Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals

By Cheng Gong, Lin Li, Zhenglu Li, Huiwen Ji, Alex Stern, Yang Xia, Ting Cao, Wei Bao, Chenzhe Wang, Yuan Wang and 5 more. Nature, 2017

4,290 citations recorded by the source corpus.

The realization of long-range ferromagnetic order in two-dimensional van der Waals crystals, combined with their rich electronic and optical properties, could lead to new magnetic, magnetoelectric and magneto-optic applications. In two-dimensional systems, the long-range magnetic order is strongly suppressed by thermal fluctuations, according to the Mermin–Wagner theorem; however, these thermal fluctuations can be counteracted by magnetic anisotropy. Previous efforts, based on defect and composition engineering, or the proximity effect, introduced magnetic responses only locally or extrinsically. Here we report intrinsic long-range ferromagnetic order in pristine Cr2Ge2Te6 atomic layers, as revealed by scanning magneto-optic Kerr microscopy. In this magnetically soft, two-dimensional van der Waals ferromagnet, we achieve unprecedented control of the transition temperature (between ferromagnetic and paramagnetic states) using very small fields (smaller than 0.3 tesla). This result is in contrast to the insensitivity of the transition temperature to magnetic fields in the three-dimensional regime. We found that the small applied field leads to an effective anisotropy that is much greater than the near-zero magnetocrystalline anisotropy, opening up a large spin-wave excitation gap. We explain the observed phenomenon using renormalized spin-wave theory and conclude that the unusual field dependence of the transition temperature is a hallmark of soft, two-dimensional ferromagnetic van der Waals crystals. Cr2Ge2Te6 is a nearly ideal two-dimensional Heisenberg ferromagnet and so will be useful for studying fundamental spin behaviours, opening the door to exploring new applications such as ultra-compact spintronics.

Subfields: Atomic and Molecular Physics, and Optics, Materials Chemistry

View Paper: https://www.semanticscholar.org/paper/e81c4f9554736915a68984f3c7ed65368e3f6314

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Citations in the corpus

Magnetism, symmetry and spin transport in van der Waals layered systems (2021, neutral): “Over the past five years, magnetic orders have been discovered in several vdW material systems 1,14–18 .”
The Computational 2D Materials Database: high-throughput modeling and discovery of atomically thin crystals (2018, background): “This comprises the first example of a pure 2D material exhibiting magnetic order and there is currently an intense search for new 2D materials with magnetic order persisting above room temperature [150 – 152].”
Probing and controlling magnetic states in 2D layered magnetic materials (2019, neutral): “15 ) , the samples were exposed to air so that there could have been some degradation of very thin samples.”
Room temperature ferromagnetism in ultra-thin van der Waals crystals of 1T-CrTe2 (2020, background): “It is known that magnetic interaction of few-layered vdW magnets may be vary from their bulk forms, depending on different systems [2, 18].”
Probing magnetism in 2D van der Waals crystalline insulators via electron tunneling (2018, neutral): “The Hamiltonian can be made quadratic with the mean field ansatz: yk ylyk yl ≈ 2〈yk yl〉yk yl + 〈yk yk〉yl yl + 〈yl yl〉yk yk (10) where we calculate the occupation numbers of the normal modes bk at low temperature as: 〈yà yà〉 = 1 &â(äuãà) − 1 ≈ &~âçäuãà åé (11)”