  Precise resistivity measurements on the ferromagnetic superconductor UGe2
under pressure p and magnetic field H reveal a previously unobserved change of
the anomaly at the Curie temperature. Therefore, the tricritical point (TCP)
where the paramagnetic-to-ferromagnetic transition changes from a second order
to a first order transition is located in the p-T phase diagram. Moreover, the
evolution of the TCP can be followed under the magnetic field in the same way.
It is the first report of the boundary of the first order plane which appears
in the p-T-H phase diagram of weak itinerant ferromagnets. This line of
critical points starts from the TCP and will terminate at a quantum critical
point. These measurements provide the first estimation of the location of the
quantum critical point in the p-H plane and will inspire similar studies of the
other weak itinerant ferromagnets.
