Superconductivity in the vicinity of ferromagnetism in oxygen free   perovskite MgCNi3: An experimental and DFT (Density Functional Theory) study
  We report synthesis, structural, magnetic, specific heat and Density Functional Theory (DFT) studies on MgCNi3 superconductor. Polycrystalline MgCNi3 samples are synthesized through standard solid state reaction route and found to crystallize in cubic perovskite structure with space group Pm3m, without any detectable trace of Ni impurity. Both AC and DC magnetization exhibited superconducting transition (Tc) at around 7.25 K. The lower critical field (Hc1) and irreversibility field (Hirr) are around 140 Oe and 11 kOe respectively at 2 K. The upper critical field (Hc2) being determined from in-field AC susceptibility measurements is 11.6 kOe and 91.70 kOe with 50% and 90% diamagnetism criteria respectively. Heat capacity (Cp) measurements are carried out under applied field of up to 140 kOe and down to 2 K. The Sommerfeld constant ({\gamma}) and Debye temperature ({\Theta}D) as determined from low temperature fitting of Cp(T) data to Sommerfeld-Debye model are 36.13 mJ/mole-K2 and 263.13 K respectively. The Bardeen-Cooper-Schrieffer (BCS) parameter (2{\Delta}/KBTc) is around 3.62, suggesting MgCNi3 to be an intermediate coupled BCS superconductor with value {\lambda} = 0.69. Although the density functional theory (DFT) calculations exhibited the compound to be non-magnetic but with spin fluctuations, the experimental isothermal magnetization MH loops at 20, 50, 100, 200 and 300 K showed some ferromagnetic nature in this compound with coercive field (Hc) of around 50 Oe at 20 K. The Ni3d states play dominant roles near the Fermi levels and there is strong hybridization between Ni3d and C2p states. It seems that MgCNi3 is superconducting in close proximity of ferromagnetism. 
Anuj Kumar, Rajveer Jha, Shiva Kumar, Jagdish Kumar, P. K. Ahluwalia,   R. P. Tandon and V. P. S. Awana
cond-mat.supr-con