Magnetisation density and covalency in ferrous fluoride

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Abstract
Polarised neutron scattering measurements have been made on single crystals of FeF2. The observation of a polarisation-dependent cross section for mixed magnetic and nuclear reflections showed that the two types of antiferromagnetic domains were unequally populated. Integrated intensity measurements were fitted to an extinction model which enabled the domain population to be established quantitatively. Moduli of the magnetic interaction vectors were deduced from polarisation ratio measurements of both mixed magnetic and nuclear and 'forbidden' magnetic reflections. The former were compared with those calculated on an ionic model using the wavefunction deduced from resonance studies and the parameters of this wavefunction were fitted to the higher-angle measurements. Deviation of the lower-angle reflections from this model were interpreted as evidence of covalency and gave a value of 0.049(10) for the sum of the covalency parameters. By comparing the experimental structure factors for the forbidden reflections with those calculated for a covalent wavefunction, individual covalent mixing parameters could be estimated.