Low-temperature magnetic properties of monoclinic pyrrhotite with particular relevance to the Besnus transition
Low-temperature magnetic properties of monoclinic pyrrhotite with particular relevance to the Besnus transition
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DOI：
10.1093/gji/ggw376
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发表时间：
2016-12
影响因子：
2.8
通讯作者：
M. Volk;S. Gilder;J. Feinberg
中科院分区：
文献类型：
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作者：
M. Volk;S. Gilder;J. Feinberg
单斜磁黄铁矿（Fe7S8）的铁磁性来自于铁空位的有序排列。它的磁性在30k左右发生显著变化，这就是所谓的贝斯努斯跃迁。对贝斯努斯转变的合理解释要么是由于晶体对称转变引起的晶体各向异性的变化，要么是由于两相体系的建立以及两相之间的磁相互作用。为了帮助解决这一差异，我们测量了单斜磁黄铁矿取向单晶在300 K时每隔5A度的磁滞回线和每隔10A度的后场曲线，以及从50 K到20 K的贝斯诺斯跃迁每隔2 K的磁滞回线。在50和30k之间，磁滞回线在晶体a轴之间具有双弯折，而平行于a轴的弯折只有一个。磁化能计算和回路的相对差值在该温度范围内显示出六倍对称性。我们提出，弯曲源于磁轴开关，这是场和温度依赖，在某种程度上类似于各向同性点，其中磁晶常数改变其符号。贝斯努斯相变的最佳特征是在6A℃温度范围（28-34 K）内剩磁和矫顽力的变化，在30 K时变化率最大。一个令人惊讶但又令人困惑的发现是，当四重对称出现时，矫顽力比在跃迁以下变得小于1。由于磁性参数的变化与晶体结构有关，我们得出贝斯努斯转变的起源是在30 K以下晶体轴的扭曲，而不是两相体系的出现。天然磁黄铁矿通过贝斯努斯转变从室温到较低温度循环的等温磁化比同等晶粒尺寸的磁铁矿减少2-4倍，在300至150 K之间，伪单畴磁黄铁矿的剩余物损失不到10%。由于PSD单斜磁黄铁矿在一些陨石中携带磁性残留物，因此在空间中低温循环然后升温到地球表面的环境条件可能对这些陨石的古强度值只有很小的影响。
Monoclinic pyrrhotite (Fe7S8) owes its ferrimagnetism to an ordered array of Fe vacancies. Its magnetic properties change markedly around 30 K, in what is known as the Besnus transition. Plausible explanations for the Besnus transition are either due to changes in crystalline anisotropy from a transformation in crystal symmetry or from the establishment of a two-phase system with magnetic interaction between the two phases. To help resolve this discrepancy, we measured hysteresis loops every 5A degrees and backfield curves every 10A degrees in the basal plane of an oriented single crystal of monoclinic pyrrhotite at 300 K and every 2 K from 50 K through the Besnus transition until 20 K. Between 50 and 30 K, hysteresis loops possess double inflections between crystallographic a-axes and only a single inflection parallel to the a-axes. Magnetization energy calculations and relative differences of the loops show a sixfold symmetry in this temperature range. We propose that the inflections stem from magnetic axis switching, which is both field and temperature dependent, in a manner somewhat analogous to an isotropic point where magnetocrystalline constants change their sign. The Besnus transition is best characterized by changes in magnetic remanence and coercivity over a 6A degrees temperature span (28-34 K) with a maximum rate of change at 30 K. A surprising yet puzzling finding is that the coercivity ratio becomes less than unity below the transition when fourfold symmetry arises. Because the changes in magnetic parameters are linked to the crystal structure, we conclude the Besnus transition owes its origin to a distortion of the crystallographic axes below 30 K rather than an apparition of a two-phase system. An isothermal magnetization of natural pyrrhotite cycled from room temperature to successively lower temperatures through the Besnus transition decreases 2-4 times less than equivalent grain sizes of magnetite, with less than a 10 per cent loss in remanence between 300 and 150 K in pseudo-single-domain (PSD) pyrrhotite. As PSD monoclinic pyrrhotite carries the magnetic remanence in some meteorites, it is likely that low-temperature cycling in space then warming to ambient conditions at the Earth's surface will have only a minor influence on palaeointensity values derived from those meteorites.