TEM study of relationships between the microstructures and magnetic properties of strongly magnetized magnetite and maghemite
TEM study of relationships between the microstructures and magnetic properties of strongly magnetized magnetite and maghemite
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强磁化磁铁矿和磁赤铁矿微观结构与磁性能关系的TEM研究
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发表时间：
2007
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通讯作者：
Dlvro R. Venlrx
中科院分区：
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作者：
Jrr.r.HN;F. Barvrrnr;W. PrrrnJ.;EwsKr .lsrr;Dlvro R. Venlrx
本研究探讨了以前被描述为I型磁铁矿的强磁化磁铁矿的微观结构与磁性之间的关系。样品主要由磁铁矿和少量磁铁矿(-3070)以及少量赤铁矿和针铁矿氧化产物(<；50/o)组成。磁铁铁矿具有弱发育的超结构，在某些区域与P4r32和P32对映体空间群一致。高分辨率成像表明，磁铁矿和磁铁矿中含有平行于{10l}的密集平面断层。位移矢量分析和高分辨率成像表明，大多数断层上存在(Y+Yr0)平移。断层不能仅仅由氧化和空位有序产生，因为它们抵消了磁铁矿的亚结构。没有显示磁铁矿亚结构偏移的边界可能代表对位同构的磁铁铁矿结构域之间的连接。虽然超晶格还不够发达，不足以详细描述空位有序模式，但我们可以确定，(Yr%0)边界通常将相同相的区域分开，而不是磁铁矿和磁铁矿的区域。平面断层形成的针状区沿L软磁方向伸展，缺陷取向与磁铁矿中磁畴壁的通常取向相对应。如前所述，跨越{10}个断层的Fe-O-Fe键角允许磁矩的直接反转。磁矢量取向的改变将需要新的磁场壁的成核或断层的移动。从一系列热磁实验中得出了三个特定的热区(20-280,280-425和425-600℃)；每个热区都与自然剩余磁化(NRM)矢量的运动有关。NRM与饱和剩余磁化强度之比较大，与闪电诱发磁化一致。在第一区，800/o的NRM被破坏，矫顽力显著降低，磁化率增加。这些变化与从样品中消除大多数堆叠故障有关。我们认为，初始的高矫直力与层错的存在直接相关，而这些特征对磁畴壁的钉扎主要解释了大而稳定的自然剩磁。磁铁矿和磁铁矿区域的形状各向异性也可能是重要的。我们初步推测，层错也可能是在闪电事件中诱发的。第二个热区以磁化率和饱和磁化强度迅速降低为特征，对应于磁赤铁矿向赤铁矿的转变。第三体制以岩浆岩和赤铁矿为主。结果表明，透射电子显微镜揭示的微观结构细节对了解矿物磁性有很大帮助。
This study investigates relationships between microstructures revealed by transmission electron microscopy (TEM) and the magnetic properties of strongly magnetized magnetite iron ore previously described as type I lodestone. The sample consists mostly of magnetite with some maghemite (-3070) and minor hematite and goethite oxidation products (<50/o). The maghemite exhibits a weakly developed superstructure that in some areas is consistent with the enantiomorphous space groups P4r32 and P\32. High-resolution imaging indicates that magnetite and maghemite contain closely spaced planar faults parallel to { l0l }. Displacement vector analyses and high-resolution imaging indicate (Y+Yr0) translations across most faults. Faults cannot arise merely by oxidation and vacancy ordering because they offset the magnetite substructure. Boundaries that show no offset of the magnetite substructure probably represent junctions between enantiomorphous maghemite domains. Although superlattices are not sufficiently well developed to allow detailed characterization of the vacancy ordering patterns, we can establish that the (Yr%0) boundaries generally separate regions of the same phase and not regions of magnetite from maghemite. Needle-shaped areas defined by planar faults are elongate along the magnetically soft I I l] direction, and defect orientations correspond to the usual orientations of magnetic domain walls in magnetite. As noted previously, Fe-O-Fe bond angles across {l0l} faults allow direct reversal of the magnetic moments. Modification of magnetic vector orientations would require either nucleation of new domain walls or the movement of faults. Three specific thermal regimes (20-280, 280-425, and 425-600 C) are derived from a series of thermomagnetic experiments; each regime is associated with the movement of the natural remanent magrretization (NRM) vector. The ratio of NRM to saturation remanent magnetization is large, consistent with lightning-discharge induced magnetization. In the first regime, 800/o of the NRM is destroyed, the coercivity is substantially reduced, and the magnetic susceptibility increases. These changes correlate with the elimination of most stacking faults from the sample. We suggest that the initial high coercivity is directly associated with the presence of stacking faults and that pinning of domain walls by these features primarily explains the large and stable natural remanence. The shape anisotropy of the magrretite and maghemite regions may also be important. We tentatively suggest that the stacking faults may also have been induced in the lightning event. The second thermal regime, characterized by rapid reduction in susceptibility and saturation magnetization, corresponds to the conversion of maghemite to hematite. The third regime is dominated by magrretite and hematite. Results demonstrate that microstructural detail revealed by TEM contributes substantially to the understanding of mineral magnetic properties.