Sudden Appearance of an Unusual Spin Density Wave At the Metal-Insulator Transition in the Perovskites RNiO ₃ (R = Pr, Nd)

The metal-insulator transition in the orthorhombic perovskites PrNiO 3 and NdNiO 3 is accompanied by the sudden 3D magnetic ordering of the Ni ions (μ Ni ≈ 0.9 μ B ). The magnetic ground state, described by a commensurate κ = (1/2, 0, 1/2) spin density wave, is unprecedented in a perovskite structure. The coexistence of F and AF interactions suggests the existence of an orbital superlattice. This results from the breakdown of the degeneracy of the Ni III ( t 2g 6 e g 1 ) state due to electronic correlations. The gap is probably of charge-transfer type rather than originated by the exchange interactions.