image: Exfoliated few-layer CrI3. (a) Atomic structures of monolayer CrI3. (b, c) Rhombohedral (b) and monoclinic (c) stacking order in bilayer CrI3. The rhombohedral structure has an out-of-plane C3 axis and a symmetric center (s6=c3+i ), while the monoclinic structure has an in-plane C2 axis and a mirror plane. (d) Optical micrograph of the exfoliated few-layer CrI3. (e) Optical contrast of the CrI3 samples with different numbers of layers (red circles). The blue solid line is calculated results based on Fresnel's equations from Xu et al. (Nature, 2017, 546, 270). (f) Magnetization of bulk CrI3 as a function of temperature, indicating that Tc = ~65 K. Layer-dependent magnetic ordering in atomically-thin CrI3 at 10 K. RMCD signal on a 2L (g), 3L (h) and 4L (i) CrI3 flake, showing antiferromagnetic behavior in bilayer CrI3 and ferromagnetic behavior in 3L and 4L CrI3. The blue arrows indicate the magnetization orientation in different layer. view more
Credit: ©Science China Press
Since the discovery of two ferromagnetic (FM) atomically thin CrI3 and Cr2Ge2Te6 in 2017 (Nature 2017, 546, 270?Nature 2017, 546, 265), intrinsic ferromagnetism in two-dimensional (2D) van der Waals (vdWs) materials, maintaining long-range magnetic orders at the atomic monolayer limit, has received growing attention. Each individual layer is FM; however adjacent layers are antiferromagnetically (AFM) coupled together. The physical property of 2D ferromagnetism CrI3 are significantly influenced by interlayer spacing and stacking order; the interlayer magnetic states are switched between FM and AFM through electric gating or electrostatic doping and pressure.
However, there remains debate on the stacking order of CrI3 at low temperature. Previous studies have reported that CrI3 is rhombohedral structure at low temperature (Nat. Mater. 2019, 18, 1303; Phys. Rev. B 2018, 98, 104307), but recent experiments and theory demonstrate that the BN-encapsulated bi- and few-layer CrI3 and CrCl3 belong to monoclinic structure (Nature 2019, 572, 497; Nat Phys, 2019, 15, 1255). Therefore, a complete understanding of lattice dynamics and stacking order of CrI3 is crucial for 2D vdW ferromagnetic materials; however, to data, this is rare.
Recently, Prof. Bo Peng from the University of Electronic Science and Technology of China and his cooperators published a paper entitled "Layer dependence of stacking order in non-encapsulated few-layer CrI3" in Science China Materials, and demonstrated the layer, polarization and temperature dependence of the Raman features of non-encapsulated 2-5 layer and bulk CrI3 (Fig. 1), illustrating that the non-encapsulated few-layer and bulk CrI3 are rhombohedral stacking order at low temperature, rather than monoclinic structure. The helicity of incident light can be maintained by Ag modes at 10 K, while it is reversed by Eg modes, which is independent of the magnetic field and only originates from the phonon symmetry. Strikingly, the spin-phonon coupling occurs below ~60 K, which modifies the Hamiltonian of Raman modes and results in a deviation behavior of the linewidth from phonon-phonon coupling modes.
This work opens up an insight into lattice stacking order and spin-phonon coupling in 2D ferromagnets, and highlights the feasibility for the manipulation of the electron spin and spin waves through spin-phonon coupling toward novel spintronic devices. This work has far-ranging impact and will stimulate the interest in several communities, e.g., layered 2D materials, spintronics, ferromagnetic 2D materials.
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See the article: http://engine.scichina.com/publisher/scp/journal/SCMs/doi/10.1007/s40843-019-1214-y?slug=fulltext
Journal
Science China Materials