Measurements of magnetic properties of natural rock-forming minerals under pressure

我们的目标是进行一项实验室实验，测量交流磁化率及其在约1 Gpa压力下的温度依赖关系。为此，我们开发了一个长约70 mm的小型装置，它由测量互感的一次和二次线圈、一个加热器线圈和一个热电偶组成，所有这些都是用耐热水泥模制的。然后，线圈系统被放置在实验室制造的活塞缸中，活塞缸可以通过方便的单轴压力产生静液压力。为了表征线圈系统在大气压下的磁行为，我们进行了一系列实验。我们用镍作为测试样品，成功地测量了磁化率随温度的变化，从而探测到了与镍(631K)相当的铁磁相变。然后，我们测量了一块天然火山岩的碎片，它含有纯磁铁矿作为唯一的磁性载体。我们可以检测到磁铁矿的T_c(853K)。这些结果表明，新的线圈系统不仅可以用来测量T_c，而且可以用来研究交流磁化率随频率的变化。后者可以为估计天然岩石中磁性矿物的粒度和识别磁性矿物提供一种新的手段。最后，我们用镍在约10兆帕的压力下进行了一系列实验。结果似乎是成功的，重现了镍的T_c，但也表明仍然很难精确地测量压力下的交流磁化率。需要指出的是，应使用固体介质作为压力变送器，而不是本研究中使用的BN粉末，以避免压力越大，BN体积变化越大而引起的线圈系统的故障。

We have aimed to conduct a laboratory experiment for measuring alternating-current (AC) magnetic susceptibility and its temperature dependence under pressure around one GPa. For this purpose, we have developed a small, c. 70mm long, devise that consists of primary and secondary coils for measuring mutual inductance, a heater coil and a thermocouple, all of which are molded with heat-resistive cement. Then, the coil system is placed within a laboratory-made piston cylinder that can give rise to hydrostatic pressure by means of a handy, uni-axial press. We have carried out a series of experiments for characterizing magnetic behaviors of the coil system at an atmospheric pressure. We have used Nickel as a test sample and succeeded in measuring magnetic susceptibility change with temperature, whereby detecting a ferromagnetic phase transition with Tc equivalent to that of Nickel (631K). Then, we have measured fragments of a natural volcanic rock that contains pure magnetite as a sole magnetic carrier. We could detect Tc of magnetite (853K). These results show that the new coil system would be useful for not only detecting Tc, but also investigating changes in AC magnetic susceptibility with frequency. The latter could provide a new means for estimating grain size as well as identifying magnetic minerals in natural rocks. We finally carried out a series of experiments using Nickel by pressurizing at c. 10 MPa. The results appear to be successful, reproducing Tc of Nickel, but also show it still hard to make precise measurements of AC susceptibility under pressure. We should point out that a solid media should be used as a pressure transmitter, instead of BN powder used in this study, in order to avoid break-down of the coil system caused by the greater volume change of BN with the more pressurized.