Local magnetic inhomogeneities in Ba(Fe1-xNix)2As2 as seen via 75As NMR

We report As-75 NMR measurements in BaFe2As2 doped with Ni. Like Co, Ni doping suppresses the antiferromagnetic and structural phase transitions and gives rise to superconductivity for sufficiently large Ni doping. The spin-lattice relaxation rate diverges at T-N with a critical exponent consistent with three-dimensional ordering of local moments. In the ordered state the spectra quickly broaden inhomogeneously with doping. We extract the average size of the ordered moment as a function of doping and show that a model in which the order remains commensurate but with local amplitude variations in the vicinity of the dopant fully explains our observations.