Effect of maghemization on the magnetic properties of nonstoichiometric pseudo-single-domain magnetite particles
Effect of maghemization on the magnetic properties of nonstoichiometric pseudo-single-domain magnetite particles
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DOI：
10.1002/2015gc005858
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发表时间：
2015-09-01
影响因子：
3.5
通讯作者：
Dunin-Borkowski, Rafal E.
中科院分区：
文献类型：
--
作者：
Almeida, Trevor P.;Muxworthy, Adrian R.;Dunin-Borkowski, Rafal E.
研究了磁化对拟单畴（PSD）尺寸范围内Fe3O4晶粒磁性能的影响及其随退火温度的变化规律。x射线衍射和透射电镜证实前驱体晶粒为Fe3O4，直径在150 ~ 250 nm之间，而穆斯堡尔光谱分析表明晶粒最初是接近化学计量的。将Fe3O4晶粒加热至120 ~ 220℃，研究其氧化为磁铁矿（γ - fe2o3）。高角度环形暗场成像和局域电子能量损失谱显示，加热到140℃时，微氧化的Fe3O4晶粒在表面表现出较高的氧含量。离轴电子全息技术允许构建单个Fe3O4和γ - fe2o3颗粒的磁感应图，揭示它们的PSD（涡旋）性质，这是由磁滞测量支持的，包括一阶反转曲线分析。晶粒的矫顽力在180℃以下随反应温度升高而升高，在200℃以上随反应温度升高而降低；这种磁性行为归因于γ - fe2o3壳层的生长，其磁性能与Fe3O4核不同。这些分离的组件之间存在交换耦合，导致涡度降低的涡状态。一旦完全氧化为γ - fe2o3，畴态恢复为涡旋，矫顽力略有降低。认为在磁化过程中，由于核/壳耦合机制，定向磁信息仍然是正确的；但是，强度信息将不会被保留。
The effect of maghemization on the magnetic properties of magnetite (Fe3O4) grains in the pseudo-single-domain (PSD) size range is investigated as a function of annealing temperature. X-ray diffraction and transmission electron microscopy confirm the precursor grains as Fe3O4 ranging from similar to 150 to similar to 250 nm in diameter, whilst Mossbauer spectrometry suggests the grains are initially near-stoichiometric. The Fe3O4 grains are heated to increasing reaction temperatures of 120-220 degrees C to investigate their oxidation to maghemite (gamma-Fe2O3). High-angle annular dark field imaging and localized electron-energy loss spectroscopy reveal slightly oxidized Fe3O4 grains, heated to 140 degrees C, exhibit higher oxygen content at the surface. Off-axis electron holography allows for construction of magnetic induction maps of individual Fe3O4 and gamma-Fe2O3 grains, revealing their PSD (vortex) nature, which is supported by magnetic hysteresis measurements, including first-order reversal curve analysis. The coercivity of the grains is shown to increase with reaction temperature up to 180 degrees C, but subsequently decreases after heating above 200 degrees C; this magnetic behavior is attributed to the growth of a gamma-Fe2O3 shell with magnetic properties distinct from the Fe3O4 core. It is suggested there is exchange coupling between these separate components that results in a vortex state with reduced vorticity. Once fully oxidized to gamma-Fe2O3, the domain states revert back to vortices with slightly reduced coercivity. It is argued that due to a core/shell coupling mechanism during maghemization, the directional magnetic information will still be correct; however, the intensity information will not be retained.