We have measured isothermal magnetization curves of the ferromagnetic superconductor UGe₂ at pressures up to 18.3 kbar and at temperatures down to 0.45 K. The pressure dependence of the Curie temperature T_Curie evaluated from a linear part of the Arrott plot is relatively in agreement with the literatures. The paramagnetic Curie temperature, however, deviates from T_Curie with increasing pressure, especially above about 10 kbar. In addition, we have found a staircase-like magnetization hysteresis loop with regular interval of magnetic fields only below about 1K. We ascribe it to the macroscopic quantum tunneling, and the analysis suggests tiny magnetic domain formation whose size is smaller than the superconducting coherence length.

PACS numbers: 74.45.+j, 74.70.Tx, 75.60.--d