The family of monolayer two-dimensional (2D) materials hosts a wide range of interesting phenomena, including superconductivity, charge density waves, topological states and ferromagnetism, but direct evidence for antiferromagnetism in the monolayer has been lacking. Antiferromagnets have attracted enormous interest recently in spintronics due to the absence of stray fields and their terahertz resonant frequency. Despite the great advantages of antiferromagnetic spintronics,  controlling and directly detecting  Neel vectors in 2D materials have been challenging. In my talk, I will show that we have developed a sensitive second harmonic generation  (SHG) microscope and detected long-range Neel antiferromagnetic (AFM) order down to the  monolayer in MnPSe3[1].    In MnPSe3, we observed the switching of an Ising-type  Neel vector  reversed by the time-reversal operation.  We rotated them by an arbitrary angle irrespective of the lattice by applying uniaxial strain. The phase transition in the presence of strain in MnPSe3 falls into the Ising universality class instead of the XY type, and the Ising Neel vector is locked to the strain. I will also talk about using the newly developed optical confocal microscopy to image other emergent orders in 2D antiferromagnets[2, 3,4].

 References:

1. Ni et al. Nature Nanotechnology 16, 782-787 (2021)  

2. Ni et al.  Phys. Rev. Lett. 127, 187201 (2021) 

3. Zhang et al. Nature Photonics 16, 311-317 (2022)

4. Ni, et al. Nano Letters 22, 3283-3288 (2022)