Stacking tunable interlayer magnetism in bilayer CrI3

Abstract: Diverse interlayer tunability of physical properties of two-dimensional layers mostly lies in the covalent-like quasi-bonding that is significant in electronic structures but rather weak for energetics. Such characteristics result in various stacking orders that are energetically comparable but may significantly differ in terms of electronic structures, e.g. magnetism. Inspired by several recent experiments showing interlayer anti-ferromagnetically coupled CrI3 bilayers, we carried out first-principles calculations for CrI3 bilayers. We found that the anti-ferromagnetic coupling results from a new stacking order with the C2/m space group symmetry, rather than the graphene-like one with R3 as previously believed. Moreover, we demonstrated that the intra- and inter-layer couplings in CrI3 bilayer are governed by two different mechanisms, namely ferromagnetic super-exchange and direct-exchange interactions, which are largely decoupled because of their significant difference in strength at the strong- and weak-interaction limits. This allows the much weaker interlayer magnetic coupling to be more feasibly tuned by stacking orders solely. Given the fact that interlayer magnetic properties can be altered by changing crystal structure with different stacking orders, our work opens a new paradigm for tuning interlayer magnetic properties with the freedom of stacking order in two dimensional layered materials.

• The paper reveals that stacking order decisively governs CrI3’s interlayer magnetic coupling, switching between FM and AFM states via advanced DFT calculations.
• The study employs van der Waals functionals to optimize both monoclinic and rhombohedral phases, ensuring robust insights into magnetic interactions.
• The findings suggest practical strategies for engineering spintronic devices and novel 2D materials by exploiting tunable interlayer magnetism.

Stacking Tunable Interlayer Magnetism in Bilayer CrI

The paper investigates the properties of bilayer chromium iodide (CrI₃) utilizing first-principle calculations to elucidate interlayer magnetism and its dependency on stacking orders. The research addresses two distinct structural phases: one with the C2/m space group symmetry (monoclinic structure) and another with the initially presumed rhombohedral structure (R3 group). The study thoroughly examines how these phases influence interlayer magnetic couplings and the corresponding electronic properties.

Key Findings and Methodology

The researchers employed density functional theory (DFT), specifically using the generalized gradient approximation and the projector augmented wave method, to analyze the stacking-dependent electronic and magnetic properties. The optB86b functional for van der Waals interactions was crucial in optimizing the structure. Various computational parameters were tested to ensure the robustness of their findings.

• Intralayer vs. Interlayer Couplings: It was found that the intralayer ferromagnetic (FM) coupling is notably strong due to a Cr-I-Cr super-exchange mechanism. Conversely, the interlayer coupling is weaker and relies on the stacking order, allowing for substantial tunability.
• Stacking Orders and Magnetic Configuration: The bilayers with the C2/m monoclinic symmetry, identified as the high-temperature (HT) phase, prefer an interlayer antiferromagnetic (AFM) order, contrary to the low-temperature (LT) rhombohedral phase that favors an FM order. This contradicts previous assumptions that the LT phase could facilitate AFM order.
• Phase Dependency: The energy differences between FM and AFM configurations suggest robust FM coupling in the LT phase and a tunable AFM state in the HT phase. These differences are influenced mildly by changes in functionals, U-J values, or other theoretical parameters, emphasizing the robustness of the finding.
• Structural Manipulation and Empirical Validation: The AFM order's relative stability in the HT phase suggests that rapid cooling and external constraints, such as capping layers, might maintain these arrangements at low temperatures. Control experiments could validate this, e.g., slower cooling and omitted constraints altering the material’s phase.

Implications and Future Directions

This study advances the understanding of tunable magnetism within two-dimensional layered materials, specifically emphasizing the role of stacking order. The implications are manifold:

• Spintronic Devices: The findings suggest that CrI₃ bilayers can be engineered into spintronic devices with tunable magnetic states, such as magnetic tunnel junctions, which exhibit significant tunneling magnetoresistance.
• Layered Material Design: The decoupling of weak interlayer AFM coupling from strong intralayer FM coupling in HT phases underscores potential strategies for advanced 2D material designs where in-plane ferromagnetism is maintained while interlayer properties are engineered.
• Exploration of Other Compounds: The principles of manipulating magnetism through stacking could extend to other layered materials, enabling novel applications in computing and materials science.
• Experimental Verification: The theoretical findings motivate further experimental pursuits to validate these stacking-induced magnetic phenomena and explore practical manipulation techniques, such as electric gating or external field applications.

Conclusion

This study on CrI₃ provides a refined perspective on the interplay between stacking order and magnetic properties within bilayers, presenting substantial evidence for the influence of crystal structure on interlayer coupling. Through sophisticated computational analyses, it proposes new paradigms for tailoring magnetic properties in 2D materials, offering promising avenues for technological applications in spintronics and beyond.