Precise Magnetization Study of UPd_2Al_3 at Ultralow Temperatures

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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.