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. Nevertheless, 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 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 and the bilayer in MnPS3 . 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 strain. By studying both a Landau theory and a microscopic model, we conclude that 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. In MnPS3, we observed the Heisenberg-type domain switching, and we also observed an anomalous layer-dependent mirror symmetry breaking.
Finally, we found that the 180 deg AFM domain walls in both compounds are highly mobile, paving the way for future control of the antiferromagnetic domains by strain or external fields
on demand for ultra-compact 2D AFM terahertz spintronics.
References:
1. Ni et al. Nature Nanotechnology 16, 782-787 (2021)