New Progress in the Field of Magnetron Electronic Structures
The application of an external magnetic field can control the electrical polarization, light polarization, temperature, geometry and other macroscopic physical properties of magnetic materials, that is, to achieve magnetoelectric, magneto-optical, magneto-caloric, and magneto-elastic effects. These effects constitute the physical basis of magnetic functional devices such as magnetic detectors, magneto-optical Kerr instruments, and magnetic refrigerators. Considering the close relationship between the macroscopic physical properties of the material and the microscopic electronic structure, the most intuitive idea is to directly control the electronic energy band structure through the magnetic field, thereby changing the electrical and optical properties of the material. Under the action of an external magnetic field, the originally degenerate electron spin state will produce Zeeman level splitting. However, this is a tiny energy level. For example, a huge magnetic field of 1 Tesla (about 30,000 times that of the earth’s magnetic field) can only produce tiny energy level splitting, which is much smaller than the thermal fluctuation at room temperature, so it cannot be used for Device design and application.
Recently, a team of researcher Zhong Zhicheng from Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences and Dr. Liao Zhaoliang from the University of Twente in the Netherlands have proposed a new type of magnetron electronic structure effect: the magnetization direction is regulated by an external magnetic field, and the spin-orbit coupling is used to achieve A dramatic change in the electronic band structure. Theoretically predicts that the energy regulation of the electronic energy band structure by the external magnetic field in this effect can be as high as 3 orders of magnitude larger than the classical Zeeman effect and higher than the thermal fluctuation at room temperature, which can be used to design new types of magnetoelectric and magneto-optical devices.
The researchers first used model analysis and found that in materials with low symmetry, strong spin-orbit coupling, and long-range ferromagnetic order, by applying an external magnetic field to change the magnetization direction, with the help of spin-orbit coupling, a huge change in the energy band structure can be achieved. (As shown in Figure 1). But the difficulty lies in the lack of material systems that meet the above conditions, until recently, two-dimensional ferromagnetic materials that can perfectly meet the above conditions have been discovered. Taking the two-dimensional ferromagnetic material CrI3 as an example, the researchers used first-principles calculations to predict that the material has a huge magnetron electronic structure effect (as shown in Figure 2). When the magnetization direction is tuned from out-of-plane to in-plane, the electronic band structure changes from a direct band gap to an indirect band gap, and the Fermi surface also changes (as shown in Figure 3). In addition, changes in magnetization direction can also drive topological phase transitions. These significant band changes can alter optical and electrical transport properties. For example, a magnetic field can be used to control the direction of the magnetization to control the fluorescence effect. In addition, the change of the Fermi surface induces a huge anisotropic magnetoresistance, and the topological phase transition changes the topological properties of the surface state of the material (as shown in Fig. 4). The changes in functional properties predicted by these theories can be confirmed by further experiments in the future.
In summary, this work proposes a new magnetron electronic structure effect, that is, by applying an external magnetic field to change the magnetization direction, a huge change in the energy band structure can be achieved, and a series of related electronic properties can be regulated. Using this effect, new spintronic devices, magnetoelectric and magneto-optical devices can be fabricated. In addition, the effect needs to satisfy three conditions: low symmetry, strong spin-orbit coupling, and long-range ferromagnetic order. Searches based on the above conditions are expected to discover more material systems with magnetron electronic structure effects.
Magnetron Electronic Structures
The above work was published in Nano Letters (Nano Lett. 2018, 18, 3844-3849) on May 22, 2018 with the title of “Spin Direction-Controlled Electronic Band Structure in Two-Dimensional Ferromagnetic CrI3” (paper link: https: https://pubs.acs.org/doi/abs/10.1021/acs.nanolett.8b01125). Dr. Jiang Peiheng and Dr. Li Lei from Ningbo Institute of Materials are the co-authors of the paper. Researcher Zhong Zhicheng from Ningbo Institute of Materials and Dr. Liao Zhaoliang from the University of Twente in the Netherlands are the co-corresponding authors. Professor Zhao Yuxin from Nanjing University participated in the discussion of topology. This work was supported by the National Key R&D Program (2017YFA0303602), the National Natural Science Foundation of China (11774360), and the Ningbo 3315 Innovation Team. All numerical calculations were performed in the Supercomputing Center of Ningbo Institute of Materials.
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