Doping effects of transition metals on superconducting properties of (Ca,\textit{RE})FeAs$_{2}$

At the previous March Meeting, we reported new iron based superconductors (Ca,\textit{RE})FeAs$_{2}$ (Ca112) (\textit{RE} $=$ La-Nd, Sm-Gd)$^{[1,2]}$. Superconducting transition was observed in all samples except for Ce-doped sample, and $T_{c}$ of La-doped sample exceeded 30 K. In this study, we have synthesized transition metals (\textit{TM }=Mn, Co, Ni) co-doped Ca112 samples. Mn co-doping suppressed superconductivity. On the contrary, enhancement of $T_{c}$ with sharp superconducting transitions was observed in most of the Co or Ni co-doped samples. $T_{c}$ of Co co-doped samples decreased with a decrease in ionic radii of \textit{RE}$^{3+}$ from 38 K for \textit{RE} $=$ La to 29 K for \textit{RE} $=$ Gd, though Eu doped sample showed exceptionally low $T_{c} \quad =$ 21 K. $J_{c}$ value of La and Co co-doped sample estimated from magnetization measurement is approximately 2.0 x 10$^{4}$ Acm$^{-2\, }$at 2 K suggesting bulk superconductivity. [1] H. Yakita \textit{et al}., $J$. \textit{Am}. \textit{Chem}. \textit{Soc}. \textbf{136} (2014) 846 [2] H. Yakita \textit{et al}. APS March Meeting 2014 C1 00090