  The exploitation of piezoelectric ferromagnetism (PFM) in two-dimensional
(2D) materials with large out-of-plane piezoelectric response is motivated not
only by technological applications but also scientific interest. In this study,
the CrONM monolayer family (N=F, Cl; M=Br, Cl) was investigated using
first-principles calculations, revealing that the Janus CrOFBr monolayer
exhibits intrinsic ferromagnetic semiconductor behavior along with a
significant out-of-plane piezoelectric effect. The calculated out-of-plane
piezoelectric strain coefficients d$_{31}$ and d$_{32}$ are up to 1.21 and 0.63
pm/V, respectively. These values are greater than those of the majority of 2D
materials. Furthermore, our findings demonstrate that applying tensile strain
can enhance the out-of-plane piezoelectric response, leading to a respective
27% and 67% augmentation in the piezoelectric strain coefficients d$_{31}$ and
d$_{32}$ compared to the unstrained configurations. This discovery holds great
potential for propelling the field of nanoelectronics forward and facilitating
the development of multifunctional semiconductor spintronic applications.
Finally, by comparing d$_{31}$ and d$_{32}$ of the CrONM monolayer family (N=F,
Cl; M=Br, Cl), we find that the magnitudes of d$_{31}$ and d$_{32}$ are
correlated with the electronegativity difference between the M and N atoms.
These findings provide valuable insights for the design of 2D piezoelectric
materials with enhanced vertical piezoelectric responses.
