Unconventional critical scaling of magnetization in uranium ferromagnetic superconductors UGe$_2$ and URhGe

摘要

We report a dc magnetization study of the critical phenomenon around theferromagnetic transition temperature T_C in high-quality single crystals ofuranium ferromagnetic superconductors UGe2 and URhGe. The critical exponents,beta for the temperature dependence of the magnetization below T_C, gamma forthe magnetic susceptibility, and delta for the magnetic isothermal at T_C havebeen determined with a modified Arrott plot, a Kouvel-Fisher plot, and thescaling analysis. Magnetization in the ferromagnetic state has strong uniaxialmagnetic anisotropy in the two compounds. However, the universality class ofthe critical phenomena do not belong to the three dimensional (3D) Isingsystem. Although the values of beta in UGe2 and URhGe are close to those in the3D magnets, the values of gamma are close to unity, that expected from the meanfield theory. Similar critical exponents have been reported previously for the3D Ising ferromagnet UIr where superconductivity appears under high pressure.The critical behavior may be limited to a very narrow Ginzburg critical regionof 1 mK because of the strong itinerant character of the 5f electrons in theferromagnetic superconductor UCoGe where the mean field behavior of themagnetization has been reported. The unconventional critical scaling ofmagnetization in UGe2, URhGe and UIr cannot be explained via previousapproaches to critical phenomena. The ferromagnetic correlation between the 5felectrons differs from that in the 3D Ising system and this difference may be akey point for the understanding of the ferromagnetism where superconductivityemerges.