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Document Type

Campus-Only Access for Five (5) Years

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Physics

Year Degree Awarded

2015

Month Degree Awarded

May

First Advisor

Prof. Mark T. Tuominen

Subject Categories

Condensed Matter Physics | Engineering Physics

Abstract

This study was conducted with the aim of improving permanent magnetic properties of existing materials and exploring non-conventional ferromagnetic properties of gold-based nanoclusters. The first chapter of this dissertation gives an introduction to relevant fundamental concepts and proceeding chapters present findings of three projects.

In the first project, shape anisotropy induced permanent magnetism in oriented magnetic thin films was investigated. Roll-to-roll nanoimprinting, a high-throughput fabrication method was utilized to fabricate densely packed Fe nanostripe-based magnetic thin films that exhibit large in-plane uniaxial anisotropy and nearly square hysteresis loops at room temperature. (BH)max exceeds 3 MGOe for samples of intermediate thickness and the anisotropy dependence on film thickness was also investigated. Temperature dependent magnetic measurements and micromagnetic simulations confirmed that the magnetization reversal is dominated by curling reversal mode.

The second project is an investigation of how magnetization reversal of hard and exchange-spring magnets is affected by substrate properties. Thin layers of SmCo5 and SmCo5/Co with varying thickness were grown at 500o C by sputtering on MgO(100) and glass with a Cr underlayer. X-ray diffraction studies reveal that in-plane hard magnetic properties is a result of SmCo5 (110), guided by the Cr (200) of the seed layer. The (BH)max of samples made on MgO(100) are higher due to high degree of SmCo5 formation with in-plane orientation by epitaxial guidance. Temperature dependent studies performed on exchange-coupled samples show that HC linearly increases with decreasing temperature, however, an exchange decoupling can be observed at lower temperatures only for the sample grown on glass.

In the third project, field-cooled and temperature dependent magnetic properties of Au25(SC6H13)18 spherical nanocluster are reported. This ensemble shows a weak exchange-biased behavior below 125 K. With increasing the cooling-field at constant temperature, the HC drops while MS rises which suggests that the magnetic state transforms from an exchange-bias like to a ferromagnetic dominated state. Temperature dependent measurements show unusual behavior in HC with the temperature, which drops first and then rises above 125 K, while magnetization rises non-monotonically. We believe this non-trivial phenomenon can be caused by magnetic phase transitions or thermally induced long-range interactions.

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