Influence of surface structure on the magnetic properties of RF plasma synthesized nickel-zinc ferrite nanoparticles

The development of magnetic materials for use in passive components for high-frequency applications will have far-reaching implications on the improved performance of power and high temperature electronic circuits. Ferrites are the most important materials for these applications because of their large resistivities, low conductive losses and reasonably high permeabilities. NiZn ferrites are the only materials that can be used in microwave applications (GHz range) such as power supply cores, because they have the highest resistivity among the ferrites. Many fundamental magnetic length scales like the single domain size, exchange length and high frequency penetration depths are on the order of 10–100 nm. Therefore the ability to tailor the microstructures of magnetic materials on the nano-scale will have far reaching implications for optimizing the performance of devices. In nanocrystalline ferrites, the symmetry and coordination of cations at the surface coordination polyhedral units may contribute differently to properties like magnetocrystalline anisotropy than in the bulk.;In this thesis, the influence of the polyhedral surface structure on the magnetic properties of NiZn ferrite nanoparticles synthesized using a TEKNA PL 50 RF induction plasma torch have been studied. The surface structure has been explored by TEM and complementary spectroscopic techniques-Mössbauer and EXAFS. The smallest nanoparticles exhibited perfectly octahedral shapes (only (111) surfaces), while the larger particles are truncated octahedral in shape (with more (111) type surfaces than (100) surfaces). The surface structure has been modeled with the various tilings of the tetrahedral and octahedral polyhedral units of the spinel structure on different surfaces. The contribution of the observed faceting behavior to the surface magnetic anisotropy of the (100) and the (111) surfaces will be discussed to interpret the dynamic transverse susceptibility and low temperature static magnetic measurements on these systems. A surface structure model has been proposed to explain surface spin canting and magnetic anisotropy for the observed (111) and (100) nanoparticle surfaces in the polydisperse as well as the chemically size-selected monodisperse nanoparticles. A decomposition model has also been proposed to explain the variations in chemistry within a single nanoparticle and this model has been used to interpret the sintering behavior of the as-synthesized nanoparticles.