Magnetic detwinning and biquadratic magnetic interaction in EuFe2As2 revealed by Eu153 NMR

In the nematic state of iron-based superconductors, twin formation often obscures the intrinsic, anisotropic, in-plane physical properties. Relatively high in-plane external magnetic fields ${H}_{\mathrm{ext}}$ greater than the typical laboratory-scale magnetic fields 10--15 T are usually required to completely detwin a sample. However, recently a very small in-plane ${H}_{\mathrm{ext}}\ensuremath{\sim}0.1$ T was found to be sufficient for detwinning the nematic domains in ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$. To explain this behavior, a microscopic theory based on biquadratic magnetic interactions between the Eu and Fe spins has been proposed. Here, using $^{153}\mathrm{Eu}$ nuclear magnetic resonance (NMR) measurements below the ${\mathrm{Eu}}^{2+}$ ordering temperature, we show experimental evidence of the detwinning under small in-plane ${H}_{\mathrm{ext}}$. Our NMR study also reveals the evolution of the angles between the Eu and Fe spins during the detwinning process, which provides experimental evidence for the existence of biquadratic coupling in the system.