Date of Award

12-2008

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

E. Ward Plummer

Committee Members

Jian Shen, Pengcheng Dai, James R. Thompson, Bin Hu

Abstract

In the past, Low Dimensional Materials by Design group at ORNL in collaboration with students from the University of Tennessee, have successfully tailored and studied magnetic nanostructures in 2D, 1D and 0D spatial confinement on Cu(111) substrates. They observed a striking collective ferromagnetic long-range order in Fe-nanodots on Cu(111) surface which can be stabilized through the indirect exchange interaction mediated by the substrate. This type of magnetic interaction was expected to have little effect on promoting a global ferromagnetic order in a randomly distributed dot assembly. It is for certain that we need a better understanding of the relative roles of magnetic anisotropy and magnetic interaction in the magnetism of reduced dimensionality, in general, and nanodot assemblies, in particular. With this accomplishment in mind, I’m trying to study the collective behavior of interacting magnetic nanoparticles.

We proposed the following experiments to quantify the relative roles of magnetic anisotropy, dipolar interaction and indirect exchange interaction on the collective magnetic behavior of nanodot assemblies. They consist of two main projects:

a) study how the indirect exchange interaction is affected by modifying the surface states. The interaction is observed through the changes in the critical temperature (Tc) of Fe dots as a function of miscut angle Cu curved substrate. Depending on the buffer layer (Xe) coverage, we observed a critical terrace width above which the Tc slightly increases and below which the Tc decreases rapidly. In other words, the (Tc) depends largely on the Cu(111) miscut angle.

b) study the role of magnetic anisotropy and dipolar interaction. We used Co dots on TiO2(110) substrate as a prototype. We observed significant perpendicular magnetic anisotropy with perpendicular easy axis for both large and small dot densities with no sign of hysteresis observed down to 2 K.

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