UC Irvine

We report a dc magnetization study of the critical phenomenon around the ferromagnetic transition temperature TC in high-quality single crystals of uranium ferromagnetic superconductors UGe2 and URhGe. The critical exponents, β for the temperature dependence of the magnetization below TC, γ for the magnetic susceptibility, and δ for the magnetic isothermal at TC, have been determined with a modified Arrott plot, a Kouvel-Fisher plot, and the scaling analysis. Magnetization in the ferromagnetic state has strong uniaxial magnetic anisotropy in the two compounds. However, the universality class of the critical phenomena do not belong to the 3D Ising system. Although the values of β in UGe2 and URhGe are close to those in the 3D magnets, the values of γ are close to unity, that expected from the mean-field theory. Similar critical exponents have been reported previously for the 3D Ising ferromagnet UIr where superconductivity appears under high pressure. The critical behavior may be limited to a very narrow Ginzburg critical region of ΔTG∼ 1 mK because of the strong itinerant character of the 5f electrons in the ferromagnetic superconductor UCoGe where the mean-field behavior of the magnetization has been reported. The unconventional critical scaling of magnetization in UGe2, URhGe, and UIr cannot be explained via previous approaches to critical phenomena. The ferromagnetic correlation between the 5f electrons differs from that in the 3D Ising system and this difference may be a key point for the understanding of the ferromagnetism where superconductivity emerges. © 2014 American Physical Society.