Magnetic properties of the superconducting ‘‘superoxide’’La2CuO4+δ

The magnetic properties of ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathrm{\delta }}$ (δ≤0.13) have been determined by means of muon-spin rotation and relaxation in transverse and zero external fields. A fraction $f_{\mathrm{SC}=0.6\mathrm{}}$ of the sample was found to be superconducting with $T_c$=35(1) K and extrapolated penetration depth λ(0)=4200 Å (powder average). The other phase, 40% of the sample volume, displayed static magnetic ordering identical to that of oxygenated ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$. The T dependence of λ is similar to that for samples of the ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$ ${\mathrm{CuO}}_4$ family.