Nature of ferromagnetic phase in pressure-induced ferromagnetic superconductor UGe2

We have measured isothermal magnetization curves of the ferromagnetic superconductor UGe2 at pressures up to 18.3kbar 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 10kbar. In addition, we have found a staircase-like magnetization hysteresis loop with regular interval of magnetic fields only below about 1 K. 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.