New antiferromagnetic order with pressure-induced superconductivity in EuFe2As2

The interplay between magnetism and superconductivity is one of the important subjects to investigate the pairing mechanism in novel superconductors. We have found a new coexistence between an antiferromagnetic order in the Fe sublattice and superconductivity of the FeAs-based ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ superconductor in the pressure range from 2.4 to 3.0 GPa by $^{57}\mathrm{Fe}$ nuclear forward scattering (NFS) using a single-crystal sample. The magnetic state in the Fe sublattice changes to a new antiferromagnetic one with superconductivity from a stripe-type antiferromagnetic one observed in the normal conducting state at 2.7 GPa. Below the superconducting transition temperature, the temperature dependence of $^{57}\mathrm{Fe}$ NFS spectra reveals that the new antiferromagnetic order develops with the superconductivity. This nontrivial coupling of two ordered states in ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ under pressure demonstrates a new and intriguing relationship between magnetism and superconductivity in Fe-based superconductors.