Strongly anisotropic antiferromagnetic coupling in EuFe 2 As 2 revealed by stress detwinning

Of all parent compounds of iron-based high-temperature superconductors, ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ exhibits by far the largest magnetostructural coupling due to the sizable biquadratic interaction between Eu and Fe moments. While the coupling between Eu antiferromagnetic (AFM) order and Fe structural/AFM domains enables rapid field detwinning, this prevents simple magnetometry measurements from extracting the critical fields of the Eu metamagnetic transition. Here, we measure these critical fields by combining x-ray magnetic circular dichroism spectroscopy with in situ tunable uniaxial stress and applied magnetic field. The combination of two tuning knobs allows us to separate the stress detwinning of structural domains from the field-induced reorientation of Eu moments. Intriguingly, we find a spin-flip transition which can only result from a strongly anisotropic interaction between Eu planes. We argue that this anisotropic exchange is a consequence of the strong anisotropy in the magnetically ordered Fe layer, which presents a form of higher-order coupling between Eu and Fe magnetism.