Evolution of the bulk properties, structure, magnetic order, and superconductivity with Ni doping inCaFe2−xNixAs2

Abstract

Magnetization, susceptibility, specific heat, resistivity, neutron and x-ray diffraction have been used to characterize the properties of single-crystalline ${\text{CaFe}}_{2\ensuremath{-}x}{\text{Ni}}_{x}{\text{As}}_{2}$ 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 $a\text{\ensuremath{-}}b$ 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 $a\text{\ensuremath{-}}b$ plane quickly decreases and then saturates. Warming and cooling data in the resistivity and neutron diffraction indicate hysteresis of $\ensuremath{\approx}2\text{ }\text{K}$. At larger doping the transition is more rounded and decreases to zero for $x\ensuremath{\approx}0.06$. 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 $a\text{\ensuremath{-}}b$ plane in going to low $T$. Heat-capacity data show that the density of states at the Fermi level increases for $x\ensuremath{\ge}0.053$ as inferred from the value of Sommerfeld coefficient $\ensuremath{\gamma}$. The regime of superconductivity is quite restrictive with a maximum ${T}_{C}$ of 15 K and an upper critical field ${H}_{c2}=14\text{ }\text{T}$. Superconductivity disappears in the overdoped region.