Incommensurate magnetic order in an axion insulator candidate EuIn2As2 investigated by NMR measurement

Magnetic topological insulators exhibit unique electronic states due to the interplay between the electronic topology and the spin structure. The antiferromagnetic metal EuIn₂As₂ is a prominent candidate material in which exotic topological phases, including an axion insulating state, are theoretically predicted depending on the magnetic structure of the Eu²⁺ moments. Here, we report experimental results of the nuclear magnetic resonance (NMR) measurements of all the nuclei in EuIn₂As₂ to investigate the coupling between the magnetic moments in the Eu ions and the conduction electrons in In₂As₂ layers and the magnetic structure. The ⁷⁵As and ¹¹⁵In NMR spectra observed at zero external magnetic field reveal the appearance of internal fields of $4.9$ and 3.6\ T respectively at the lowest temperature, suggesting a strong coupling between the conduction electrons in the In₂As₂ layer and the ordered magnetic moments in the Eu ions. The ⁷⁵As NMR spectra under in-plane external magnetic fields show broad distributions of the internal fields produced by an incommensurate fan-like spin structure which turns into a forced ferromagnetic state above 0.7\ T. We propose a spin reorientation process that an incommensurate helical state at zero external magnetic field quickly changes into a fan state by applying a slight magnetic field.