Evidence of magnetism-induced topological protection in the axion insulator candidate EuSn 2 P 2

Significance Understanding the mutual influence between magnetic and topological properties in a solid system is a fundamental challenge for quantum materials research. The so-called axion insulator is one such case, with exotic properties related to the magnetoelectric effect. In this work, by combining chemical and magnetic-state sensitive electron and X-ray spectroscopies with first-principle calculations we reveal that EuSn 2 P 2 holds the characteristics of an axion insulator and displays “hidden” electronic properties arising from its layer-dependent ferromagnetic character. As a function of the termination, we observe specific electronic states, holding a spin arrangement representative of a topological insulator. Furthermore, the direction of magnetization enables topological protection, a key property related to axion-based physics.