Antiferromagnetic ordering in a 90 K copper oxide superconductor

Using elastic neutron scattering, we evidence a commensurate antiferromagnetic Cu(2) order (AF) ${(T}_N\sim 330\hspace{0.5em}\mathrm{K},$ $\mu \sim 0.14\hspace{0.5em}{\mu }_{\mathrm{B}})$ in the superconducting (SC) high-$T_c$ cuprate ${\mathrm{YBa}}_2({\mathrm{Cu}}_{1-y}{\mathrm{Co}}_y{)}_3{\mathrm{O}}_{7+\delta }$ ${(y=0.013,T}_c=93\mathrm{}\hspace{0.5em}\mathrm{K}).\mathrm{}$ We find that the spin excitation spectrum is still dominated by a magnetic resonance peak at 41 meV as in the Co-free system, but with a reduced spectral weight. The substitution of Co leads to a state where AF and SC cohabit at a microscopic scale, without any strong interference.