Different universality classes of isostructural UTX compounds (T = Rh, Co, Co_0.98Ru_0.02; Ⅹ = Ga, Al)

摘要

Magnetization isotherms of the 5f-electron ferromagnets URhGa, UCoGa. and UCo_(0.98)Ru_(0.02)Al were measured at temperatures in the vicinity of their Curie temperature to investigate the critical behavior near the ferromagnetic phase transition. These compounds adopt the layered hexagonal ZrNiAl-type structure and exhibit huge uniaxial magnetocrystalline anisotropy. The critical β, γ, and δ exponents were determined by analyzing Arrott-Noakes plots, Kouvel-Fisher plots, critical isotherms, scaling theory, and Widom scaling relations. The values obtained for URhGa and UCoGa can be explained by the results of the renormalization group theory for a two-dimensional (2D) Ising system with long-range interactions similar to URhAl reported by other investigators. On the other hand, the critical exponents determined for UCo_(0.98)Ru_(0.02)Al are characteristic of a three-dimensional (3D) Ising ferromagnet with short-range interactions suggested in previous studies also for the itinerant 5f-electron paramagnet UCoAl situated near a ferromagnetic transition. The change from the 2D to the 3D Ising system is related to the gradual delocalization of 5f electrons in the series of the URhGa, URhAl, and UCoGa to UCo_(0.98)Ru_(0.02)Al and UCoAl compounds and appears close to the strongly itinerant nonmagnetic limit. This indicates possible new phenomena that may be induced by the change of dimensionality in the vicinity of the quantum critical point.