Authors:	Krautscheid, Pascal
Title:	Magnetic domain walls and spin current-driven magnetization manipulation in confined geometries probed with high resolution SEMPA
Online publication date:	11-Jun-2018
Year of first publication:	2018
Language:	english
Abstract:	The prevalent data storage devices today rely on a controlled magnetization modification of thin ferromagnetic films or elements on a nanometer scale. The traditional approach of Oersted field-induced magnetic switching combined with advances in magnetic sensing enabled an exponential increase in data density and proposed concepts predict further advances in this field. However, general read/write latency and random-access performance stagnated and the data storage device became a bottleneck in computer architecture. Subsequently, an increased interest in unexploited mechanisms led to the development of new concepts including the spin polarized current-induced manipulation of well-defined magnetic states. For instance, the racetrack memory device proposed by Parkin in 2008 employes the current-induced displacement of domain structures along thin and narrow magnetic wires. This requires first the investigation of suitable geometries where the spin configuration can be reliably tailored and controlled and secondly an understanding of the fundamental physical parameters that govern the magnetization dynamics under current excitations. In this thesis, we considered two expected requirements for domain wall-based spintronics and studied the stability of tailored domain wall states as well as effective approaches to change technologically relevant system parameters. First, we studied the controlled formation of magnetic domain walls in ferromagnetic rings made of iron for various sizes by varying the thickness and inner diameter in a regime relevant for devices using a high resolution scanning electron microscope with polarization analysis (SEMPA). Micromagnetic simulations at 0K were performed mimicking the nucleation process and accounting for the metastability of intermediate magnetic spin configurations. Accordingly, it has been shown that the lowest energy state determined by comparing the magnetostatic and exchange energy of a transverse and vortex domain wall configuration is not necessarily accessible at low temperatures. Furthermore, a careful analysis of the experimental data revealed that in addition to the geometry, the influence of materials properties, defects and thermal activation all need to be taken into account in order to understand and reliably control the experimentally accessible states, as needed for device applications. To further understand approaches of magnetization manipulation, the interaction of a spin polarized current with a flux-closure magnetic vortex state for variously doped Permalloy-alloys was studied. A controlled displacement of the well-defined vortex core region was observed for up to four different energetically identical magnetic states and the acting spin torque contribution was subsequently isolated to determine the non-adiabatic parameter, ξ. The measurement was repeated for different dysprosium dopant-concentrations and an increase in ξ concurrent with an increase in the independently measured Gilbert-damping parameter, α, was observed. However, ξ/α was constant within the studied dopant concentration range and thus faster domain wall motion at lower current densities as required for efficient spin transfer torque-driven domain wall-based devices can not be accomplished by a low Dy dopant concentration and has to be achieved with a different approach.
DDC:	530 Physik 530 Physics
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 08 Physik, Mathematik u. Informatik
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-4436
URN:	urn:nbn:de:hebis:77-diss-1000020315
Version:	Original work
Publication type:	Dissertation
License:	In Copyright
Information on rights of use:	https://rightsstatements.org/vocab/InC/1.0/
Extent:	114 Seiten
Appears in collections:	JGU-Publikationen