CLUSTER STUDY OF THE NEUTRON-SCATTERING FROM FACTOR FOR ANTIFERROMAGNETIC KNIF3 AND NIO

Motivated by difficulties in understanding the magnetism of the insulating parents of high-T(c) superconductors, we have studied the less covalent, and thus simpler, antiferromagnetic (AF) insulators NiO and KNiF3. We also consider the apparently covalent material La2NiO4, Which is closely related to the high-T(c) superconductor parent La2CuO4. Despite many studies of KNiF3 and NiO via cluster calculations, we found that a satisfactory ab initio cluster theory of the neutron form factor is lacking. We have carried out such a calculation in the unrestricted Hartree-Fock (UHF) approximation, taking the basic cluster as (NiF6)4- and (NiO6)10- for KNiF3 and NiO, respectively, treating the remaining lattice in the point-charge model. We show that correlation effects and Pauli repulsion corrections to the point charges are negligible in these cases. After correcting for the zero-point spin fluctuations, the UHF form factor agrees well with experiment in KNiF3, where the absolute value of the form factor is known for small scattering vectors q. The UHF calculations agree satisfactorily with the relative form factor data for NiO, which cover a large range of Absolute value of q ( the absolute experimental values are not available); the agreement in shape includes the variations with q due to asphericity of the spin density. We also found that the UHF results on La2NiO4, obtained by using similar methods, disagree sharply with experiment.