Exploration of exotic quantum physics in magnetic systems of reduced dimensionality is among the central themes of condensed matter physics. Here we investigate the nature of magnetic couplings, topological excitations, and critical phenomena in monolayer $\mathrm{MnPS}{\mathrm{e}}_3$, a representative two-dimensional antiferromagnet. We first establish, based on first-principles calculations, that the $\mathrm{S}=5/2\mathrm{M}{\mathrm{n}}^{2+}$ ions possess single-ion anisotropy, favoring in-plane magnetization, and exhibit isotropic antiferromagnetic pairwise couplings. Next, our atomistic magnetics simulations identify antiferromagnetically coupled meron pairs to be the characteristic topological excitations, whose size, polarity, and dynamics can be delicately tuned by an external magnetic field. Significantly, a critical temperature of $\sim 30$ K is indicated in both the susceptibility and specific heat, and detailed scaling analyses on the spin correlations and stiffness unambiguously attribute the critical behavior to the Berezinskii-Kosterlitz-Thouless universality class. Our findings thereby provide an ideal candidate antiferromagnetic counterpart of the recently identified monolayer XY ferromagnet of $\mathrm{CrC}{\mathrm{l}}_3$, the former may harbor distinct topological excitations and multicriticality.

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• Magnetic phase transitions
• Magnetic vortices
• Density functional calculations
• Heisenberg model