Anisotropic magnetic form factor in a detwinned single crystal of BaFe2 As2

We have performed neutron diffraction measurements at 12 K on a single crystal of BaFe2As2, which serves as the parent compound of iron-based superconductors. To investigate the in-plane anisotropy of the magnetic form factor in the antiferromagnetic phase, the single crystal was detwinned. The magnetic structure factor and magnetic form factor are well explained by the spin densities comprising 3dyz electrons with a fraction of approximately 40% and electrons in the other four 3d orbitals each with a fraction of approximately 15%. It is a direct observation of the largely anisotropic spin density relative to the small orthorhombic lattice distortion, (a-b)/(a+b)∼0.3%.