Observation of strain-locked in-plane spin orders in a 2D XY antiferromagnet

It is believed that no long-range magnetic order exists in a 2D magnet with the XY anisotropy under Mermin–Wagner theorem. Surprisingly, we directly observe the long-range antiferromagnetic (AFM) N\'eel orders in an XY magnet MnPSe$_3$ down to monolayer by optical second-harmonic generation (SHG) microscopy. We further demonstrate the directions of the in-plane spins are decoupled to the crystalline axes and actually locked by the external strains. A simple Ginzburg–Landau theory is applied to explain our observation, where an inevitable tiny strain in the real world would drive the system from XY Universality class to Ising Universality class and help stabilize the long-range orders. Our work also demonstrate the SHG microscopy as a powerful tool to imaging AFM spin structures in 2D magnets.