Electrical & Computer Engineering, Department of
First Advisor
Mathias Schubert
Date of this Version
7-2020
Document Type
Thesis
Abstract
Modern material growth techniques allow for nano-engineering highly complex three dimensionally nanostructured materials. These nano-engineered materials possess highly anisotropic physical properties that are significantly different from that of their bulk counterparts. The magnetization properties of nano-engineered materials can be modified through a close range interaction known as magnetic exchange. These materials are referred to as magnetic exchange-coupled materials. Exchange-coupled magnetic materials are composite magnetic materials where the magnetization of one material is influenced by the magnetization state of the neighboring materials.
The author describes the creation of a representative sample set of exchange-coupled nanoengineered magnetic materials. These materials are created by glancing angle deposition (GLAD), which is a physical vapor deposition process. By means of a self shadowing, highly ordered nanostructures are created which are columnar in shape and form a quasi-thin film. The nanocolumnar films are heterostructured by subsequent material depositions of cobalt and permalloy (Ni80Fe20). Atomic layer deposition (ALD) is utilized to introduce ultra thin dielectric gates between cobalt and permalloy in order to modify the exchange-coupling. ALD is further used to passivate the overall structures with conformal Al2O3 coatings to prevent oxidation.
The structural properties of the nanoengineered materials are determined by scanning electron microscopy and generalized ellipsometry. The magnetic properties of the nanoengineered materials are determined at room temperature by vibrating sample magnetometry and by vector magneto-optical generalized ellipsometry (VMOGE). Vector magneto-optical generalized ellipsometry is a setup that combines generalized ellipsometry with a programmable vector magnet. The eight pole vector magnet is capable of producing magnetizing fields of arbitrary amplitude within the given physical limits and arbitrary spatial orientation to perform hysteresis loop measurements on quasi thin film samples. Mueller matrix generalized ellipsometry data are gathered from the sample with and without the magnetizing field present. Through a differencing process, the magnetically induced changes to the data are isolated. The data is analyzed with a layered optical model to determine the three dimensional magnetically induced changes to the dielectric tensor. These changes to the dielectric tensor of the magnetized material are proportional to the magnetization of the material.
To quantify the magnetization properties of the materials the author develops hysteresis based models. The hysteresis models are compared to the characterization techniques that were performed on the representative sample set. The comparison of data for the representative sample set demonstrate the tailoring of magnetic properties, in particular the energy product, based on the geometry and material selection. The studies performed on the representative sample set by specialized means have found that the highly anisotropic magnetic properties are able to tuned by material selection and geometry. In particular, the use of GLAD in coordination with ALD allows for the control of the magnetic reversal by tuning the exchange-coupling between material subsections in the representative sample set.
Advisor: Mathias Schubert
Comments
A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Electrical Engineering, Under the Supervision of Professor Mathias Schubert. Lincoln, Nebraska: July 2020
Copyright 2020 Chad Briley