Apparent low A-site moment in Fe3O4

Magnetic structure amplitudes of 49 independent reflections in <110> zone in natural magnetite (Fe3O4) have been measured upto sin(theta/lambda)=0.83A-1 using the polarised neutron technique. The magnetic moment density integrated over circular areas about the A-site in the (110) plane projection leads to a lower value for the localised moment (3.82 μB) than expected for the Fe3+ ions. The A-site magnetic form factor points obtained from an analysis of A-site-only reflections scale closely to Fe3+ free ion form factor with this value of moment though there is some ambiguity about (220) form factor. The B-site form factors are deduced from the remaining reflections. the apparent reduction of the localised A-site moment is considered to be a consequence of covalency in the crystal. It is argued that predominant transfer of moment takes place through T2 molecular orbitals. An unpolarised neutron powder diffraction study of the same specimen leads to little higher moment values for the A and B-sites than from the above analysis