Transformation kinetics & magnetism of magnetite nanoparticles.

摘要

This dissertation presents the results of a study of the nucleation and growth kinetics and magnetic properties of iron-oxide based nanoparticles that are formed by crystallization of a Na-Ca borate amorphous precursor. In addition to the interesting phase transformation kinetics and concentration dependent growth phenomena observed in this system, it also provides an opportunity to study finite-size effects on the magnetic properties of single domain particles. The crystallization of iron oxide nanoparticles and the phase identity (magnetite or maghemite) upon heat treatment was investigated over a range of time, temperature, and redox conditions. X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron diffraction (ED) were used to structurally characterize the resulting nanoparticles. Magnetite formation was favored under more reducing conditions whereas maghemite was formed under more oxidizing conditions. Under all conditions investigated, the particle size fell in the narrow range of about 2.5 to 4.5 nm and showed no evidence of further growth with time. This is believed to result from the increased viscosity in the iron-depleted diffusion field surrounding each particle. The magnetic properties were characterized using a superconducting quantum interference device (SQUID) and Mossbauer spectrometer (MS). Magnetization measurements were made from room temperature down to ∼10 K under field-cooled (FC) and zero-field-cooled (ZFC) conditions. The average blocking temperature obtained from these measurements is generally consistent with the measured particle size for reasonable values of the anisotropy constant. Unlike maghemite, bulk magnetite exhibits a metal-insulator transition, first reported by Verwey in 1939. The effect was attributed by Verwey and others to a charge-ordering transition. However, very recent data suggest that the Verwey transition is instead associated with a structural transition from inverse to normal spinel. In the present work, the magnetization data for samples produced under more reducing conditions show clear evidence of a Verwey transition in 4--4.5 nm magnetite nanoparticles. However, the transition is found to be shifted from the bulk value of ∼120 K to 85--95 K, which is attributed to finite-size effects.