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 (T_C) is sufficiently suppressed with increasing pressure and T_C suddenly disappears at the critical pressure P_c ∼ 5.2 GPa. Above P_c, we observed the large enhancement of the A coefficient of AT ² resistivity term, which is defined below T∗(P). Near P_c, 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 P_c 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 T_C support the change of nature of transition from second-order to first-order in URhAl near P_c. Furthermore, the pressure dependences of A(P) and ρ₀(P) are asymmetric around P_c, and these behaviors might be explained by a remarkable Fermi-surface change, which accompanies the ferromagnetic-paramagnetic quantum phase transition in URhAl.