Direct observation of van der Waals stacking dependent interlayer magnetism (1906.03383v1)

Abstract: Controlling the crystal structure is a powerful approach for manipulating the fundamental properties of solids. Unique to two-dimensional (2D) van der Waals materials, the control can be achieved by modifying the stacking order through rotation and translation between the layers. Here, we report the first observation of stacking dependent interlayer magnetism in the 2D magnetic semiconductor, chromium tribromide (CrBr3), enabled by the successful growth of its monolayer and bilayer through molecular beam epitaxy. Using in situ spin-polarized scanning tunneling microscopy and spectroscopy, we directly correlated the atomic lattice structure with observed magnetic order. We demonstrated that while individual CrBr3 monolayer is ferromagnetic, the interlayer coupling in bilayer depends strongly on the stacking order and can be either ferromagnetic or antiferromagnetic. Our observations provide direct experimental evidence for exploring the stacking dependent layered magnetism, and pave the way for manipulating 2D magnetism with unique layer twist angle control.

• The paper demonstrates that stacking order in 2D CrBr3 bilayers governs the switch between ferromagnetic and antiferromagnetic interlayer coupling.
• It uses molecular beam epitaxy and spin-polarized STM to reveal distinct coercive fields (~30 mT for monolayers and ~45 mT for H-type stacking in bilayers).
• The findings highlight the potential for engineering tunable magnetic properties in 2D materials for future spintronic applications.

Direct Observation of Stacking Dependent Interlayer Magnetism in 2D CrBr$_3$

The paper, "Direct observation of van der Waals stacking dependent interlayer magnetism" explores the intricate relationship between stacking order and magnetic properties in two-dimensional (2D) van der Waals materials, specifically focusing on chromium tribromide (CrBr$_3$) bilayers. Utilizing molecular beam epitaxy (MBE), the authors successfully cultivated CrBr$_3$ monolayers and bilayers, enabling them to directly probe the microscopic relationship between crystal structure and magnetism.

Methodology and Findings

1. Growth and Structural Analysis: CrBr$_3$ monolayers and bilayer islands were synthesized on highly oriented pyrolytic graphite (HOPG) using MBE. The quality and structure were verified via reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM), confirming the expected honeycomb lattice with high fidelity.
2. Magnetic Properties: Spin-polarized scanning tunneling microscopy (SP-STM) was employed to understand the magnetic properties. Measurements conducted under a magnetic field demonstrated that CrBr$_3$ monolayers possess ferromagnetic behavior with an out-of-plane easy axis. The coercive field was determined to be ~30 mT, indicative of robust ferromagnetism.
3. Stacking Configurations and Magnetic Coupling: Distinct stacking structures were observed within the bilayer systems:

• H-type Stacking: Characterized by a 180° rotation between layers, exhibited ferromagnetic interlayer coupling. A rectangular hysteresis loop with a coercive field of ~45 mT was observed, similar to the monolayer behavior.
• R-type Stacking: Demonstrated identical orientation of layers. The observation of a magnetoresistance plateau for magnetic fields > ±0.5 T and a dI/dV contrast implies antiferromagnetic interlayer coupling, allowing for field-dependent switching from antiferro- to ferromagnetic states.

Implications and Future Directions

These experimental results underscore the influence of stacking order on interlayer magnetism, facilitated by superexchange interactions contingent on the Cr-Br-Br-Cr bonding angle and distance. The discovery broadens the understanding of 2D magnets, highlighting the potential of stacking order manipulation to predictably engineer magnetic properties in van der Waals materials.

The research provides a foundational step towards the application of 2D materials in spintronic devices, where layer-dependent property tuning is critical. Future work could explore twisted bilayer heterostructures and their potential in customizing magnetic textures at the nanoscale. Moreover, examining similar phenomena in other van der Waals materials could unravel additional magnetic configurations, further enriching the landscape of 2D magnetic systems.