Characterisation of nanocrystalline g-Fe2O3 with synchrotron radiation techniques

Synchrotron radiation X-ray scattering and X-ray absorption spectroscopy techniques have been used to characterise the size distribution, crystallographic structure and the local atomic environment of nanocrystalline Fe2O3 particles in order to correlate structure with magnetic properties. The material has been prepared using the microwave plasma technique and Fe(CO)5 as a precursor. The synthesis parameters have been varied to obtain particles with a narrow particle size distribution and a typical weighted mean diameter of 5 nm. A subset of samples has in addition been coated with methylacrylic acid in order to prevent particle growth. The particles crystallise in the cubic maghemite (-Fe2O3) structure with iron occupying tetrahedral and octahedral sites in the inverse spinel lattice. Due to vacancies on octahedral sites, competing magnetic interactions of disordered crystal sublattices as well as surface effects, the spin lattices experience frustration and disorder. An accurate determination of the local geometrical structure and possible structural disorder should provide the basis for detailed modelling of the magnetic structure. In addition to the determination of local structure using EXAFS, the radial distribution function G(r) of the -Fe2O3 reference (figure 1) as well as of a nanocrystalline sample (figure 2) has been derived using high quality data from the ESRF beamline ID 31 and a scattering range up to q=2.6 nm-1. The magnetic blocking temperature as well as AC susceptibility of the ultra-fine particles have been determined and related to particle structure and particle size.