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

Motivated by difficulties in understanding the magnetism of the insula ting parents of high-T(c) superconductors, we have studied the less co valent, and thus simpler, antiferromagnetic (AF) insulators NiO and KN iF3. We also consider the apparently covalent material La2NiO4, Which is closely related to the high-T(c) superconductor parent La2CuO4. Des pite many studies of KNiF3 and NiO via cluster calculations, we found that a satisfactory ab initio cluster theory of the neutron form facto r is lacking. We have carried out such a calculation in the unrestrict ed Hartree-Fock (UHF) approximation, taking the basic cluster as (NiF6 )4- and (NiO6)10- for KNiF3 and NiO, respectively, treating the remain ing lattice in the point-charge model. We show that correlation effect s 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 a bsolute value of the form factor is known for small scattering vectors q. The UHF calculations agree satisfactorily with the relative form f actor 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 d ensity. We also found that the UHF results on La2NiO4, obtained by usi ng similar methods, disagree sharply with experiment.