  Magnetization isotherms of the 5f-electron ferromagnets URhGa, UCoGa and
UCo0.98Ru0.02Al were measured at temperatures in the vicinity of their Curie
temperature in order 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 \b{eta}, {\gamma} and {\delta} 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 2D Ising
system with long-range interactions similar to URhAl reported by other
investigators. On the other hand, the critical exponents determined for
UCo0.98Ru0.02Al are characteristic of a 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, UCoGa to UCo0.98Ru0.02Al 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.
