Neutron-diffraction study of the antiferromagnetic form factor ofLa2NiO4

Neutron-diffraction techniques have been used to study the antiferromagnetic form factor of a ${\mathrm{La}}_2$ ${\mathrm{NiO}}_4$ single crystal at 15 K. The antiferromagnetic form factor was obtained by measuring the integrated intensities of 16 magnetic reflections. A plateau is clearly seen at low Q (up to 3 A${\mathrm{̊}}^{\mathrm{-}1}$) in the experimentally determined form factor, similar to those observed for ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$ and ${\mathrm{Sr}}_2$ ${\mathrm{CuO}}_2$ ${\mathrm{Cl}}_2$. The magnetization density deduced from the experimental data exhibits strong ansiotropy in the Ni-O plane. The experimental data are compared with covalency models and a recent spin-polarized band calculation.