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Title: Domain wall motion and magnetization switching driven by magnetic/laser fields
Author: Wang, Yichuan
Awarding Body: University of York
Current Institution: University of York
Date of Award: 2018
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The work presented in this thesis mainly focus on the domain wall motion in planar Permalloy nanowires and magnetization switching in ferromagnetic Co/Pt thin films studied with both static Kerr imaging and dynamic pump and probe time-resolved magneto-optical Kerr effect (TR-MOKE) measurement techniques. The aim is to provide insightful support for the future novel data storage devices based on magnetic nanostructures. A spatially resolved wide-field magneto-optical Kerr imaging system has been successfully built during this project. The dependence of the coercivity of the Permalloy nanowires on different notch depth and geometries have been investigated systematically. The results show that the depinning field is strongly dependent on the detailed notch geometry. We have found that even the notches with same triangular feature, but different orientations will have a large depinning field difference. Our micromagnetic simulations support the experimental observations and show the correlation between the notch geometry, and the domain wall pinning and depinning processes. The laser-induced precession dynamics in Co/Pt thin films with multiple layers have been investigated by pump and probe TR-MOKE. By fitting the experimental curves via phenomenological formula, the effective Gilbert damping constants are retrieved. The results show that Gilbert damping constant has a significant external field and layer repetition number dependences. The more Co/Pt layer repeats number, the higher value of constant. This enhancement of the Gilbert damping constant value with more Co/Pt layer repeats could be due to the interfacial effect or decoherence. As the lattice mismatching in the Co/Pt structures increases with the thickness, the increased dislocation may promote electron hopping between two different sites which enhances the intrinsic damping in the thicker films. Combing the TR-MOKE system with the Kerr imaging, the magneto-optical responses of the ferromagnetic Co/Pt films with different Co thicknesses under the action of femtosecond laser beam have been explored systematically. We have developed a new approach to study the helicity dependence in the all optical switching (AOS) by varying the degree of helicity. Our results demonstrate unambiguously that the laser helicity plays an essential role in the AOS. We have further established a detailed relationship between domain switching percentage and the degree of the pumping laser helicity.
Supervisor: Wu, Jing Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available