Magnetic and topological phase transition in the symmetry-breaking 1T′-FeSe2 monolayer

Identifying two-dimensional (2D) intrinsic magnetic materials is of great significance for revolutionized spintronic application and fundamental research. Through comprehensive first-principles calculations, we uncover a dynamical and thermally stable monolayer 2D transition metal dichalcogenide compound FeSe2 with an uncommon 1T′ structure and dimerized Fe–Fe bonds. More interestingly, the electronic structure of the 1T′-FeSe2 monolayer depends on the magnetic configurations. The ground state is a ferromagnetic (FM) metal with an obvious magnetocrystalline anisotropy and a high Curie temperature of nearly 400 K. In contrast, the nonmagnetic and antiferromagnetic (AFM) states are insulators, implying the FM to paramagnetic transition will be accompanied by a metal–insulator transition. Furthermore, the FM order transforms to AFM order under a 2.5% in-plane tension, accompanied by a metal–insulator transition. Intriguingly, the AFM trivial insulating state further evolves to AFM topological insulating state by further stretching the in-plane area with a tensile strain of ∼9.1%, which is attributed to the nonsymmorphic symmetry resulting from structural transition by breakdown of the dimerized Fe–Fe bonds. The present work not only is of great scientific interest in exploring unusual magnetic monolayer materials and fascinating phase transitions but also reveals the potential applications of 1T′-FeSe2 monolayers in nanoscale devices.