Hyeonsik Cheong
Magnetism in low dimensional systems is a fascinating topic for the fundamental physics as well as for possible applications in future spintronic devices. Although ferromagnetic 2-dimensional (2D) materials are attracting the most interest, antiferromagnetic 2D materials are equally interesting for the rich physics they reveal. However, antiferromagnetic ordering is much more difficult to investigate because the lack of net magnetization hinders easy detection of antiferromagnetic ordering. Neutron scattering, which is a powerful tool to detect antiferromagnetic order in bulk materials, cannot be used for atomically thin samples due to the small sample volume. Raman spectroscopy has proven to be a powerful tool to detect antiferromagnetic ordering by monitoring magnetically induced changes in the Raman spectrum. In this talk, I will review recent achievements in the study of antiferromagnetism in 2 dimensions using Raman spectroscopy. FePS3 exhibits an Ising-type antiferromagnetic ordering down to the monolayer limit, in good agreement with the Onsager solution for 2-dimensional order-disorder transition. The transition temperature remains almost independent of the thickness from bulk to the monolayer limit, indicating that the weak interlayer interaction has little effect on the antiferromagnetic ordering. On the other hand, NiPS3, which shows an XXZ-type antiferromagnetic ordering in bulk, exhibits antiferromagnetic ordering down to 2 layers with a slight decrease in the transition temperature, but the magnetic ordering is suppressed in the monolayer limit. A Heisenberg-type antiferromagnet MnPS3 also exhibits ordering down to 2 layers with a small decrease in the transition temperature. Furthermore, a recent discovery of a peculiar excitonic transition that exhibit a dramatic decrease of the linewidth below the transition temperature will be reported.