NEUTRON-DIFFRACTION STUDY OF THE MAGNETIC-ORDERING IN THE INSULATING REGIME OF THE PEROVSKITES RNIO3 (R=PR AND ND)

Neutron-diffraction experiments and polarization analysis techniques r eveal that the metal-insulator transition in the orthorhombic perovski tes PrNiO3 and NdNiO3 is accompanied by a sudden three-dimensional mag netic ordering of the Ni sites (mu(Ni) almost-equal-to 0.9mu(B)). The electronic localization is followed by a magnetic ground state consist ing of an unusual antiferromagnetic structure, with k = (1/2 , 0, 1/2) relative to the orthorhombic crystal cell. This propagation vector im plies the symmetrical coexistence of ferro- and antiferromagnetic coup lings along the three pseudocubic axes. This suggests the existence of a nonuniform orbital distribution of the single e(g) electron. The or bital superlattice may result from the breakdown of the degeneracy of the Ni(III)(t2g6e(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 cr ystallographic Nd site: half of these ions become ordered and the othe r half remain disordered as in a simple paramagnet. This can be explai ned by the combination of polarization effects due to the exchange fie ld from nickel moments and the breaking down of point symmetry at Ni s ite.