A change of the Fermi surface across the metamagnetic transition under pressure in UGe₂

We have performed the de Haas-van Alphen (dHvA) and ac-susceptibility experiments on a ferromagnet UGe₂ under pressure for the field along the easy a-axis. The p-H phase diagram was determined, indicating three kinds of phase: paramagnetic and weakly and strongly polarized phases. The detected dHvA frequencies as well as the cyclotron masses are found to be very different between the weakly and strongly polarized states. A change of the Fermi surface occurs when the weakly polarized phase is changed into the strongly polarized phase with increasing magnetic field.