Ferromagnetic spin fluctuation in LaFeAsO1-xFx

The F-doped LaFeAsO-a recently discovered superconductor with the high T(c) of 26 K-has been studied using the resistivity (rho), magnetic-susceptibility (chi), and heat-capacity (C(p)) measurements in the F doping range 0 <= x <= 0.14 (LaFeAsO(1-x)F(x)). In the low-temperature region, a T(3) ln T term in C(p) and a T(2) term in chi-which are derived from the spin fluctuation-are observed. The nearly ferromagnetic nature evidenced by a large Wilson ratio (6.5 for x=0 and 11.2 for x=0.025) suggests that the superconductivity in the LaFeAsO system is mediated by ferromagnetic spin fluctuation.