The emergence of van der Waals (vdW) magnets greatly enriched our understanding of intrinsic magnetism in the 2D limit, beyond the conventional Ising model and XY model pictures. Different spin configurations in diverse materials enable the experimental realization of the well-established magnetism models. For example, Ising-type ferromagnetism (FM) is found in monolayer CrI₃, while the XXZ-type antiferromagnetism (AFM) in bulk NiPS₃is suppressed in monolayer NiPS₃, both are consistent with the theoretical predictions. However, the existing vdWs magnets are still rare. Therefore, it is imperative to find more vdWs magnetic materials with different bandgaps and novel magnetic properties for applications in a wide variety of 2D magnetic and spintronics devices.

Through DFT calculations, Prof. Yang’s group found a 2D multiferroic materials CuCrP₂S₆and synthesized the single crystal successfully. 2D CuCrP₂S₆is a type-I multiferroic candidate (or a ferromagnetically ferroelectric material), where the ferromagnetic and ferroelectric polarization stems from the Cr and Cu elements, respectively. In experiments, the ferroelectricity was observed in 13.3 nm-thick CuCrP2S6, using piezo force microscopy (PFM) at room temperature. The ferromagnetism was observed from the hysteresis loop of the massively stacked nanosheets（Nanoscale, 2019,11, 5163).

Figure 1. The magnetic hysteresis loops of AgVP₂Se₆

Recently, Yang‘s group and Ye’s group found a new 2D quaternary vdW ferromagnetic semiconductor AgVP₂Se₆. The bulk ferromagnetic transition temperature T_Cof AgVP₂Se₆is 18.5 K. When the thickness of the AgVP₂Se₆flakes is reduced, enhanced out-of-plane magnetic anisotropy is observed. The 6.7 nm AgVP₂Se₆flake exhibits a rectangular hysteresis loop with a large coercive field of 750 Oe at 2 K and an undiminished T_Cof 19 K, proving itself a promising 2D hard-ferromagnetic semiconductor for future 2D spintronic devices. This work was published in Advanced Functional Materials (DOI: 10.1002/adfm.201910036).

Figure 2. The magnetic structures of CrPS₄under the magnetic field along the c axis.

Prof. Yang’s group also collaborated with Prof. Zheng group from Zhejiang University and Dr. Maxim Avdeev of Australian Nuclear Science and Technology Organisation to study vdW antiferromagnet CrPS₄. Complimentary magnetic, torque, and neutron diffraction measurements have been made to elucidate the metamagnetic transitions and the magnetic structure of high-quality CrPS₄single crystals. By fitting of neutron diffraction results, an A-type AFM ground state is confirmed in bulk CrPS₄below 38 K. When H is applied along the c axis (H∥c), a spin-flop transition is observed at around 0.7 T, corresponding to a canted interlayer AFM in the direction perpendicular to the c axis. The canted interlayer AFM phase eventually turns into out-of-plane ferromagnetic ordering, manifested by a spin-flip transition at about 8.5 T. This work was published in Advanced Materials (DOI: 10.1002/adma.202001200).

The above studies received support from the National Natural Science Foundation of China, Ministry of Science and Technology of China, Chinese Academy of Sciences, and Beijing Municipal Science and Technology Commission.