Study of the magnetite to maghemite transition using microwave permittivity and permeability measurements
Study of the magnetite to maghemite transition using microwave permittivity and permeability measurements
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DOI：
10.1088/0953-8984/28/10/106002
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发表时间：
2016-03-16
影响因子：
2.7
通讯作者：
Porch, Adrian
中科院分区：
文献类型：
--
作者：
Cuenca, Jerome Alexander;Bugler, Keith;Porch, Adrian
微波腔微扰（MCP）技术被用来识别从磁铁矿（Fe 3 O 4）到磁赤铁矿（γ-Fe 2 O3）的亚稳态形式的转变。在这项研究中，Fe 3 O 4在60至300摄氏度的温度下退火以改变氧化。在退火之后，测量氧化铁粉末的复介电常数和磁导率。用X射线衍射（XRD）、X射线光电子能谱（XPS）和振动样品磁强计（VSM）证实了向γ-Fe 2 O3的转变。XRD、XPS和VSM表明，起始粉末与Fe 3 O 4一致，并且在超过200 ℃下退火的粉末转变为γ-Fe 2 O3。MCP测量给出了在2.5-10.2 GHz的频率范围内的两个相位的复介电常数和磁导率的大的差异。磁导率随退火温度的降低，但磁损耗表现出频率依赖性行为。复介电常数测量结果表明，在所有测量频率下的介电常数和损耗都大幅下降，以及以相变温度为中心的突出损耗峰。我们解释的损失峰值是由于中间多相混合物的场效应的后果。此外，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Fe-ocr（2+）阳离子对微波频率下的极化提供了显著贡献，相比之下，Fe-ocr（3+）的影响是微不足道的。损耗的变化可以解释为两相有效电导率差异的组合（即Fe 3 O 4表现出电子跳跃传导，而γ-Fe 2 O3中空位的存在使其无效）。这表明，非侵入式MCP测量作为一种高度灵敏和通用的方法，用于观察铁的这种相变以及氧化态对其他铁氧化物极化的潜在影响。
The microwave cavity perturbation (MCP) technique is used to identify the transition from magnetite (Fe3O4) to the meta-stable form of maghemite (gamma-Fe2O3). In this study Fe3O4 was annealed at temperatures from 60 to 300 degrees C to vary the oxidation. Subsequent to annealing, the complex permittivity and magnetic permeability of the iron oxide powders were measured. The transition to gamma-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 degrees C were transitioning to gamma-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 Fe-ocr(2+) cations in the lattice provide a significant contribution to polarization at microwave frequencies and the effects of Fe-ocr(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 gamma-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.