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

我们的目标是进行一个实验室实验，测量在压力约1 GPa下的交流（AC）磁化率及其温度依赖性。为此，我们开发了一个小的，约70毫米长，设计包括用于测量互感的初级和次级线圈，加热器线圈和热电偶，所有这些都是用耐热水泥成型的。然后，将线圈系统放置在实验室制造的活塞缸中，该活塞缸可以通过方便的单轴压力机产生静水压力。我们进行了一系列的实验来表征在大气压下线圈系统的磁性行为。我们以镍作为测试样品，成功地测量了磁化率随温度的变化，从而检测到具有与镍（631K）等效Tc的铁磁相变。然后，我们测量了天然火山岩的碎片，其中包含纯磁铁矿作为唯一的磁性载体。我们可以检测到磁铁矿的Tc （853K）。这些结果表明，新的线圈系统不仅可以用于检测Tc，还可以用于研究交流磁化率随频率的变化。后者可为估计天然岩石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10.10 MPa的压力下进行了一系列的实验。结果似乎是成功的，再现了镍的Tc，但也表明，在压力下精确测量交流磁化率仍然很困难。需要指出的是，压力变送器应使用固体介质，而不是本研究中使用的BN粉末，以避免由于BN的压力越大，体积变化越大而导致线圈系统击穿。

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.