Local magnetic anisotropy in BaFe${}_2$As${}_2$: A polarized inelastic neutron scattering study

The anisotropy of the magnetic excitations in BaFe${}_2$As${}_2$ was studied by polarized inelastic neutron scattering, which allows one to separate the components of the magnetic response. Despite the in-plane orientation of the static ordered moment, we find the in-plane polarized magnons to exhibit a larger gap than the out-of-plane polarized ones, indicating very strong single-ion anisotropy within the layers. It costs more energy to rotate a spin within the orthorhombic a-b plane than to rotate it perpendicular to the FeAs layers.

Figure 1

(a) Visualization of the crystal and magnetic structure of BaFe${}_2$As${}_2$ in the orthorhombic setting. (b) Definition of the reference frame for the neutron polarization analysis on the (1 0 1) reflection as an example (only the Fe ions are depicted in the unit cell).

Figure 2

Temperature dependence of the elastic magnetic scattering cross section at $\mathbf{Q}=(1$ 0 1) revealing that almost the entire magnetic signal stems from ${\sigma }_y$. The values have been obtained by Eqs. (1). The fitting of a power law to the ${\sigma }_y$ data yields a Néel temperature of 137 K.

Figure 3

Inelastic constant-$\mathbf{Q}$ scans at the magnetic Brillouin-zone centers (1 0 $l$) for different $l$ values and at (1 0 0) measured at 2 K. (Blue) dots represent the magnetic scattering cross section ${\sigma }_y$, while (red) squares depict ${\sigma }_z$. The cross sections have been obtained by measuring the SF${}_z$ and SF${}_y$ channels, respectively, and applying Eq. (1). The solid lines are spline-interpolated spin-wave theory calculations folded with the experimental resolution.

Figure 4

Temperature dependence of the inelastic magnetic scattering at $\mathbf{Q}=(1$ 0 1) and an energy transfer of 10 meV documenting the transition from isotropic to anisotropic fluctuations at $T_N$. The solid lines serve as a guide to the eye.