Neutron-diffraction study of the magnetic ordering in the insulating regime of the perovskites R${\mathrm{NiO}}_3$ (R=Pr and Nd)

Neutron-diffraction experiments and polarization analysis techniques reveal that the metal-insulator transition in the orthorhombic perovskites ${\mathrm{PrNiO}}_3$ and ${\mathrm{NdNiO}}_3$ is accompanied by a sudden three-dimensional magnetic ordering of the Ni sites (${\mathrm{\mu }}_{\mathrm{Ni}}$≊0.9${\mathrm{\mu }}_{\mathit{B}}$). The electronic localization is followed by a magnetic ground state consisting of an unusual antiferromagnetic structure, with k=(1/2,0,1/2) relative to the orthorhombic crystal cell. This propagation vector implies the symmetrical coexistence of ferro- and antiferromagnetic couplings along the three pseudocubic axes. This suggests the existence of a nonuniform orbital distribution of the single ${\mathit{e}}_{\mathit{g}}$ electron. The orbital superlattice may result from the breakdown of the degeneracy of the ${\mathrm{Ni}}^{\mathrm{III}}$(${\mathit{t}}_{2\mathit{g}}^6$${\mathit{e}}_{\mathit{g}}^1$) state due to electronic correlation. This type of spin arrangement has not been observed in other perovskite oxides. These findings are confirmed by the magnetic behavior of the single crystallographic Nd site: half of these ions become ordered and the other half remain disordered as in a simple paramagnet. This can be explained by the combination of polarization effects due to the exchange field from nickel moments and the breaking down of point symmetry at Ni site.