Neutron-scattering study of spin-density wave order in the superconductingstate of excess-oxygen-doped La2CuO4+y

We report neutron-scattering measurements of spin-density wave order within the superconducting state of a single crystal of predominately stage-4 La2 CuO4+y with a T-c (onset) of 42 K. The low-temperature elastic magnetic sca ttering is incommensurate with the lattice and is characterized by long-ran ge order in the copper-oxide plane with the spin direction identical to tha t in the insulator. Between neighboring planes, the spins exhibit short-ran ge correlations with a stacking arrangement reminiscent of that in the undo ped antiferromagnetic insulator. The elastic magnetic peak intensity appear s at the same temperature within the errors as the superconductivity, sugge sting that the two phenomena are strongly correlated. These observations di rectly reveal the persistent influence of the antiferromagnetic order as th e doping level increases from the insulator to the superconductor. In addit ion, our results confirm that spin-density wave order for incommensurabilit ies near 1/8 is a robust feature of the La2CuO4-based superconductors. [S01 63-1829(99)02029-9].