Fig.1 Interlayer distance and interlayer number regulating interlayer magnetism. a: The interlayer electron kinetic energy term, Pauli and Coulomb repulsion competition determine the interlayer magnetic ground state. b: Total-energy differences between the interlayer AFM/FM configurations as a function of interlayer Se-Se distance. c: Evolution of energy differences between interlayer FM and AFM configurations as a function of the layer number. T d: Experimental confirmation of ferromagnetism of CrSe2 enhanced with the number of layers. e: CrSe2 remains stable after exposure to air for several months.
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Nature Materials
Van der Waals epitaxial growth of air-stable CrSe2 nanosheets with thickness-tunable magnetic order
It is found that in weakly interlayer coupled CrI3 system, the horizontal sliding change stack can regulate interlayer magnetism, and the vertical change of interlayer distance can also be another dimension to regulate two-dimensional magnetism. The researchers found that MX2 (M=V, Cr, Mn; X=S, Se, Te) in the double layer, the phenomenon of interlayer antiferromagnetic — ferromagnetic transition will occur with the change of the layer spacing, which is the result of the competition between the kinetic energy of electrons moving in the layer, the Pauli repulsion of electrons and the Coulomb repulsion of electrons[1] (FIG. 1 a-b).
This work is the first to show that interlayer distance can be used as a new degree of freedom to regulate two-dimensional magnetism, highlighting the importance of non-metallic elements and their interlayer Pauli repulsion not considered in previous models.
We found that the above magnetic coupling mechanism is also applicable to the layers. With the increase of layer thickness, the kinetic energy of electrons moving along the direction of CrSe2 layer increases, gradually represses the interlayer antiferromagnetic coupling caused by Pauli and Coulomb repulsion, and macro ferromagnetism gradually takes the upper hand. In cooperation with the experimental groups, we confirmed the above theoretical image, which is the first international confirmation that CrSe2 is a two-dimensional magnetic material with thickness-dependent magnetic coupling transition between layers and is highly stable in air[2] (FIG. 1 c-e). It has initially overcome the difficulty of insufficient air stability of two-dimensional magnetic materials that had puzzled the scientific community for a long time. As soon as this work was published, it quickly gained academic attention. Professor Andrew T.S. Wee from the National University of Singapore quoted and positively appraised this work for a long time, and Ki Kang Kim from Sungkyunkwan University in Korea wrote a review article affirming this work.
1. Wang, C. et al. Bethe-Slater-curve-like behavior and interlayer spin-exchange coupling mechanisms in two-dimensional magnetic bilayers. Phys. Rev. B 102, 020402 (2020).
2. Li, B. et al. Van der Waals epitaxial growth of air-stable CrSe2 nanosheets with thickness-tunable magnetic order. Nature Materials 20, 818-825 (2021).