Antiferromagnetism inLa2CuO4−y

Powder neutron diffraction studies of undoped ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{y}}$ have revealed new superlattice peaks below ∼220 K. The absence of corresponding x-ray superlattice lines and an observed susceptibility anomaly near 220 K suggest the occurrence of antiferromagnetism. From the magnetic peak intensities we deduce a structure consisting of ferromagnetic sheets of Cu spins alternating along the [100] orthorhombic axis, with the spins aligned along the [001] orthorhombic axis, The low-temperature magnetic moment is approximately 0.5${\mathrm{\mu }}_{\mathrm{B}}$/(Cu atom). The tetragonal-orthorhombic transition at 505 K has also been studied.