Spin-Fluctuation Effects Near the Quantum Phase Transition of the Ising-Type Itinerant Ferromagnet URhAl

We performed high-pressure resistivity measurements for an Ising-type itinerant ferromagnet URhAl at low temperatures down to 40 mK. In URhAl, the Curie temperature (TC) is sufficiently suppressed with increasing pressure and TC suddenly disappears at the critical pressure Pc ∼ 5.2 GPa. Above Pc, we observed the large enhancement of the A coefficient of AT 2 resistivity term, which is defined below T*(P). Near Pc, the exponent (n) of resistivity, ρ(T) = ρ0 + A’T n, approaches n ∼ 5/3, which is suggested from the spin-fluctuation theory for a three dimensional itinerant ferromagnetic system. We observed the non-Fermi-liquid behavior of resistivity under high pressure far above Pc up to ∼ 7.5 GPa. On the other hand, the critical behaviors of T*(P) and A(P) for URhAl do not obey the spin-fluctuation theory for a ferromagnetic quantum critical point, where the Curie temperature vanishes as a second-order phase transition. The sudden disappearance of TC support the change of nature of transition from second-order to first-order in URhAl near Pc. Furthermore, the pressure dependences of A(P) and ρ0(P) are asymmetric around Pc, and these behaviors might be explained by a remarkable Fermi-surface change, which accompanies the ferromagnetic-paramagnetic quantum phase transition in URhAl.