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 cations in the lattice provide a significant contribution to polarization at microwave frequencies and the effects of 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.
Highlights
Magnetic particles have gained great interest recently in applications ranging from electromagnetic absorbers [1,2,3,4] to numerous biomedical applications including magnetic drug delivery [5] and treatment of hyperthermia [6, 7]
We have shown that the Microwave cavity perturbation (MCP) method is excellent for high-sensitivity, non-invasive materials characterization for the Fe3O4 to γ-Fe2O3 transition in annealed powders
We have presented data from the standard methods of x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometry (VSM) techniques on these powders to corroborate the findings found with MCP
Summary
Magnetic particles have gained great interest recently in applications ranging from electromagnetic absorbers [1,2,3,4] to numerous biomedical applications including magnetic drug delivery [5] and treatment of hyperthermia [6, 7]. Synthesis of γ-Fe2O3 is most commonly through subsequent oxidation of Fe3O4, with the exception of the dehydroxylation of iron oxide-hydroxides such as lepidocrocite (γ-FeOOH) or goethite (α-FeOOH) [10, 11]. Oxidation of Fe3O4 in air at temperatures around 200–300 °C induces the phase change to γ-Fe2O3 [12,13,14]. These two oxides are of interest due to their structural similarity but γFe2O3 is the more thermally and chemically stable of the two [15, 16]
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