Nanoscale magnetic particles. New methods to surface protected metallic and immiscible bimetallic clusters/particles

Abstract

The metal vapour-solvent low temperature matrix method has been used to prepare nanoscale iron powders. The morphology of the fine particles was characterized using transmission electron microscopy. Crystallite and particle sizes were determined via XRD and BET techniques. The magnetic properties of the powders were investigated using Mössbauer spectroscopy and SQUID magnetometry. X-ray photoelectron spectroscopy was used to probe the surface of the fine particles. The use of a non-polar solvent (hexane) to trap the atoms resulted in air-sensitive iron powders with some incorporation of carbon and hydrogen. Crystallite sizes of approximately 5 nm were observed and these systems behaved superparamagnetically at room temperature. The powders were easily oxidized to α-Fe 2O 3. Codeposition of iron with perfluorotri-n-butyl-amine (PFTA) resulted in the formation of FeF 2 as the major product. Reduced values of T N and θ c were observed for FeF 2 phase. The fine powders were found to be air and moisture sensitive forming FeF 3, FeF 3·H 2O and γ-Fe 2O 3 upon exposure to air. The bimetallic system of immiscible metals, Fe and Ag, was investigated by co-deposition of these metals with pentane. Separate phases of Ag and Fe were observed to be in these powders. Mössbauer investigations indicated that isolated iron atoms accounted for 5–27% of the iron present indicating some solid solubility of these two metals. Enhanced hyperfine fields were observed for the iron clusters. Fe 3O 4, formed during handling of the powders, had reduced hyperfine fields for both A and B sites as well as reduced Verwey transition temperatures. Iron powders prepared in pentane and trapped with 1-dodecanethiol exhibited unique behaviour after heat treatment. Small iron particles with a coating of FeS were formed after heat treatment at 300–600°C for 2 h under argon. Air sensitivity of the powders was greatly reduced after the formation of the sulphide surface layer. The possible existence of exchange interactions between the two phases was investigated using magnetic measurements and was found to be small but significant. These experiments demonstrate that metal atom clustering in low temperature matrices, followed by trapping/ligation and or heat treatment can yield a variety of new nanoscale materials. Crystallite sizes and outer coating material can be controlled and manipulated. Immiscible bimetallic particles, core-shell structures, and many other unique phases are attainable.