Perpendicular exchange bias in ferromagnetic/antiferromagnetic multilayers and magnetization switching

Exchange bias is an important magnetic interaction existing at the interface between ferromagnetic (FM) and antiferrromagnetic (AFM) coupled layers. This phenomenon has attracted an extensive amount of attention due to its technological applications and rich physics. Better understanding of the exchange bias and related magnetic behaviors of artificial magnetic structures will have significant impact on improving device performances and gaining insight into fundamental magnetic physics.;Previous studies on exchange bias have been focused on applying the cooling field along the sample easy axis, and most of the angular dependent magnetization reversal studies of thin film structures are limited in plane without consideration of shape anisotropy. In this dissertation, we focus on our research of establishment of perpendicular exchange bias through the control of cooling field direction. Investigation of temperature and constituent layer thickness effects demonstrates that the exchange bias induced by perpendicular field cooling has very similar behaviors to the conventional longitudinal exchange bias. Although the magnitudes of the induced anisotropies have quantitative differences for the two field cooling geometries, the perpendicular exchange bias still reflects the same interfacial nature of the FM/AFM interaction. In our research, systematic angular dependent magnetization reversal measurements have been performed for both perpendicular and longitudinal exchange biases. Angular dependent hysteresis loops with distinctive characteristics from convention have been observed for out-of-plane measurements. To explain the magnetization reversal process and quantify the observed angular dependence, a three-dimensional magnetization reversal model based on S-W coherent rotation hypothesis has been developed. After taking into account the competitive field cooling induced anisotropies and the thin film shape anisotropy, the three-dimensional coherent rotation model alone can quantitatively explain the observed out-of-plane angular dependence for both field cooling geometries. Magnetization reversal is found to be accomplished by coherent rotation out of the plane determined by anisotropic axis and applied field. The model is also applicable in predicting the more general thin-film magnetization reversal with out-of-plane applied fields.;Our study provides direct evidence that through appropriate field cool procedure exchange bias can be established and controlled in any directions in the space. The realization of exchange bias in conventional soft FM thin films indicates that perpendicular anisotropy can be cost-effectively achieved instead of using high cost perpendicular films for recording and sensor applications.