Validation of a generalized model for the description of polycrystalline exchange-biased magnetic thin films

Applications based on nano- and microscaled systems have become indispensable in many aspects in everyday life, industry, research and medicine. In this regard, thin film technology plays a crucial role with the physical properties of solids changing significantly when they are reduced to thin layers with thicknesses in the nanometer range. In this regime, a solid’s microstructure as well as boundary and surface effects start to dominate. A prominent example is the exchange bias effect, which results due to the combination of a thin ferro- and antiferromagnet in a defined preferred direction of the ferromagnetic magnetization owing to exchange interaction between interfacial magnetic moments. This is generally reflected in the broadening and the horizontal shift of the ferromagnetic hysteresis. Commonly, the phenomenon is exploited within read heads based on the giant- and tunnel magnetoresistance effect for the implementation in magnetic storage and sensor devices. Additionally, it further has great potential to be integrated in emerging spintronic applications and versatile miniaturized technology platforms applicable, e.g., in a biomedical context. In the case of polycrystalline systems, the exchange bias effect is strongly determined by the size distribution of antiferromagnetic grains as well as their crystal structure. The thesis at hand investigates the validity of a generalized theoretical description of the observable phenomenology which discretizes the continuous distribution of antiferromagnetic grain sizes into thermally stable and unstable grains in dependence on experimental temperatures and times, e.g., during measurement. For this purpose, an existing model approach was mathematically reformulated and further developed. Phenomenological relations have been derived from it, interlinking macroscopically measurable quantities with averaged parameters of the microstructure. Experimentally, prototypical exchange-biased bilayers have been fabricated via sputter deposition upon variation of different fabrication parameters. Structural and magnetic characterization techniques have been used to specify deposition conditions resulting in homogeneous and columnar grain growth simultaneously guaranteeing a preserved crystallinity, layer quality and distribution of interfacial contact areas. This allowed to tailor the grain size distribution with the help of the antiferromagnetic layer’s thickness and to systematically investigate the contributions of different grain classes in dependence on measurement angle and time. In addition to the validation of the generalized model approach, it was further evidenced that the antiferromagnetic order that actively contributes to the interfacial exchange coupling extends only partially over the actual structural volumes of the antiferromagnetic grains. Furthermore, the simultaneous presence of thermally unstable and stable grains was found to be a necessary condition for an asymmetric magnetization reversal resulting from different probabilities for the nucleation of magnetic domains on one of the hysteresis branches. Finally, the remagnetization behavior upon the application of non-saturating magnetic field sequences has been investigated experimentally and theoretically. Thereby, a viscous magnetization decrease upon increasing driving field has been observed, which was correlated in agreement with the generalized model description to a dominating contribution of thermally unstable antiferromagnetic grains.