Jacob Elkins

Graduation Semester and Year

2020

Language

English

Document Type

Thesis

Degree Name

Master of Science in Materials Science and Engineering

Department

Materials Science and Engineering

First Advisor

P J Liu

Abstract

Co is an attractive element in the development of nanostructured magnetic materials due to its relatively high moment (1.72-1.75 µB) and strong exchange and spin-orbit interactions, and many of these materials have potential in applications such as data recording media, hyperthermia treatments, and nanostructured permanent magnets. In this thesis, magnetic nanostructures of CoFe2O4, Co, and CoNi with controlled size, shape, composition, and crystal structure are synthesized via bottom-up wet chemical methods and their magnetic properties are systematically characterized. Additionally, nanocomposites of exchange-coupled FeCo/CoFe2O4 and Co/FeCo core-shell nanostructures with enhanced magnetizations are prepared and investigated. In order to understand the prominent surface effects, CoFe2O4 nanoparticles are investigated by comparing nanoparticles of sizes 3.5 nm and 16 nm. It is found that the 3.5 nm nanoparticles in an assembly have a coercivity of 23 kOe at 5 K, which is attributed to the exchange coupling of the core spins to the surface disordered spins. The magnetization value is enhanced to 108 emu/g by synthesizing FeCo/CoFe2O4 core-shell nanocomposites of size 10.5 nm, and compared to pure CoFe2O4 nanoparticles, these core-shell nanoparticles are shown to have enhanced dipolar and exchange interactions. More importantly, the alignment of these nanoparticles in an assembly is shown to have a significant effect on the coercivity, reaching as high 22.4 kOe at 10 K for an assembly aligned under an external field. Co nanowires were synthesized using a thermal decomposition method, and their diameters were controlled by varying the concentration of surfactant. A diameter as low as 10 nm was achieved and the coercivity reached 10.4 kOe for an aligned assembly. The 10 nm nanowires were then coated with FeCo using an electroless plating method, which increased the magnetization saturation from 150 emu/g for the pure Co to a max of 182 emu/g for the Co/FeCo nanocomposite. In addition to the Co nanowires, Co1-xNix nanowires and nanoparticles were synthesized using a solvothermal approach. The Ni concentration was varied by controlling the Ni precursor concentration. The effects of the Ni concentration on the morphology, structure, and magnetic properties are investigated. Systematic investigation of the nanoparticles shows that the CoNi can retain nanowire morphology and hcp crystal structure up to a 30% Ni concentration, with further increase of Ni leading to the growth of polyhedral fcc nanoparticles. The magnetic performance of the CoNi nanoparticles with varying Ni concentration in assemblies is then investigated.

Keywords

magnetic nanoparticles, superparamagnetism, shape anisotropy

Disciplines

Engineering | Materials Science and Engineering

Comments

Degree granted by The University of Texas at Arlington

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