Critical behavior of magnetization in URhAl: Quasi-two-dimensional Ising system with long-range interactions
Abstract
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 ss 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 ss = 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/ss.
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 (ss = 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)similar to 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.