Doping-Enhanced Antiferromagnetism in Ca1-xLaxFeAs2

In iron pnictides, high temperature superconductivity emerges after suppressing antiferromagnetism by doping. Here we show that antiferromagnetism in Ca${}_{1-x}$La${}_x$FeAs${}_2$ is robust against and even enhanced by doping. Using ${}^{75}$As-nuclear magnetic resonance and nuclear quadrupole resonance techniques, we find that an antiferromagnetic order occurs below the N'eel temperature $T_{\mathrm{N}}$ = 62 K at a high doping concentration ($x$ = 0.15) where superconductivity sets in at the transition temperature $T_{\mathrm{c}}$ = 35 K. Unexpectedly, $T_{\mathrm{N}}$ is enhanced with increasing doping, rising up to $T_{\mathrm{N}}$ = 70 K at $x$ = 0.24. The obtained phase diagram of this new system enriches the physics of iron-based high-$T_{\mathrm{c}}$ superconductors.