Abstract
Powder neutron diffraction studies of undoped La2Cu04 —~ 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 0.5pa/Cu-atom. The tetragonal-orthorhombic transition at 505 K has also been studied.
It is interesting that the ordering does not further double the orthorhombic unit cell, and in fact occurs at the wave vector which corresponds closely to the nesting-Fermi-surface instability in the tetragonal (undistorted) phase. ' A crucial unanswered question in this context is whether the magnetic moment on the copper ions survives in the superconducting compositions. Further neutron scattering experiments may help to resolve this issue.
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