title: Study of the magnetite to maghemite transition using microwave permittivity and permeability measurements.
abstract: The microwave cavity perturbation (MCP) technique is used to identify the transition from magnetite (Fe3O4) to the meta-stable form of maghemite (γ-Fe2O3). In this study Fe3O4 was annealed at temperatures from 60 to 300 °C to vary the oxidation. Subsequent to annealing, the complex permittivity and magnetic permeability of the iron oxide powders were measured. The transition to γ-Fe2O3 was corroborated with x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometry (VSM). XRD, XPS and VSM implied that the starting powder was consistent with Fe3O4 and the powders annealed at more than 200 °C were transitioning to γ-Fe2O3. The MCP measurements gave large differences in both complex permittivity and magnetic permeability of the two phases in the frequency range of 2.5-10.2 GHz. Magnetic permeability decreased with annealing temperature, though magnetic losses showed frequency dependent behaviour. Complex permittivity measurements showed a large decrease in both dielectric constant and losses at all measurement frequencies, as well as a prominent loss peak centred around the phase transition temperatures. We interpret the loss peak as being a consequence of field effects due to an intermediate multi-phase mixture. Additionally, almost no frequency dependence was observed. The reduction in complex permittivity implies that the Feoct(2+) cations in the lattice provide a significant contribution to polarization at microwave frequencies and the effects of Feoct(3+) are nominal in comparison.. The change in loss can be explained as a combination of the differences in the effective conductivity of the two phases (i.e. Fe3O4 exhibits electron-hopping conduction whereas the presence of vacancies in γ-Fe2O3 nullifies this). This shows that the non-invasive MCP measurements serve as a highly sensitive and versatile method for looking at this phase transition in iron and potentially the effects of oxidation states on the polarization in other iron oxides.
authors.affiliation: Cardiff Wolfson Centre for Magnetics, School of Engineering, Cardiff University, Cardiff, CF24 3AA, UK.
authors.forename: Jerome Alexander
authors.initials: JA
authors.lastname: Cuenca
authors.forename: Keith
authors.initials: K
authors.lastname: Bugler
authors.forename: Stuart
authors.initials: S
authors.lastname: Taylor
authors.forename: David
authors.initials: D
authors.lastname: Morgan
authors.forename: Paul
authors.initials: P
authors.lastname: Williams
authors.forename: Johann
authors.initials: J
authors.lastname: Bauer
authors.forename: Adrian
authors.initials: A
authors.lastname: Porch
mesh_terms: D005290:Ferric Compounds / Q000737:chemistry*; D052203:Ferrosoferric Oxide / Q000737:chemistry*; D060328:Magnetic Phenomena; D008872:Microwaves; D008958:Models, Molecular; D008968:Molecular Conformation; D010084:Oxidation-Reduction; D010539:Permeability; D014732:Vibration
chemical_list: D005290:Ferric Compounds; C000499:ferric oxide; D052203:Ferrosoferric Oxide