We have measured the magnetization of an uranium chalcogenide compound β-US 2 under high pressure by piston cylinder and indenter type cells up to 2.42 GPa in a commercial SQUID magnetometer. The compound is a narrow-gap semiconductor that does not order magnetically down to 0.3 K at 1 bar. The appearance of a pressure-induced phase above 2 GPa was suggested by a previous study with the electrical resistivity measurement under high pressure. In this study, it was found that a ferromagnetic state is induced above 1 GPa. The pressure phase diagram were obtained. The values of the spontaneous magnetic moment at 0 K are less than 0.1 µ B /U, suggesting that the pressure-induced phase is a very weak ferromagnetic state. Considering the CEF singlet ground state in β-US 2 , it is suggest that the ferromagnetic state is an "induced-magnetic phase" often observed in some praseodymium compounds such as Pr 3 Tl. The experimental results are discussed in terms of the mean field theory.

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In this paper we summarize recent experiments that address the symmetry of the hidden ordered state in URu 2 Si 2. Possible space groups for the hidden order and corresponding multipolar order parameters are restricted under the current conditions considered to be reasonable; 1) The hidden order transition is of second order, 2) The 4-fold local symmetry at U, Ru and Si sites is preserved in the hidden ordered state, 3)The ordering wave vector is Q = (001). There are controversial experimental results regarding time-reversal symmetry up to now. Thus, on conditions 1)-3), possible cases where time-reversal symmetry is preserved and broken are addressed.