Unpaired Spin Density in Ordered Fe₃Al

A precise determination of the magnetic form factor of the ordered alloy Fe₃Al has been made by diffraction of polarized neutrons from a single crystal. Some 43 reflections in the angular range sinθ / λ≤0.9 A^-1 have been examined and show characteristic departures from a smooth single-valued function, indicating that the unpaired electron density in the unit cell is not spherically symmetric. The data are analyzed in two ways: by comparison with form factors calculated from free-atom Hartree-Fock wave functions including crystal-field splitting effects, and by two-dimensional Fourier projections of the unpaired spin density. The analysis indicates that, while the two types of iron atom in the lattice have similar radial spin densities, their orbital symmetry is different. The results are discussed with reference to various theories of the electronic structure in transition metals.