Narrow superconducting window in LaFe 1 − x Ni x AsO

We have studied Ni-substitution effect in ${\text{LaFe}}_{1\ensuremath{-}x}{\text{Ni}}_{x}\text{AsO}$ $(0\ensuremath{\le}x\ensuremath{\le}0.1)$ 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 $0.03\ensuremath{\le}x\ensuremath{\le}0.06$ with the maximum ${T}_{c}$ 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 ${T}_{c}$ in comparison with ${\text{LaFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ system is discussed.