Simultaneous Ordering of Holes and Spins inLa2NiO4.125

We report a single-crystal neutron diffraction study of the incommensurate magnetic ordering that occurs in ${\mathrm{La}}_2$Ni${\mathrm{O}}_{4.125}$ below 110 K. Besides the magnetic first and third harmonic Bragg peaks, we have also observed second harmonic peaks associated with charge ordering. The magnitude of the incommensurate splitting, $\epsilon$, is strongly temperaure dependent. Lock-in behavior indicates that $\epsilon$ tends to rational fractions, while regions of continuous variation suggest a devil's staircase. Analysis of these features indicates that the holes, induced by the excess oxygen, order in domain walls that form antiphase boundaries between antiferromagnetic domains.