Magnetic phase diagram and the pressure and field dependence of the Fermi surface in UGe₂

The ac susceptibility and de Haas-van Alphen (dHvA) effect in UGe₂ are measured at pressures P up to 17.7 kbar for the magnetic field B parallel to the a axis, which is the easy axis of magnetization. Two anomalies are observed at B_x(P) and B_m(P)(B_x>B_m at any P), and the P-B phase diagram is presented. The Fermi surface and quasiparticle mass are found to vary smoothly with pressure up to 17.7 kbar unless the phase boundary B_x(P) is crossed. The observed dHvA frequencies may be grouped into three according to their pressure dependences, which are largely positive, nearly constant, or negative. It is suggested that the quasi particle mass moderately increases as the boundary B_x(P) is approached. DHvA effect measurements are also performed across the boundary at 16.8 kbar.