Pressure-induced ferromagnetism with strong Ising-type anisotropy in YbCu2Si2

We report dc magnetic measurements on YbCu2Si2 at pressures above 10 GPa using a miniature ceramic anvil cell. YbCu2Si2 shows a pressure-induced transition from the nonmagnetic to a magnetic phase at 8 GPa. We find a spontaneous dc magnetization in the pressure-induced phase above 9.4 GPa. The pressure dependence of the ferromagnetic transition temperature T-C and the spontaneous magnetic moment mu(0) at 2.0 K have been determined. The value of mu(0) in the present macroscopic measurement is less than half of that determined via Mossbauer experiments. The difference may be attributed to a spatial phase separation between the ferromagnetic and paramagnetic phases. This separation suggests that the pressure-induced phase boundary between the paramagnetic and ferromagnetic states is of first order. Further, we have studied the magnetic anisotropy in the pressure-induced ferromagnetic state. The effect of pressure on the magnetization with magnetic field along the magnetic easy c axis is much larger than for field along the hard a axis in the tetragonal structure. The pressure-induced phase has strong Ising-type uniaxial anisotropy, consistent with the two crystal electric field models proposed for YbCu2Si2.