Local magnetic anisotropy in BaFe$_2$As$_2$: a polarized inelastic neutron scattering study

TL;DR

This study uses polarized inelastic neutron scattering to investigate magnetic anisotropy in BaFe$_2$As$_2$, revealing strong single-ion anisotropy and differing energy gaps for in-plane and out-of-plane magnons.

Contribution

It provides detailed insights into the magnetic excitation anisotropy and the energy costs of spin rotations within the layered structure of BaFe$_2$As$_2$.

Findings

In-plane polarized magnons have a larger energy gap than out-of-plane ones.

Strong single-ion anisotropy exists within the layers.

Rotating spins within the a-b plane requires more energy than perpendicular to the layers.

Abstract

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 {\it a-b} plane than rotating it perpendicular to the FeAs layers.