  Magnetization, susceptibility, specific heat, resistivity, neutron and x-ray
diffraction have been used to characterize the properties of single crystalline
CaFe2-xNixAs2 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
~2 K. 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. The regime
of superconductivity is quite restrictive, with a maximum TC of 15 K and an
upper critical field Hc2=14 T. Superconductivity disappears in the overdoped
region.
