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.