  The Dzyaloshinskii-Moriya interaction (DMI), which only exists in
noncentrosymmetric systems, is responsible for the formation of exotic chiral
magnetic states. The absence of DMI in most two-dimensional (2D) magnetic
materials is due to their intrinsic inversion symmetry. Here, using
first-principles calculations, we demonstrate that significant DMI can be
obtained in a series of Janus monolayers of manganese dichalcogenides MnXY in
which the difference between X and Y on the opposites sides of Mn breaks the
inversion symmetry. In particular, the DMI amplitudes of MnSeTe and MnSTe are
comparable to those of state-of-the-art ferromagnet/heavy metal (FM/HM)
heterostructures. In addition, by performing Monte Carlo simulations, we find
that at low temperatures the ground states of the MnSeTe and MnSTe monolayers
can transform from ferromagnetic states with worm-like magnetic domains into
the skyrmion states by applying external magnetic field. At increasing
temperature, the skyrmion states starts fluctuating above 50 K before an
evolution to a completely disordered structure at higher temperature. The
present results pave the way for new device concepts utilizing chiral magnetic
structures in specially designed 2D ferromagnetic materials.
