Date of Award
Doctor of Philosophy
James R. Thompson
M. Breinig, T. A. Callcott, J. L. Musfeldt, T. E. Haynes
Magnetization measurements have been performed on nanoparticle arrays of Fe, Co, Ni, and FePt in single-crystal substrates. Materials are formed by ion implantation into a layer followed by thermal annealing to give electrically isolated but crystallographically textured arrays. Studies show competition between eﬀective single-particle anisotropy (due to magnetocrystalline, shape, surface, and stress eﬀects) and macroscopic anisotropy due to dipolar interactions.
Results of these measurements are compared to expectations found in the well-known Stoner-Wohlfarth model of non-interacting, uniaxial particles. While this model is extensively used in magnetic materials research, the missing eﬀects from dipolar interactions between particles give very diﬀerent results than those measured here. To gauge the importance of interactions with respect to anisotropy, the magnetic length scales of Holz and Scherer are used.
Finite size eﬀects associated with shrinking lengths to the nanometer scale have also been measured. Some examples of these eﬀects are an enhanced moment in Ni nanoparticles due to a transition from ferromagnetic to paramagnetic behavior and an enhanced critical exponent β in FePt.
Micromagnetic simulations have been used to model these arrays. They conﬁrm the eﬀects of stabilized magnetization from dipolar interactions found in experiment. Also, they predict a diﬀering behavior between more typical 2D arrays and the thicker arrays formed in this project.
Sorge, Korey Dean, "Magnetic Properties of Nanoparticle Systems of Fe, Co, Ni, and FePt, Created by Ion Implantation. " PhD diss., University of Tennessee, 2002.