagnetic/antiferromagnetic interface. Besides these characteristic properties, EB systems have often been correlated to asymmetrically shaped remagnetization curves, indicating non-equivalent reversal processes in the two separated hysteresis branches. Various studies attempted to understand the origin of this asymmetry, mostly interpreted as the interplay of the UDA and other prevailing axial anisotropies. Whereas the theoretical explanations are based on the analytical Stoner Wohlfarth model approach, assuming a ferromagnetic macro spin reversing by coherent rotation, the contributions of nonuniform reversals, i.e., incoherent rotation, domain nucleation, and domain wall expansion, have been proven in domain-imaging experiments. Especially, the latter ones are dominating in patterned magnetic geometries. Therefore, this dissertation is dedicated to achieving an in-depth understanding of the role of domain nucleation in asymmetric magnetization reversal processes in patterned EB systems, with a strong emphasis on periodic one-dimensionally confined EB microstructures. Magneto-optical Kerr measurements revealed a difference in the nucleation probabilities along the hysteresis loop branches in various types of EB topographically elevated and/or magnetically patterned microstructures with in-plane directed magnetization. The macroscopic magnetic properties and asymmetric nucleation probabilities have been discussed qualitatively within micromagnetic MuMax3 simulations and an empirical model for polycrystalline EB layer systems. In out-of-plane magnetized microstripes, the combination of the EB with the Dzyaloshinskii–Moriya interaction results in the rise of a branch-dependent, chiral asymmetric reversal process, quantitatively and qualitatively studied in vectorial vibrating sample magnetometry, Kerr and X-ray photoelectron emission microscopy investigations.