Advisor(s)

Latika Menon

Contributor(s)

Don Heiman, Moneesh Upmanyu

Date of Award

2011

Date Accepted

12-2011

Degree Grantor

Northeastern University

Degree Level

M.S.

Degree Name

Master of Science

Department or Academic Unit

College of Engineering, Department of Mechanical Engineering

Keywords

mechanical engineering, materials science, nanoscience, condensed matter physics, GaMnAs, magnetoresistance, MnAs, nanomagnetism, nanoparticles, superparamagnetic

Disciplines

Mechanical Engineering | Nanoscience and Nanotechnology

Abstract

The unique properties of single domain magnetic nanoparticles have been studied since the 1950's. When a material is geometrically confined to nanoscale dimensions its characteristic properties can change dramatically. In this thesis the magnetic and magnetotransport characteristics of GaMnAs and MnAs nanostructured systems are explored in several novel experimental studies. The ternary magnetic semiconductor GaMnAs was investigated under 3 dimensions (3D) of spatial confinement.1 Up until this research, GaMnAs hadn't been successfully fabricated, or studied, as a nanodot. By creating a novel patterning approach using anodic porous alumina, the superparamagnetism of confined GaMnAs was able to be acquired using SQUID magnetometry. I also studied GaAs crystalline nanocomposites fabricated with dispersed MnAs nanoparticles. The embedded nanoparticles created a disordered electronic structure displaying a linear-in-H magnetoresistance up to very high fields (14 T). Through further thermomagnetic characterization studies and data analyses, a fascinating relation was realized. It turned out that a non-saturating linear magnetoresistance is governed strictly by the average macroscopic carrier mobility and not carrier concentration. This is a very important observation, for it realizes a universal model for the nature of linear magnetoresistance phenomena in disordered crystalline materials. Finally, we developed a non-invasive method to quantify the size distribution of nanoparticles in the GaAs matrix. This numerical method makes use of thermomagnetic measurements from SQUID magnetometry to generate a plot for the distribution probability vs. particle diameter. Implementation on the MnAs-GaAs samples resulted in a log normal distribution of superparamagnetic particles through the matrix. Overall, this thesis outlines studies that make definitive steps forward in our understanding of the superparamagnetism of magnetic nanodots, magnetotransport in linear magnetoresistive nano-composite systems and the quantified calculation of a composite thin films nanoparticle size distribution.

Document Type

Master's Thesis

Rights Information

copyright 2011

Rights Holder

Steven Bennett


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