The 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 UGe$_2$ are measured at pressures {\it P} up to 17.7 kbar for the magnetic field {\it B} parallel to the {\it a} axis, which is the easy axis of magnetization. Two anomalies are observed at {\it B$_x$}({\it P}) and {\it B}$_m$({\it P}) ({\it B$_x$} $>$ {\it B}$_m$ at any {\it P}), and the {\it P}-{\it 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 {\it B$_x$}({\it 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 quasiparticle mass moderately increases as the boundary {\it B$_x$}({\it P}) is approached. DHvA effect measurements are also performed across the boundary at 16.8 kbar.