Structural phase transitions, mechanical properties, and electronic band structures of room-temperature ferromagnetic monolayers ScMP2 (M = Mn and Cr)

Two-dimensional (2D) auxetic properties and ferromagnetism are two major focuses of research for next-generation nanomechanical and spintronic devices. However, 2D materials with the two properties at the same time are rarely reported. Based on first-principles calculations, we propose the 2D bimetallic phosphide ScMP2 (M = Mn and Cr), including three stable tetragonal monolayers. The monolayer ScMnP2 exhibit the square structural phase while there is a structural phase transition between the square ScCrP2 and the rectangular ScCrP2, which originates from the mechanism of ferroelasticity and ferroelectricity. Both monolayers ScCrP2 exhibit in-plane auxetic properties, and the value of the negative Poisson’s ratio can reach –0.322. All the three monolayers show a ferromagnetic ground state with high Curie temperature, and are a Dirac half-metal without spin–orbit coupling (SOC). When considering SOC, a nontrivial bandgap can be found in the monolayer ScMnP2, which further becomes a half-Chern insulator. Importantly, the structural phase transitions from the square lattice to the rectangular lattice in the monolayers ScCrP2 will enable a significant improvement in the values of negative Poisson’s ratio and magnetic anisotropic energy simultaneously. Taking the multiferroic (ferroelastic, ferroelectric, and ferromagnetic), auxetic, and topological properties into consideration, the 2D system of ScMP2 provide promising applications for future multifunctional nanodevices.