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 $\text{Fe}_{\text{oct}}^{2+}$ cations in the lattice provide a significant contribution to polarization at microwave frequencies and the effects of $\text{Fe}_{\text{oct}}^{3+}$ 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.
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机译：微波腔扰动（MCP）技术用于识别从磁铁矿（Fe3O4）到亚稳型磁铁矿（γ-Fe2O3）的过渡。在这项研究中，Fe3O4在60至300°C的温度下退火以改变氧化程度。退火后，测量氧化铁粉末的复介电常数和磁导率。 X射线衍射（XRD），X射线光电子能谱（XPS）和振动样品磁强法（VSM）证实了向γ-Fe2O3的转变。 XRD，XPS和VSM表示起始粉末与Fe3O4一致，并且在200℃以上退火的粉末正在转变为γ-Fe2O3。 MCP测量在2.5–10.2 GHz的频率范围内，两相的复介电常数和磁导率都有很大差异。磁导率随退火温度降低，尽管磁损耗表现出频率依赖性。复介电常数的测量结果表明，在所有测量频率下介电常数和损耗都大大降低，并且以相变温度为中心出现了明显的损耗峰。我们将损耗峰解释为由于中间多相混合物而引起的场效应。另外，几乎没有观察到频率依赖性。复介电常数的降低意味着晶格中的$ \ text {Fe} _ {{text {oct}} ^ {2 +} $阳离子对微波频率下的极化和$ \ text {Fe } _ {\ text {oct}} ^ {3 +} $是名义上的。损耗的变化可以解释为两相有效电导率差异的组合（即，Fe3O4表现出电子跳跃传导，而γ-Fe2O3中空位的存在使这一点无效）。这表明，非侵入性MCP测量是一种高度灵敏且用途广泛的方法，用于查看铁中的这种相变以及潜在的氧化态对其他铁氧化物中极化的影响。
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