The study on magnetic order in 2D vdW antiferromagnet CoPS3

Abstract

The study on two-dimensional (2D) magnets coupled by van der Waals (vdW) force can be traced back to 50 years ago, however, interest has renewed recently due to the discovery of magnetism in vdW magnets down to the 2D limit, which does not only provide an ideal platform to study the basic magnetic model but also has potential applications in ultra-high density data storage and information processing. Among them, 2D antiferromagnets have attracted much attention due to zero net magnetic moments and are insensitive to external disturbances, making them ideal candidates for the building blocks in information storage. In this thesis, we focus on 2D transition metal phosphorus trisulfides (TMPS3, TM = Mn, Fe, Co, Ni), which are a class of vdW antiferromagnetic (AF) materials. Our target material is CoPS3, which is a representative of 2D planar antiferromagnet. Its single-layer carries the promise of monolayer AF platforms for the ultimately thin spintronics. This thesis reports our experimental study on the magnetic properties of CoPS3 down to the monolayer limit with optical spectroscopy.

This thesis can be divided into two parts. In the first part, we focus on the study of the long-range magnetic order and magnetostrictive effects in CoPS3 with polarization-resolved Raman spectroscopy. We have successfully observed the paramagnetic-AF phase transition in monolayer CoPS3. Phenomenally, the splitting of the double-degenerate Eg phonon mode of the Co-atom correlation vibration and the appearance of a new peak (P2') in the LL configuration indicates the presence of quasi-long-range ordering below TKT in 2D XY-type antiferromagnet. In the second part, we investigate the magnetic anisotropy of monolayer CoPS3 using polarization-resolved broadband pump-probe reflectance spectroscopy. We experimentally demonstrated the correlation between the zigzag AF order and observed linear dichroism effects. This correlation not only provides an effective method for probing the magnetic order in few-layer antiferromagnets but also offers the possibility of ultrafast optical measurements of AF dynamics and their critical behavior.

Our findings provide evidence towards the understanding of the fundamental mechanism of magnetostrictive effects and magnetic linear dichroism (MLD) in 2D AF materials, and demonstrate that circular polarization-resolved Raman spectroscopy and broadband pump-probe reflectance spectroscopy are effective tools for the indirect detection of 2D antiferromagnetism down to the monolayer limit. In addition, we believe that our results will help clear some obstacles to the future fabrication of AF storage and spintronic devices.