Electric-field switching of two-dimensional van der Waals magnets

Abstract: Controlling magnetism by purely electrical means is a key challenge to better information technology1. A variety of material systems, including ferromagnetic (FM) metals2,3,4, FM semiconductors5, multiferroics6,7,8 and magnetoelectric (ME) materials9,10, have been explored for the electric-field control of magnetism. The recent discovery of two-dimensional (2D) van der Waals magnets11,12 has opened a new door for the electrical control of magnetism at the nanometre scale through a van der Waals heterostructure device platform13. Here we demonstrate the control of magnetism in bilayer CrI3, an antiferromagnetic (AFM) semiconductor in its ground state12, by the application of small gate voltages in field-effect devices and the detection of magnetization using magnetic circular dichroism (MCD) microscopy. The applied electric field creates an interlayer potential difference, which results in a large linear ME effect, whose sign depends on the interlayer AFM order. We also achieve a complete and reversible electrical switching between the interlayer AFM and FM states in the vicinity of the interlayer spin-flip transition. The effect originates from the electric-field dependence of the interlayer exchange bias.

• The paper shows a robust magnetoelectric effect in bilayer CrI₃ with a linear ME coefficient of around ±100 ps/m enabling effective electric control of magnetic order.
• The research demonstrates reversible switching between antiferromagnetic and ferromagnetic phases using moderate gate voltages and a critical magnetic field.
• The study employs dual-gate devices with hBN dielectrics and graphene electrodes, ensuring precise electric field modulation without unwanted doping effects.

Overview of Electrical Switching of Two-Dimensional Van der Waals Magnets

The paper presents a thorough exploration into the electrical control of magnetism within two-dimensional (2D) van der Waals (vdW) magnets, specifically focusing on bilayer chromium iodide (CrI₃). The study is situated within a burgeoning field which seeks to marry the semiconducting properties of 2D materials with magnetic functionality, driven by the advances in layered vdW heterostructures. The researchers harness the unique magnetoelectric (ME) properties of bilayer CrI₃ to demonstrate the feasibility of electrically driven magnetic order switching, paving the way for its potential implementation in next-generation spintronic and nonvolatile memory devices.

Key Findings

1. Magnetoelectric Effect in CrI₃: The paper highlights the large linear ME effect observed in antiferromagnetic (AFM) bilayer CrI₃. This effect is induced through a spin-dependent interlayer charge transfer catalyzed by an applied electric field. Remarkably, the ME coefficient approaches values comparable to the best-known single-phase materials, with experimental results indicating a volumetric ME coefficient of approximately ±100 ps/m.
2. Magnetic Phase Control: The bilayer CrI₃ serves as a showcase for the reversible electrical switching between distinct magnetic phases. The study reports a transition from an AFM to a ferromagnetic (FM) phase, facilitated by moderate gate voltages in the presence of a critical applied magnetic field. This transition is notably influenced by the electric-field-dependent change in interlayer exchange bias, where the critical spin-flip field increases linearly with the applied electric field up to 0.5 V/nm.
3. Hysteresis and Phase Transition: The research outlines the hysteresis observed during the magnetic phase transitions, characteristic of a first-order nature. Electrical fields as low as 0.2 V/nm initiate the switch between FM and AFM phases, elucidating both electric and magnetic order dependencies.
4. Fabrication and Methodology: The devices employed in this study are dual-gate CrI₃ field-effect structures, where hBN gate dielectrics encapsulate the CrI₃ layers. Graphene layers serve as gate and contact electrodes, enabling the modulation of the electric fields and eliminating doping influences on observed phenomena.

Implications and Future Directions

The implications of this work extend to various facets of spintronic and memory device engineering. The ability to control magnetic states through electrical means enables the design of low-power, nonvolatile magnetic storage devices. Furthermore, the integration of such vdW heterostructures into existing electronic architectures could catalyze advancements in spintronic devices, including spin valves and transistors.

From a theoretical standpoint, the findings underscore the potential of vdW materials in exploring magnetic interactions and ME phenomena at reduced dimensionalities. The delineation between FM and AFM phases via electric fields introduces avenues for comprehensive studies into symmetry breaking and quantum phase interactions inherent to layered materials.

Future explorations could benefit from a deeper investigation into the microscopic mechanisms underlying the ME effects observed within bilayer CrI₃. Potential avenues include leveraging other 2D ferromagnetic or antiferromagnetic materials to glean insights into the scalability and material-specific characteristics of electrically controllable magnetism. Furthermore, exploring temperature dependency and ambient stability of these ME effects may provide additional leverage in understanding and harnessing these phenomena for real-world applications.