Precise Magnetization Study of UPd_2Al_3 at Ultralow Temperatures

采用自行研制的“电容法”磁力计，首次研究了铀化合物UPd_2Al_3在静磁场高达20 T、温度低至40 mK条件下的等温磁化过程。六角形结构的B//基面在18T左右出现明显的变磁跃迁。此外，在4.2 K以下首次观察到迟滞现象。磁滞率随温度的降低而增大，在40 mK时可达0.02 T左右。磁滞率和磁化强度的急剧转变表明磁滞率是一级的。温度越低，过渡场越大。

Isothermal magnetization processes of a Uranium compound UPd_2Al_3 is first studied at temperatures down to 40 mK in static magnetic fields up to 20 T using a superconducting magnet and a hybrid magnet with a 'Capacitance Method' type magnetometer newly developed by ourselves. A sharp metamagnetic transition is shown around 18T for B// basal plane of hexagonal structure. Moreover, a hysteresis have been first observed below 4.2 K. The hysteresis becomes larger with decreasing temperature and is as large as about 0.02 T at 40 mK. This hysteresis and sharp transition of the magnetization suggest that this transition is of the first order. The transition field becomes higher with lowering temperature.