Neutron diffraction measurement of covalency and spin deviation: a test of theory and experimental method using Ni2+in KNiF3

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Abstract
Two main effects reduce the apparent moment associated with a cation in an ordered antiferromagnet. Covalency tends to spread the moment on to the adjacent anions, resulting in a loss due to cancellation at the ligand, and the nature of the antiferromagnetic ground state gives rise to zero-point deviations from fully aligned spins. The latter effect has proved very difficult to measure experimentally with accuracy. In principle, the moment associated with the cation may be measured by using neutron diffraction by extrapolation of the magnetic Bragg peak intensities to forward scattering. The present measurements were carried out on an ideal antiferromagnet, KNiF3, in order to test the theories of covalency and spin deviations. The magnitude of both effects may be predicted in this compound.