Large out-of-plane piezoelectric response in ferromagnetic monolayer NiClI

The combination of piezoelectricity and ferromagnetic (FM) order in a two-dimensional (2D) material, namely, 2D piezoelectric ferromagnetism (PFM), may open up unprecedented opportunities for device applications. Here, we predict an in-plane FM semiconductor Janus monolayer NiClI with a considerably large magnetic anisotropy energy of 1.439 meV, exhibiting dynamical, mechanical, and thermal stabilities. The NiClI monolayer possesses larger in-plane piezoelectricity (d11 = 5.21 pm/V) comparable to that of MoS2. Furthermore, NiClI has large out-of-plane piezoelectricity (d31 = 1.89 pm/V), which is highly desirable for ultrathin piezoelectric device application. It is proved that large out-of-plane piezoelectricity is robust against electronic correlation, which confirms reliability of large d31. Finally, being analogous to NiClI, PFM with large out-of-plane piezoelectricity can also be achieved in the Janus monolayers of NiClBr and NiBrI, with the predicted d31 of 0.73 and 1.15 pm/V, respectively.