Magnetism and magnetotransport of magnetic nanoparticle arrays.

摘要

As the size of particles scales down to nanometer regime, the physical properties are totally different from bulk counterparts. In magnetic materials, magnetic properties show strong size dependence, originating from finite size and surface effects. In this dissertation, I show that both magnetization and magnetotransport in magnetic nanoparticles can be dominated by surface magnetic configurations. In systems with competing antiferromagnetic (AFM) exchange interaction and ferromagnetic (FM) exchange interaction, a reduction of the total saturation magnetization is accompanied by a fast drop of magnetization at low temperatures for nanoparticles. Depending on the surface termination, AFM exchange interaction may contribute stronger at the surface than inside the particle, leading to such effects. Charge transport properties in nanoparticles are determined by inter-particle distances, which can be tuned by annealing conditions. The magnetoresistance (MR) is dominated by spin dependent scattering at the nanoparticle surfaces with spin disorder, and therefore MR can be fitted perfectly by a Langevin-like function regardless of annealing temperatures.