SUDDEN APPEARANCE OF AN UNUSUAL SPIN-DENSITY WAVE AT THE METAL-INSULATOR-TRANSITION IN THE PEROVSKITES RNIO3 (R=PR, ND)

The metal-insulator transition in the orthorhombic perovskites PrNiO3 and NdNiO3 is accompanied by the sudden 3D magnetic ordering of the Ni ions (mu(Ni) almost-equal-to 0.9 mu(B)). The magnetic ground state, described by a commensurate k = (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) (t2g6 e(g)1) state due to electronic correlations. The gap is probably of charge-transfer type rather than originated by the exchange interactions.