PHASE-SEPARATION AND WEAK FERROMAGNETISM IN LIGHTLY-OXYGEN-DOPED LA2CUO4-CRYSTALS IN LOW MAGNETIC-FIELDS(X SINGLE)

Ferromagnetic regions formed in lightly-oxygen-doped La2CuO4+x single crystals have been detected by measuring magnetic susceptibility in lo w (<50 Oe) magnetic fields. These regions produce a characteristic cur ve of (chi)(T,H) (symmetric in shape approaching a maximum value of si milar to 10(-5) emu/g-enormous for this system) which is observed only at sufficiently low magnetic fields. Time and frequency dependencies of (chi)(T) along with x-ray diffraction data provide evidence in favo r of a contribution from oxygen ordering to this effect. Crystals with differing excess oxygen content and mobility have been studied and th e largest effect has been found in crystals with high oxygen mobility entering the superconducting state.