Magnetization distribution in the tetragonal phase of ${\text{BaFe}}_2{\text{As}}_2$

We have determined the spatial distribution of the magnetization induced by a field of 9 T in the tetragonal phase of ${\text{BaFe}}_2{\text{As}}_2$ using polarized neutron diffraction. Magnetic structure factors derived from the polarization dependence of the intensities of Bragg reflections were used to make a maximum-entropy reconstruction of the distribution projected on the 110 plane. The reconstruction shows clearly that the magnetization is confined to the region around the iron atoms and that there is no significant magnetization associated with either As or Ba atoms. The distribution of magnetization around the Fe atom is significantly nonspherical with a shape which is extended in the $⟨111⟩$ directions in the projection. These results show that the electrons which give rise to the paramagnetic susceptibility are confined to the Fe atoms their distribution suggests that they occupy $3d$ $t_{2g}$-type orbitals with $\approx 60\mathrm{\%}$ in those of $xy$ symmetry.

Figure 1

(Color online) Paramagnetic scattering amplitudes measured for Fe in ${\text{BaFe}}_2{\text{As}}_2$ at 200 K. The solid curve shows the $⟨j_0⟩$ form factor for neutral Fe, Ref. 28, scaled to the paramagnetic magnetization of $6.65\times {10}^{-3}{\mu }_{\text{B}}/\text{Fe}$. The dashed (blue) and the dotted (green) curves show $⟨j_2⟩$ and $⟨j_4⟩$ form factors, which multiply the anisotropic terms in the magnetic scattering on the same scale.

Figure 2

(Color online) Maximum-entropy reconstruction of the magnetization distribution in tetragonal ${\text{BaFe}}_2{\text{As}}_2$ at 200 K projected down $\left[1\overline{1}0\right]$. Contours are drawn at intervals of ${10}^{-2}{\mu }_{\text{B}}{\text{Å}}^{-2}$.