Strongly anisotropic antiferromagnetic coupling in EuFe 2 As 2 revealed by stress detwinning
Of all parent compounds of iron-based high-temperature superconductors, EuFe 2 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 order and Fe structural/antiferromagnetic domains enables rapid field detwinning, this prevents simple magnetometry measurements from extracting the critical fields of the Eu metamagnetic transitions. Furthermore 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 stressdetwinning of structural domains from the field-induced reorientation of Eu moments. Intriguingly, from the critical field values we find that the Eu interplanar interaction is strongly anisotropic. We argue that this anisotropic exchange term is likely a consequence of the strong anisotropy in the magnetically ordered Fe layer, which highlights the non-trivial interplay between Eu-Fe magnetism in this system.