  We report the novel critical behavior of magnetization in low carrier
concentration systems UTeS and USeS that exhibit the large negative
magnetoresistance around the ferromagnetic transition temperatures T_C ~ 85 and
23 K, respectively. UTeS and USeS crystallize in the same orthorhombic
TiNiSi-type crystal structure as those of uranium ferromagnetic superconductors
URhGe and UCoGe. We determine the critical exponents, beta for the spontaneous
magnetization M_s, gamma for the magnetic susceptibility chi, and delta for the
magnetization isotherm at T_C with several methods. The ferromagnetic states in
UTeS and USeS have strong uniaxial magnetic anisotropy. However, the critical
exponents in the two compounds are different from those in the
three-dimensional Ising model with short-range magnetic exchange interactions.
Similar sets of the critical exponents have been reported for the uranium
ferromagnetic superconductors UGe_2 and URhGe, and uranium intermetallic
ferromagnets URhSi, UIr and U(Co_0.98Os_0.02)Al. The universality class of the
ferromagnetic transitions in UTeS and USeS may belong to the same one for the
uranium compounds. The novel critical phenomenon associated with the
ferromagnetic transition is observed not only in the uranium intermetallic
ferromagnets with the itinerant 5f electrons but also in the low carrier
concentration systems UTeS and USeS with the localized 5f electrons. The large
negative magnetoresistance in UTeS and USeS, and the superconductivity in UGe_2
and URhGe share the similarity of their closeness to the ferromagnetism
characterized by the novel critical exponents.
