  We have used resonant x-ray diffraction to develop a detailed description of
antiferromagnetic ordering in epitaxial superlattices based on two-unit-cell
thick layers of the strongly correlated metal LaNiO3. We also report reference
experiments on thin films of PrNiO3 and NdNiO3. The resulting data indicate a
spiral state whose polarization plane can be controlled by adjusting the Ni
d-orbital occupation via two independent mechanisms: epitaxial strain and
quantum confinement of the valence electrons. The data are discussed in the
light of recent theoretical predictions.
