Predicted ferromagnetic monolayer MoTeX (X = Cl, Br, I) with large vertical piezoelectric response: a first-principles study

Abstract The emerging class of two-dimensional piezoelectric ferromagnetic (PFM) systems, combining intrinsic ferromagnetism and piezoelectricity, has attracted significant attention for their potential in multifunctional spintronic devices. In this work, we systematically investigate the electronic structure, magnetic properties, and piezoelectric performance of Janus MoTeX (X = Cl, Br, I) monolayers using first-principles calculations and Monte Carlo simulations. Our results demonstrate that these MoTeX monolayers are stable intrinsic ferromagnetic semiconductors. The Janus-structured MoTeCl, MoTeBr, and MoTeI monolayers exhibit out-of-plane piezoelectric coefficients ( d 31 ) of 1.42, 1.08, and 0.50 pm V −1 , respectively, surpassing most reported 2D materials. This enhanced piezoelectricity originates from the broken inversion symmetry along the vertical direction. Notably, MoTeCl, MoTeBr, and MoTeI monolayers display high Curie temperatures ( T c ), with values of 190, 219, and 249 K. Furthermore, Janus MoTeX monolayers exhibit excellent mechanical flexibility. The application of biaxial tensile strain engineering significantly improves the PFM performance of MoTeCl monolayers: a 2% strain elevates T c to room temperature and induces a transition of the easy magnetization axis from in-plane to out-of-plane, substantially enhancing their practical applicability. These findings highlight Janus MoTeX monolayers as promising candidates for developing next-generation multifunctional spintronic devices.