December 3, 2019 feature
The direct observation of van der Waals stacking-dependent interlayer magnetism
Materials scientists aim to control the crystal structure of a solid—in a powerful approach to manipulate their fundamental properties. Researchers can achieve this control in van der Waals (vDW) materials by modifying the stacking order through rotation and translation between the vDW layers. In a recent study published in Science, Weijong Chen and a research team in the interdisciplinary departments of physics, advanced materials, nanoelectronics devices and quantum computing, and materials science and engineering in China and the U.S. observed stacking-dependent interlayer magnetism in the two-dimensional magnetic semiconductor chromium tribromide (CrBr3).
They achieved this through the successful growth of a monolayer and bilayer of the material using molecular beam epitaxy (MBE). The researchers used in situ spin-polar scanning tunneling microscopy and spectroscopy to directly correlate the atomic lattice structure with the observed magnetic order. They observed the individual monolayer of CrBr3 to be ferromagnetic but the interlayer coupling in the bilayer depended on the stacking order to either be ferromagnetic or antiferromagnetic. Observations made in the work will pave the way to manipulate 2-D magnetism with layer twist angle control.
Understanding the type of van der Waals (vdW) stacking is critical to determine the properties of layered vdW materials. Weak interlayer vdW interactions can allow scientists to control rotational and translational degrees of freedom between layers to create a host of new materials with distinct stacking symmetries and functionalities. While previous work focused on the electronic and optical properties of vdW stacking, scientists have made recent discoveries of magnetism in two-dimensional materials using mechanical exfoliation and molecular epitaxy techniques. Among the newly discovered 2-D magnetic materials, the family of chromium trihalide CrX3 (where X can be chlorine, bromine or iodine) have received great attention. Such magnetic structures can lead to a number of emerging phenomena including giant tunneling magnetoresistance, electrical control of 2-D magnetism and giant non-reciprocal optical second-harmonic generation.
In contrast to chromium triiodide (CrI3), researchers found the interlayer coupling in atomically thin chromium tribromide (CrBr3)to be ferromagnetic. In the present work, Chen et al. therefore used in situ spin-polarized scanning tunneling microscopy and spectroscopy to establish a direct correlation between the interlayer magnetic coupling and stacking structures in CrBr3. The team initially grew CrBr3 films on freshly cleaved, highly oriented pyrolytic graphite (HOPG) substrates using molecular beam epitaxy (MBE). They monitored the sample surface during growth in situ with reflection high-energy electron diffraction (RHEED). The stripe-like RHEED patterns confirmed a 2-D crystalline monolayer thin film of CrBr3, which Chen et al. verified using scanning tunneling microscopy.
On further deposition, the materials scientists enabled bilayer CrBr3 islands to form as periodically spaced triangular clusters. The crystal structure of the CrBr3 molecule contained Cr atoms arranged in a honeycomb lattice, surrounded by an octahedron of six Br atoms. They determined the thickness of the monolayer to be 6.5 Angstrom (Å) using atomic force microscopy (AFM). Both large-scale topography (surface geometry) and atomically resolved STM images showed high-quality growth of the CrBr3 monolayer films. The team measured magnetic properties of the thin film using spin-polarized STM measurements and additionally confirmed the existence of ferromagnetism. For this, Chen et al. measured a series of tunneling spectra (dI/dV) by sweeping the magnetic field back and forth. The observation suggested that epitaxial CrBr3 monolayers grown on HOPG (highly oriented pyrolytic graphite) maintained semiconductor ferromagnetic properties. After confirming the atomic structure and ferromagnetism of monolayer CrBr3, Chen et al. focused on the CrBr3 bilayer.
In the MBE-grown bilayers, the scientists observed H- and R-type stacking structures, where the R-type maintained both layers aligned in the same orientation while the H-type allowed 180-degree rotation between the bilayers. The structural alignments gave rise to distinct interlayer magnetic coupling. For instance, in H-type stacked bilayer CrBr3, the interlayer coupling was ferromagnetic. Whereas the R-type stacked bilayer showed antiferromagnetically coupled behavior in its ground states giving rise to two additional configurations of magnetization. On further examination of interlayer coupling, the scientists observed the two-plateau behavior to demonstrate magnetic field-driven transition from antiferromagnetic to ferromagnetic character.
In this way, the scientists demonstrated distinct interlayer magnetism of the MBE-grown bilayer CrBr3 from antiferromagnetic coupling in R-type stacking to ferromagnetic coupling in H-type stacking to indicate the broad tunability of magnetism across the stacking orders of 2-D materials. Chen et al. credited the interlayer coupling in bilayer CrBr3 to superexchange interaction controlled by directional hybridization between p-orbitals of bromine (Br) and d-orbitals of chromium (Cr). Since the bond angles and bond distance of the Cr-Br-Br-Cr exchange path strongly depended on the stacking order, they expect the interlayer magnetism to depend on the interlayer distance and atomic site position relative to the specific stacking structure.
While the exact growth mechanisms remain to be investigated, Chen et al. illustrated the importance of polytypism (polymorphism or variety) in vDW materials and its role in 2-D magnetism. The new work calls to closely examine stacking structures in mechanically exfoliated CrX3 samples to understand the distinctly observed properties of interlayer magnetic coupling. The researchers expect the work principle to manipulate 2-D magnetism by engineering unique spatially dependent spin textures for a variety of applications with vDW materials.
Yuwei Shan et al. Stacking symmetry governed second harmonic generation in graphene trilayers, Science Advances (2018). DOI: 10.1126/sciadv.aat0074
Tao Jiang et al. Valley and band structure engineering of folded MoS2 bilayers, Nature Nanotechnology (2014). DOI: 10.1038/nnano.2014.176
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