Short-range cluster spin glass near optimal superconductivity in   BaFe$_{2-x}$Ni$_{x}$As$_{2}$
  High-temperature superconductivity in iron pnictides occurs when electrons are doped into their antiferromagnetic (AF) parent compounds. In addition to inducing superconductivity, electron-doping also changes the static commensurate AF order in the undoped parent compounds into short-range incommensurate AF order near optimal superconductivity. Here we use neutron scattering to demonstrate that the incommensurate AF order in BaFe$_{2-x}$Ni$_{x}$As$_{2}$ is not a spin-density-wave arising from the itinerant electrons in nested Fermi surfaces, but consistent with a cluster spin glass in the matrix of the superconducting phase. Therefore, optimal superconductivity in iron pnictides coexists and competes with a mesoscopically separated cluster spin glass phase, much different from the homogeneous coexisting AF and superconducting phases in the underdoped regime. 
Xingye Lu, David W. Tam, Chenglin Zhang, Huiqian Luo, Meng Wang, Rui   Zhang, Leland W. Harriger, T. Keller, B. Keimer, L.-P...
cond-mat.supr-con