Magnetic topological materials provide a unique playground for exploring exotic quantum phenomena in condensed matter physics, such as the axion insulator state and the quantum anomalous Hall effect. The rare-earth Zintl compound EuIn₂As₂, in which topological InAs layers are sandwiched by magnetic Eu layers, is recently pointed out to be an intrinsic magnetic topological insulator by firstprinciples calculations. However, although the emergence of this topological state depends on the magnetic structure of Eu layers, the details of the magnetic states remain controversial.

To clarify the magnetic structure, we investigate the magnetic state of EuIn₂As₂ by using NMR measurements [1]. Our ⁷⁵As NMR measurements reveal that an incommensurate magnetic state with a non-uniform semi-circle fan structure appears by applying an in-plane magnetic field. In addition, the reorientation process in the ordered moments under the external field, in which more spins are concentrated in the direction perpendicular to the applied field, is observed as an increase of the NMR intensity. On the other hand, the magnetic structure smoothly transforms to the forced ferromagnetic state when the magnetic field is applied perpendicular to the plane. These results indicate that an incommensurate magnetic structure weakly pinned in the crystal axes appears in this compound. We conclude that a fine control of the magnetic structure by applying a chemical or physical pressure is required to realize the topological states found by the first-principles calculations.

Furthermore, we try the carrier doping of this compound. Actually, previous transport and angle-resolved photoemission spectroscopy measurements have shown that EuIn₂As₂ is highly hole doped, which is needed to be compensated to realize the axion insulator nature. Chemical doping has been attempted to compensate the hole carrier, which is, however, often accompanied by unintended changes in the magnetic property. For example, P-doping in EuIn₂As₂ changes the AFM order into a spin glass state, as well as the disappearance of the Néel peak in the temperature dependence of the magnetic susceptibility.

To tune the Fermi level, we dope Ca ions to Eu sites of EuIn₂As₂ and characterize electronic state by transport measurements [2]. Unexpectedly, we find that the hole carrier is decreased by the isovalent Ca doping due to a slight increase of the valence charge of the Ca ions, giving rise to an effective electron doping. We further find that, although the magnetic transition temperature decreases with increasing the Ca ions, both the temperature dependence of the magnetic susceptibility and the topological Hall effect are observed in all the Ca-doped samples as observed in our previous measurements done in the pristine EuIn₂As₂ [3], suggesting that the AFM magnetic structure remains almost the same. Our results indicate that non-magnetic Ca doping can modify the band structure without largely changing the magnetic ground state of EuIn₂As₂, providing an opportunity to realize the axion insulating state by only tuning the carrier density

References

• [1] H. Takeda et al., npj Quantum Materials 9, 67 (2024).
• [2] J. Yan et al., Phys. Rev. B 110, 115111 (2024).
• [3] J. Yan et al., Phys. Rev. Research 4, 013163 (2022).

Authors

• H. Takeda, J. Yan, J. Si^a, Z. Jiang^b, H. Ma, Y. Uwatoko, B.-T. Wang^b, X. Luo^b, Y. Sun^b, and M. Yamashita
• ^aSongshan Lake Materials Laboratory
• ^bChinese Academy of Sciences