The effects of dislocations on crystallographic twins and domain wall motion in magnetite at the Verwey transition
The effects of dislocations on crystallographic twins and domain wall motion in magnetite at the Verwey transition
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DOI：
10.1186/s40623-018-0981-7
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发表时间：
2019-01-15
影响因子：
3
通讯作者：
Newell, Andrew J.
中科院分区：
文献类型：
--
作者：
Lindquist, Anna K.;Feinberg, Joshua M.;Newell, Andrew J.
纯磁铁矿在120 ~ 125k附近发生一级相变（Verwey相变），矿物的对称性由立方向单斜转变。这种转变导致了细尺度晶体孪晶的形成，并伴随着磁性质的深刻变化。Verwey转变对于环境磁学和古磁学的各种应用至关重要，因为它的表达可以诊断化学计量（或接近化学计量）磁铁矿的存在，并且通过Verwey转变的循环往往会消除大部分多畴剩磁。内部和外部应力明显影响Verwey转变的开始。位错产生局部的内部应力场，并被引用为变形样品中改变的Verwey转变的可能来源。为了进一步研究这种行为，在透射电子显微镜下观察了实验室变形的磁铁矿样品，因为它通过维维转变冷却。在洛伦兹显微镜的菲涅耳模式下操作显微镜，可以对相变过程中位错、磁畴壁和低温孪晶形成之间的相互作用进行成像。为了将观察到的变化与更容易测量的大块样品磁性行为联系起来，还使用SQUID磁力仪进行了低温磁性测量。这项研究使我们第一次在一个缺陷丰富的地区观察到Verwey转变。在相变过程中，位错及其相关的应力场阻碍了单斜磁铁矿孪晶结构的发展，并通过Verwey相变增加了磁铁矿样品在冷却和加热后的剩余物。
Pure magnetite experiences a first-order phase transition (the Verwey transition) near 120-125K wherein the mineral's symmetry changes from cubic to monoclinic. This transformation results in the formation of fine-scale crystallographic twins and is accompanied by a profound change in magnetic properties. The Verwey transition is critical to a variety of applications in environmental magnetism and paleomagnetism because its expression is diagnostic for the presence of stoichiometric (or nearly stoichiometric) magnetite and cycling through the Verwey transition tends to remove the majority of multidomain magnetic remanence. Internal and external stresses demonstrably affect the onset of the Verwey transition. Dislocations create localized internal stress fields and have been cited as a possible source of an altered Verwey transition in deformed samples. To further investigate this behavior, a laboratory-deformed magnetite sample was examined inside a transmission electron microscope as it was cooled through the Verwey transition. Operating the microscope in the Fresnel mode of Lorentz microscopy enabled imaging of the interactions between dislocations, magnetic domain walls, and low-temperature crystallographic twin formation during the phase transition. To relate the observed changes to more readily measurable bulk sample magnetic behavior, low-temperature magnetic measurements were also taken using SQUID magnetometry. This study allows us, for the first time, to observe the Verwey transition in a defect-rich area. Dislocations, and their associated stress fields, impede the development of monoclinic magnetite twin structures during the phase transition and increase the remanence of a magnetite sample after cooling and warming through the Verwey transition.