Narrow superconducting window in LaFe1-xNix AsO

We have studied Ni-substitution effect in LaFe1−xNixAsO (0≤x≤0.1) by the measurements of x-ray diffraction, electrical resistivity, magnetic susceptibility, and heat capacity. The nickel doping drastically suppresses the resistivity anomaly associated with spin-density-wave ordering in the parent compound. Superconductivity emerges in a narrow region of 0.03≤x≤0.06 with the maximum Tc of 6.5 K at x=0.04, where enhanced magnetic susceptibility shows up. The upper critical field at zero temperature is estimated to exceed the Pauli paramagnetic limit. The much lowered Tc in comparison with LaFeAsO1−xFx system is discussed.