  The family of two-dimensional (2D) van der Waals transition metal phosphorus
trichalcogenides has received a renewed interest due to their intrinsic 2D
antiferromagnetism, which proves them as unprecedented and highly tunable
building blocks for spintronics and magnonics at the single-layer limit.
Herein, motivated by the exciting potential of atomic-substitution demonstrated
in Janus transition metal dichalcogenides, we investigate the crystal,
electronic and magnetic structure of selenized Janus monolayers based on
MnPS$_3$ and NiPS$_3$ from first-principles. In addition, we calculate the
magnon dispersion and perform real-time real-space atomistic dynamic
simulations to explore the propagation of spin waves in MnPS$_3$, NiPS$_3$,
MnPS$_{1.5}$Se$_{1.5}$ and NiPS$_{1.5}$Se$_{1.5}$. Our calculations predict a
drastic enhancement of magnetic anisotropy and the emergence of large
Dzyaloshinskii-Moriya interactions, which arises from the induced broken
inversion symmetry in the 2D Janus layers. These results pave the way to the
development of Janus 2D transition metal phosphorus trichalcogenides and
highlight their potential for magnonic applications.
