Antiferromagnetic order and its interplay with superconductivity in CaK(Fe_1-xMn_x)₄As₄

The magnetic order for several compositions of CaK(Fe1-xMnx)(4)As-4 has been studied by nuclear magnetic resonance (NMR), Mossbauer spectroscopy, and neutron diffraction. Our observations for the Mn-doped 1144 compound are consistent with the hedgehog spin vortex crystal (hSVC) order which has previously been found for Ni-doped CaKFe4As4. The hSVC state is characterized by the stripe-type propagation vectors (pi 0) and (0 pi) just as in the doped 122 compounds. The hSVC state preserves tetragonal symmetry at the Fe site, and only this SVC motif with simple antiferromagnetic (AFM) stacking along c is consistent with all our observations using NMR Mossbauer spectroscopy, and neutron diffraction. We find that the hSVC state in the Mn-doped 1144 compound coexists with superconductivity, and by combining the neutron scattering and Mossbauer spectroscopy data we can infer a quantum phase transition, hidden under the superconducting dome, associated with the suppression of the AFM transition temperature (T-N) to zero for x approximate to 0.01. In addition, unlike several 122 compounds and Ni-doped 1144, the ordered magnetic moment is not observed to decrease at temperatures below the superconducting transition temperature (T-c).