Evolution of the bulk properties, structure, magnetic order, and superconductivity with Ni doping in CaFe2-x Nix As2

Magnetization, susceptibility, specific heat, resistivity, neutron and x-ray diffraction have been used to characterize the properties of single-crystalline CaFe2-x Nix As2 as a function of Ni doping for x varying from 0 to 0.1. The combined first-order structural and magnetic phase transitions occur together in the undoped system at 172 K with a small decrease in the area of the a-b plane along with an abrupt increase in the length of the c axis in the orthorhombic phase. With increasing x the ordered moment and transition temperature decrease but the transition remains sharp at modest doping while the area of the a-b plane quickly decreases and then saturates. Warming and cooling data in the resistivity and neutron diffraction indicate hysteresis of ≈2K. At larger doping the transition is more rounded and decreases to zero for x≈0.06. The susceptibility is anisotropic for all values of x. Electrical resistivity for x=0.053 and 0.06 shows a superconducting transition with an onset of nearly 15 K which is further corroborated by substantial diamagnetic susceptibility. For the fully superconducting sample there is no long-range magnetic order and the structure remains tetragonal at all temperature but there is an anomalous increase in the area of the a-b plane in going to low T. Heat-capacity data show that the density of states at the Fermi level increases for x ≥ 0.053 as inferred from the value of Sommerfeld coefficient γ. The regime of superconductivity is quite restrictive with a maximum TC of 15 K and an upper critical field Hc2 =14T. Superconductivity disappears in the overdoped region.