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Date of Award

9-2011

Document Type

Campus Access

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Physics

First Advisor

Mark T. Tuominen

Second Advisor

Narayanan Menon

Third Advisor

Anthony D. Dinsmore

Subject Categories

Condensed Matter Physics | Electromagnetics and Photonics | Nanoscience and Nanotechnology

Abstract

This thesis discusses experimental research and theoretical analysis on exploring the physics and techniques of manipulation of magnetization states of ferromagnetic nanorings in both homogeneous and non-homogeneous applied magnetic field. Magnetization states and their switching processes are fundamental properties of magnetic systems. The ring shape is particularly interesting because of the existence of the closed-flux vortex state, which can be used to encode binary information. The understanding and control of the magnetization switching of ferromagnetic nanorings could lead to new designs of practical magnetic data storage devices.

The work in this thesis is grouped into three main activities: theoretical analysis and micromagnetic simulation, fabrication techniques, and characterization of magnetization switching by applied magnetic field.

Ferromagnetic rings with different geometric parameters were fabricated by electron beam lithography (EBL), electron beam evaporation and lift-off techniques. EBL patterning on double layer photo resist improved lithography and lift-off resolution.

The experiments with applied homogeneous and non-homogeneous fields were able to manipulate the ferromagnetic nanorings through different magnetic configurations. The key accomplishment of this work is we experimentally achieved direct switching between two magnetic vortex states of opposite circulation of magnetization, by using an applied azimuthal (circular) Oersted magnetic field. Such field was generated by applying current through the center of a ring using a platinum atomic force microscopy tip. We used magnetic force microscope imaging to demonstrate the controllability of magnetic switching from onion state to vortex state and for the first time, direct switching between two opposite vortex states. Moreover, we investigated the switching mechanisms associated with nucleation, annihilation, and propagation of domain walls. The magnetic switching properties were found to be sensitive to the ring geometrical parameters. Smaller rings require less circular field to complete the switching than bigger rings. Asymmetric rings require less circular field to complete the switching than symmetric rings with the same dimensions.

Theoretical analysis and micromagnetic simulations were conducted on symmetric and asymmetric nanorings, with the purpose of helping us better understand the physics behind the experimental results. Those systematic studies investigated the energy and stability of different magnetization states, the switching field required as a function of the ring geometric designs, as well as switching mechanism and the evolutions among different magnetic states, in both in-plane and azimuthal Oersted magnetic fields. We found the simulations results are in a good agreement with the characterization results.

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