Modeling the complex magnetic phase diagram of the axion insulator candidate EuIn2As2 in a magnetic field*

The axion insulator candidate EuIn2As2 exhibits a broken-helix magnetic ground state [S. X. M. Riberolles et al., Nature communications, 12(1), 1-7. (2021)] that breaks inversion but respects the product of twofold rotation and time-reversal (2’) symmetry. This 2’ symmetry is predicted to lead to exotic protected gapless surface states on select crystal faces. Motivated by the recent neutron scattering experiments that report a complex magnetic behavior in an external magnetic field, we design and study a minimal symmetry-constrained spin model that exhibits a broken-helix ground state. In addition to first, second, and third neighbor bilinear exchange interactions, we identify a first-neighbor biquadratic exchange coupling as the crucial ingredient to capture the unique physics of the material. We map out the phase diagram in a magnetic field, focusing on the evolution of the different helical magnetic domains observed in the ground state. We find that different domains are characterized by different symmetries in a magnetic field, pointing to the intriguing possibility of protected gapless modes located at internal domain wall boundaries in the material.