Magnetic form factor of iron in ${\text{SrFe}}_2{\text{As}}_2$

The iron magnetic form factor in ${\text{SrFe}}_2{\text{As}}_2$ has been determined by neutron diffraction and by density-functional theory (DFT). As noted previously, the magnitude of the calculated moment using DFT is sensitive to the Fe-As distance. However, the shape of the calculated form factor is practically insensitive to the Fe-As distance, and further we show that the form factor closely resembles that of bcc iron, and agrees well with experiment. The spin density exhibits some anisotropy due to geometry and As hybridization.

Figure 1

(Color online) ${\text{SrFe}}_2{\text{As}}_2$ orthorhombic crystal structure with atomic positions as indicated; (Fe1) and (Fe2) are Fe atoms with antiparallel magnetic moment directions. The gray plane is the one that was chosen for the spin-density contour plot of Fig. 3.

Figure 2

(Color online) Spin density along the Fe-As direction as calculated from LDA with the experimental atomic position. Although As does not have a net magnetic moment in the antiferromagnetic structure, its spin-density distribution depends on the magnetic ordering of Fe. The inset shows spin densities inside the Fe muffin-tin sphere along specific directions demonstrating substantial anisotropy.

Figure 3

(Color online) Two-dimensional spin-density plot indicating hybridization between Fe and As atoms and anisotropy. Since the spin density is strongly concentrated in the small radius inside Fe atoms (see Fig. 2) a small spin-density contour range was chosen to reveal the low spin-density structure. The black line between Fe and As is the axis for Fig. 2

Figure 4

(Color online) (a) Comparison of the form factors calculated with the experimental atomic positions $\left(z_{\text{As}}=0.3612\right)$ and theoretical energy optimized one $\left(z_{\text{As}}=0.3499\right)$. The arrow points to scattering vectors that provide evidence for fairly strong anisotropy in the form factor. (b) Experimental form factors for ${\text{SrFe}}_2{\text{As}}_2$ and BCC Fe, and the calculated form factors with the experimental As position.