  The critical behavior of dc magnetization in the uranium ferromagnet URhAl
with the hexagonal ZrNiAl-type crystal structure has been studied around the
ferromagnetic transition temperature T_C. The critical exponent beta for the
temperature dependence of the spontaneous magnetization below T_C, gamma for
the magnetic susceptibility, and delta for the magnetic isotherm at T_C have
been obtained with a modified Arrott plot, a Kouvel-Fisher plot, the critical
isotherm analysis and the scaling analysis. We have determined the critical
exponents as beta = 0.287 +- 0.005, gamma = 1.47 +- 0.02, and delta = 6.08 +-
0.04 by the scaling analysis and the critical isotherm analysis. These critical
exponents satisfy the Widom scaling law delta=1+gamma/beta. URhAl has strong
uniaxial magnetic anisotropy, similar to its isostructural UCoAl that has been
regarded as a three-dimensional (3D) Ising system in previous studies. However,
the universality class of the critical phenomenon in URhAl does not belong to
the 3D Ising model (beta = 0.325, gamma = 1.241, and delta = 4.82) with
short-range exchange interactions between magnetic moments. The determined
exponents can be explained with the results of the renormalization group
approach for a two-dimensional (2D) Ising system coupled with long-range
interactions decaying as J(r)~r^-(d+sigma) with sigma = 1.44. We suggest that
the strong hybridization between the uranium 5f and rhodium 4d electrons in the
U-Rh_I layer in the hexagonal crystal structure is a source of the low
dimensional magnetic property. The present result is contrary to current
understandings of the physical properties in a series of isostructural UTX
uranium ferromagnets (T: transition metals, X: p-block elements) based on the
3D Ising model.
