The generation of pseudo-magnetic fields in strained graphene leads to quantized Landau levels in the absence of an external magnetic field, providing the potential to achieve a zero-magnetic-field analogue of quantum Hall effect. Here, we report the realization of pseudo-magnetic field in epitaxial graphene by building monolayer CrCl2/graphene heterointerface. The CrCl2 crystal structure exhibits spontaneous breaking of three-fold rotational symmetry, yielding anisotropic displacement field at the interface. Using scanning tunneling spectroscopy, we have discovered a sequence of pseudo-Landau levels associated with massless Dirac fermions. A control experiment performed on CrCl2/NbSe2 interface confirms the origin as the pseudo-magnetic field in the graphene layer that strongly interacts with the CrCl2. More interestingly, the strength of the pseudo-magnetic fields can be tuned by the twist angle between the monolayer CrCl2 and graphene, with a variation of up to threefold, depending on the twist angle of 0° to 30°. This work presents a rare 2D heterojunction for exploring PMF-related physics, such as valley Hall effect, with the advantage of easy and flexible implementation.
Yangliu Wu, Zhaozhuo Zeng, Haipeng Lu, Xiaocang Han, Chendi Yang, Nanshu Liu, Xiaoxu Zhao, Liang Qiao, Wei Ji*, Renchao Che, Longjiang Deng*, Peng Yan* and Bo Peng*
Abstract:
Multiferroic materials have been intensively pursued to achieve the mutual control of electric and magnetic properties. The breakthrough progress in 2D magnets and ferroelectrics encourages the exploration of low-dimensional multiferroics, which holds the promise of understanding inscrutable magnetoelectric coupling and inventing advanced spintronic devices. However, confirming ferroelectricity with optical techniques is challenging in 2D materials, particularly in conjunction with antiferromagnetic orders in single- and few-layer multiferroics. Here, we report the discovery of 2D vdW multiferroic with out-of plane ferroelectric polarization in trilayer NiI2 device, as revealed by scanning reflective magnetic circular dichroism microscopy and ferroelectric hysteresis loops. The evolution between ferroelectric and antiferroelectric phases has been unambiguously observed. Moreover, the magnetoelectric interaction is directly probed by magnetic control of the multiferroic domain switching. This work opens up opportunities for exploring new multiferroic orders and multiferroic physics at the limit of single or few atomic layers, and for creating advanced magnetoelectronic devices.
Nanshu Liu, Cong Wang, Changlin Yan, Changsong Xu, Jun Hu, Yanning Zhang, and Wei Ji
Abstract:
Interlayer magnetism was tuned by many interlayer means, e.g., stacking, distance, and external fields in two-dimensional (2D) magnets. As an exception, the interlayer magnetism of CrSBr few layers was, however, experimentally changed by applied intralayer strains [Nat. Nanotechnol. 17, 256 (2022)], the mechanism of which is yet to be unveiled. Here, we uncovered its mechanism by investigating in-plane strained bilayer CrSBr using density functional theory calculations. Under in-plane tensile strain, wavefunction overlaps are strengthened for Br p electrons within each CrSBr layer, which delocalizes intralayer electrons and, as a consequence, promotes interlayer electron hopping. A negative interlayer Poisson’s ratio also enlarges interlayer spacing for bilayer CrSBr, which reduces the interlayer Pauli repulsion. This joint effect, further verified by examining interlayer sliding and interfacial element substitution, leads to an interlayer antiferromagnetic to ferromagnetic transition, consistent with the previous experimental observation. This mechanism enables a route to tune interlayer magnetism by modifying intralayer electron localization in 2D magnets.
Nanshu Liu, Cong Wang, Changlin Yan, Changsong Xu, Jun Hu, Yanning Zhang, and Wei Ji
Abstract:
A recent experiment reported type-II multiferroicity in monolayer (ML) NiI2 based on a presumed spiral magnetic configuration (Spiral-B), which is, as we found here, under debate in the ML limit. Freestanding ML NiI2 breaks its C3 symmetry, as it prefers a striped antiferromagnetic order (AABB-AFM) along with an intralayer antiferroelectric (AFE) order. However, substrate confinement may preserve the C3 symmetry and/or apply tensile strain to the ML. This leads to another spiral magnetic order (SpiralIVX), while 2L shows a different order (SpiralVX) and Spiral-B dominates in thicker layers. Thus, three multiferroic phases, namely, SpiralB+FE, Spiral-IVX +FE, Spiral-VX+FE, and an anti-multiferroic AABB-AFM+AFE one, show layer-thickness-dependent and geometry-dependent dominance, ascribed to competitions among thickness-dependent Kitaev, biquadratic, and Heisenberg spin–exchange interactions and single-ion magnetic anisotropy. Our theoretical results clarify the debate on the multiferroicity of ML NiI2 and shed light on the role of layer-stacking-induced changes in noncollinear spin–exchange interactions and magnetic anisotropy in thickness-dependent magnetism.
