The propagation and nucleation of magnetic domain walls in multi-turn counter sensor devices
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Abstract
Magnetic domain walls are quasi-particles indicating the rotation of the magnetization direction on a constrained length scale. In magnetic nanoconduits, domain walls can be manipulated by an applied magnetic field, and the dynamics of the various processes occurring is still under active investigation. Despite being a state of the art research topic, domain walls are already utilized in nonvolatile sensors. In this sensor type, domain walls allow a true power on functionality as demanded by the industry for improved automatization. Currently, the field operating window of these sensors remains limited due to the lack of physical understanding of the parameters controlling it. This gap in our knowledge hinders an improvement of current devices and therefore blocks a route to a higher scaling of the technology. In this thesis, we present a detailed study of the propagation and the nucleation of domain walls in various looping structures relevant for applications and realize innovative concepts for the advancement of the field. To grasp insight on the physics controlling the propagation and nucleation field values, parameters such as the cross-sectional shape, the thickness, and the width of wires, as well as material stack, deposition conditions, and patterning processes, are modified. Applied field sequences are created that allow the comfortable measurement of both field values with a various range of measurement techniques such as the magneto-optical Kerr effect or the giant magneto-resistance effect. The propagation field of domain walls appears mostly insensitive to geometrical changes but affected by changes of the material used as the conduit. The propagation of domain walls shows a much higher sensitivity to pinning sites created by defects of the patterning processes thus constituting a challenge due to their arbitrary occurrence. In contrast, the nucleation field exhibits a large hyperbolic and linear scaling, respectively, with the width and the thickness of the wire thus yielding a more comfortable handle for the improvement of the field operating window. The results appear in good agreement with micromagnetic simulations that allow the identification of the physical processes at the origin of the saturation of the scaling of the nucleation field. Also, the field operating window shows a robustness towards modifications of the deposition conditions but a significant decrease after the use of electron beam patterning. The combination of the results from this comprehensive study highlights the impact of the investigated parameters and enables the systematic tuning of the ones needed for improvement of the field operating window. An innovative concept using the parallel coprime counting of closed-loop structures is then introduced. The closed-loop contains an essential cross-shape element, which appears unreliable under the rotation of an applied field without the placement of a syphon element on each arm of the cross. With the use of micromagnetic simulations, the individual angular dependence of the cross and the syphon are generated. The obtained physical processes provide with the field operation windows of the device concept for each element. The overlapping of their contribution yields a numerically working concept and its dependence on parameters such as syphon angle or cross-center dimension. The idea is then experimentally realized with variations in the cross and syphon geometry, and the results are in good agreement with the simulations. Finally, a complete built device helps to identify the nucleation in the center of the cross as a limiting factor for the improvement of the operating window of this concept, which was not observed in the previous simulations. This experimental realization demonstrates the feasibility of a magnetic domain wall based million-turn counting sensor device.