Doping effects of Cr on the physical properties of BaFe1.9-xNi0.1CrxAs2

We present a systematic study on the heavily Cr doped iron pnictides BaFe1.9_xNi0.1CrxAs2 by using elastic neutron scattering, high-resolution synchrotron x-ray diffraction (XRD), resistivity, and Hall transport measurements. When the Cr concentration increases from x = 0 to 0.8, neutron diffraction experiments suggest that the collinear antiferromagnetism persists in the whole doping range, where the Neel temperature T-n coincides with the tetragonal-to-orthorhombic structural transition temperature T-s, and both of them keeps around 35 K. The magnetic ordered moment, on the other hand, increases within increasing x until x = 0.5, and then decreases with further increasing x. Detailed refinement of the powder XRD patterns reveals that the Cr substitutions actually stretch the FeAs4 tetrahedron along the c axis and lift the arsenic height away from the Fe-Fe plane. Transport results indicate that the charge carriers become more localized upon Cr doping, then changes from electron type to hole type around x = 0.5. Our results suggest that the ordered moment and the ordered temperature of static magnetism in iron pnictides can be decoupled and tuned separately by chemical doping.