Pressure-temperature-field phase diagram in the ferromagnet U₃P₄

The pressure-temperature-field phase diagram and quantum fluctuation effect were investigated in the itinerant ferromagnet U₃P₄ by resistivity, ac susceptibility, and Hall effect measurements under high pressure. The zerotemperature ferromagnetic-to-paramagnetic transition is located at P_c ∼ 4.0 GPa. The tricritical point exists at P^∗ = 3.8 GPa and T^∗ = 32K, where the ferromagnetic transition changes from second- to first-order. As a quantum fluctuation effect, the low-temperature resistivity at P_c follows T^5/3 dependence, which was theoretically described in the case of a three-dimensional ferromagnet. This result indicates that the ferromagnetic-to-paramagnetic transition at P_c may be considered as weakly first-order. Another critical behavior, the huge enhancement of ρ₀ observed at P_c, cannot be interpreted as the ferromagnetic fluctuation effect. The Hall effect measurements suggest that the magnetic structure in the ordered state changes under high pressure.