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

We report 75As 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 TN 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