Date of Award

2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering (PhD)

College, School or Department Name

Department of Materials Science and Engineering

Advisor

Yongmei M. Jin

Abstract

Structures, properties and functionalities of magnetic domain walls in thin film, nanowires and atomic chains are studied by micromagnetic simulations and ab initio calculations in this dissertation. For magnetic domain walls in thin films, we computationally investigated the dynamics of one-dimensional domain wall line in ultrathin ferromagnetic film, and the exponent α = 1.24 ± 0.05 is obtained in the creep regime near depinning force, indicating the washboard potential model is supported by our simulations. Furthermore, the roughness, creep, depinning and flow of domain wall line with commonly existed substructures driven by magnetic field are also studied. Our simulation results demonstrate that substructures will decrease the roughness exponent ζ, increase the critical depinning force, and reduce the effective creep energy barrier. Current induced domain-wall substructure motion is also studied, which is found quite different from current induced domain wall motion.

For magnetic domain walls in nanowires, field and current induced domain wall motion is studied, and some relevant spintronic devices are proposed based on micromagnetic simulations. Novel nanometer transverse-domain-wall-based logic elements, 360° domain wall generator and shift register are proposed. When spinpolarized current is applied, the critical current for domain wall depinning can be substantially reduced and conveniently tuned by controlling domain wall number in the pile-up at pinning site, in analogy to dislocation pile-up responsible for Hall-Petch effect in mechanical strength. Furthermore, threshold currents for domain wall depinning and transportation through circular geometry in planar nanowire induced by spin transfer torques and spin-orbit torques are theoretically calculated. In addition, magnetic vortex racetrack memory which combines both conceptions of magnetic vortex domain walls and racetrack is also proposed using micromagnetic simulations.

For magnetic domain walls in Ni atomic chains, a truly magnetic domain wall structure and the single domain switching process are investigated by both ab initio studies and spin dynamics simulations. Spin moment softening effect caused by the hybridization effect between two spin channels is considered. The atomic domain wall as narrow as 4 atom-distance with slight spin moment softening effect indicates a relatively evident ballistic magnetoresistance effect, and the large EB indicates the strong stability of single domain state.

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