Switching 2D Magnetic States via Pressure Tuning of Layer Stacking

Abstract: The physical properties of two-dimensional van der Waals (2D vdW) crystals depend sensitively on the interlayer coupling, which is intimately connected to the stacking arrangement and the interlayer spacing. For example, simply changing the twist angle between graphene layers can induce a variety of correlated electronic phases, which can be controlled further in a continuous manner by applying hydrostatic pressure to decrease the interlayer spacing. In the recently discovered 2D magnets, theory suggests that the interlayer exchange coupling strongly depends on layer separation, while the stacking arrangement can even change the sign of the magnetic exchange, thus drastically modifying the ground state. Here, we demonstrate pressure tuning of magnetic order in the 2D magnet CrI3. We probe the magnetic states using tunneling and scanning magnetic circular dichroism microscopy measurements. We find that the interlayer magnetic coupling can be more than doubled by hydrostatic pressure. In bilayer CrI3, pressure induces a transition from layered antiferromagnetic to ferromagnetic phases. In trilayer CrI3, pressure can create coexisting domains of three phases, one ferromagnetic and two distinct antiferromagnetic. The observed changes in magnetic order can be explained by changes in the stacking arrangement. Such coupling between stacking order and magnetism provides ample opportunities for designer magnetic phases and functionalities.

• The paper demonstrates that hydrostatic pressure enhances interlayer coupling in 2D CrI₃, switching its state from antiferromagnetic to ferromagnetic.
• Using a magnetic tunnel junction and RMCD microscopy, the study quantifies pressure-induced structural shifts supported by Raman spectroscopy.
• The research reveals complex coexistence of magnetic phases in trilayer CrI₃ under pressure, highlighting potential for tunable layered magnetic orders.

Pressure-Tuned Magnetic State Switching in 2D CrI₃

The paper "Switching 2D Magnetic States via Pressure Tuning of Layer Stacking" presents a comprehensive study on the manipulation of magnetic states in two-dimensional (2D) van der Waals (vdW) magnets using hydrostatic pressure as a tuning mechanism. The central focus is on CrI₃, a 2D material known for its intriguing magnetic properties, including transitions between ferromagnetic and antiferromagnetic states.

The investigation hinges on the sensitivity of 2D vdW materials to interlayer coupling, which is strongly influenced by stacking arrangements and interlayer separation. Historically, the manipulation of electronic phases through the modulation of interlayer spacing has been demonstrated in materials such as graphene. However, in the specific context of CrI₃, a mechanical adjustment of stacking configurations is shown to significantly impact the magnetic order—a property that the authors explore through pressure experiments on bilayer and trilayer CrI₃.

The experimental setup employs a magnetic tunnel junction (MTJ) configuration, with CrI₃ sandwiched between graphene contacts and encapsulated in hexagonal boron nitride (hBN) to mitigate sample degradation. By applying hydrostatic pressure up to 2.7 GPa, the researchers probe the magnetic states using tunneling current measurements, supplemented by reflective magnetic circular dichroism (RMCD) microscopy upon decompression.

Key results indicate that in bilayer CrI₃, hydrostatic pressure can more than double the interlayer magnetic coupling, effectively transforming the state from antiferromagnetic at low pressure to ferromagnetic at high pressure. The measured critical field for the spin-flip transition reveals a significant escalation under pressure, indicative of heightened exchange interactions owing to reduced interlayer spacing. This transition is substantiated by RMCD measurements showcasing hysteresis loops akin to ferromagnetic order, aligning with Raman spectroscopy results that signal a structural shift, likely from monoclinic to rhombohedral stacking.

In trilayer CrI₃, the scenario becomes more complex, with pressure facilitating the emergence of coexisting domains of three magnetic phases. The findings suggest that structural changes at one of the two interfaces contribute to varying interfacial interactions across these domains. The spin-flip transition fields demonstrate marked variability, identifying distinct magnetic phases that are pressure-dependent. Intriguingly, pressure reveals new transient states that could be attributed to stacking configuration changes, offering a potential for manipulating ferromagnetic and antiferromagnetic orders via structural modulation.

The paper concludes that the ability to switch magnetic states through pressure modulation signals a promising avenue for designing novel magnetic phases and functional materials. Furthermore, the CrI₃ system acts as a prototypical model for exploring reconfigurable magnetic structures, with implications for the development of magnetic textures and control over electronic phenomena in layered materials. Future work might target atomically resolved imaging to unearth stacking configurations and refine the understanding of phase transitions and domain boundaries, which would accelerate advances in pressure-induced magnetic tuning and van der Waals heterostructure engineering.