Static magnetism in superconducting stage-4 La₂CuO_4+y(_y = 0.12)

Zero-field (ZF) and transverse-field (TF) μSR measurements have been performed on a superconducting La₂CuO_4+y crystal with y = 0.12 where excess oxygen is intercalated to form a stage-4 structure. Measurements are made on the same single-crystal sample studied in the neutron scattering measurements of Lee et al. [Phys. Rev. B 60 (1999) 3643] who find that both superconductivity and static incommensurate magnetic modulation develop below T_c ∼ T_N ∼ 42 K. The amplitude of the muon precession signal observed in ZF-μSR increases progressively below T_N and saturates at T → 0 to the value which indicates that a static magnetic field from Cu moments exists in only a part of muon sites corresponding to a magnetic site fraction of V_M = 41%. The observed frequency ∼ 3.5 MHz of the precession signal suggests the development of a large (at least 0.4 μ_B) static Cu moment near the "magnetic" muon sites. Comparing the TF-μSR results with computer simulations, we obtained V_M(T → 0) = 42%.