  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 novel device applications. Here, we
predict an in-plane FM semiconductor Janus monolayer NiClI with considerably
large magnetic anisotropy energy (MAE) of 1.439 meV, exhibiting dynamic,
mechanical and thermal stabilities. The NiClI monolayer possesses larger
in-plane piezoelectricity ($d_{11}$$=$5.21 pm/V) comparable to that of
$\mathrm{MoS_2}$. Furthermore, NiClI has huge out-of-plane piezoelectricity
($d_{31}$$=$1.89 pm/V), which is highly desirable for ultrathin piezoelectric
device application. It is proved that huge out-of-plane piezoelectricity is
robust against electronic correlation, which confirms reliability of huge
$d_{31}$. 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 $d_{31}$ of 0.73 pm/V and 1.15 pm/V, respectively.
The predicted huge out-of-plane piezoelectric response makes Janus monolayer
NiClI a good platform for multifunctional semiconductor spintronic
applications, which is also compatible with the bottom/top gate technologies of
conventional semiconductor nanoelectronic devices.
