Magnetic detwinning and biquadratic magnetic interaction in EuFe2As2 revealed by 153Eu NMR

TL;DR

This study demonstrates that small in-plane magnetic fields can detwin EuFe2As2 by revealing biquadratic magnetic interactions through $^{153}$Eu NMR measurements, providing new insights into the magnetic coupling in iron-based superconductors.

Contribution

The paper provides experimental evidence of magnetic detwinning at low fields and confirms the presence of biquadratic Eu-Fe spin coupling in EuFe2As2.

Findings

Small in-plane magnetic fields induce detwinning in EuFe2As2.

$^{153}$Eu NMR reveals the evolution of Eu and Fe spin angles during detwinning.

First experimental confirmation of biquadratic Eu-Fe magnetic interaction.

Abstract

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 $H_{\rm ext}$ greater than the typical lab-scale magnetic fields 10--15 T are usually required to completely detwin a sample. However, recently a very small in-plane $H_{\rm ext} \sim$ 0.1 T was found to be sufficient for detwinning the nematic domains in EuFe$_2$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 $^{153}$Eu nuclear magnetic resonance (NMR) measurements below the Eu$^{2+}$ ordering temperature, we show experimental evidence of the detwinning under small in-plane $H_{\rm ext}$. Our NMR study also reveals the evolution of the angles between the Eu and Fe spins during the detwinning process, which provides the first experimental evidence for the existence of biquadratic coupling in the system.