Anisotropic Dzyaloshinskii-Moriya Interactions with Symmetry Preservation Discovered in 2D Magnets

Magnetic skyrmions are non-collinear magnetic vortex quasiparticles with topologically protected real space. They have many advantages such as nanometer size, stable structure, easy control, and small driving threshold current. They are expected to become the next generation of high capacity, high speed read and write, Low power consumption, non-volatile information storage and information carrier for logic operations. The formation, stability and motion of magnetic skyrmions are closely related to a magnetic interaction (antisymmetric exchange coupling, also known as Dzyaloshinskii-Moriya interaction (DMI)), which, as a fundamental magnetic interaction, has profound intrinsic Physical properties have received extensive attention in the fields of basic and applied scientific research in the past 20 years. At the same time, the two-dimensional van der Waals magnets discovered in recent years have also attracted widespread attention from scientists due to their unique physical properties. The quantum functional materials team led by researcher Yang Hongxin from the Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences proved through theoretical research and in cooperation with the experimental team that magnetic skyrmions, bisemitrons and chiral domain walls can all interact in isotropic DM. 2D eigenmagnets with Cnv point groups (such as Janus magnets and monolayer multiferroics) and van der Waals heterostructures based on 2D magnets [Phys. Rev. B 101, 184401(2020), Editors’ Suggestion; Phys. Rev. B 101, 184401 (2020); Phys. Rev. B 102, 220409(R) (2020); Physical Review B 103 (10), 104410 (2021), Editors’ Suggestion; Phys. Rev. Res. 3, 043011 (2021)]. However, anisotropic DM interactions and antiferromagnetic topological magnetic structures in 2D magnets have not yet been reported.

Recently, researcher Yang Hongxin led a team to make important progress in the study of anisotropic DM interaction and topological magnetic structure in two-dimensional magnets. The related work is titled “Anisotropic Dzyaloshinskii–Moriya Interaction and Topological Magnetism in Two-Dimensional Magnets Protected by Crystal Symmetry”. Title published in Nano Letters 22, 2334 (2022).

The team found that the two-dimensional magnetic material AX2 with space group (A: 3d transition metal element; X: the fourth and fifth main group elements) naturally has symmetry-protected anisotropic DM interactions. AX2 has center inversion symmetry breaking and S4z symmetry, which makes the DM interaction between the nearest-neighbor magnetic elements in the x and y directions must have the same size and opposite chirality. The team screened 23 candidate magnetic materials from the two-dimensional material database that meet the symmetry requirements, and used the DM interaction algorithm independently developed by the team (supercellular short spin wave DMI algorithm and spin helical state generalized Bloch). Theorem DMI algorithm), it is proved that there are anisotropic DM interactions protected by symmetry in these 23 candidate materials, as shown in Figure 1 and Figure 2. The team also found that as the A-type element changes from V to Ni, the material undergoes a transition from a ferromagnetic ground state to an antiferromagnetic ground state. Further analysis found that the exchange coupling of the material system is determined by the energy level splitting caused by the tetrahedral crystal field and the 3d electron arrangement. This result also implies that the AX2 system provides a platform for the study of antiferromagnetic topological magnetic structures. Further, based on the magnetic interaction calculated by first-principles calculations, the team used the atomic-scale spin model to perform micromagnetic simulations to reveal the emergence of various novel topological chiral magnetic structures in the AX2 material family, including diamagnetic sigma Mingons, antiferromagnetic antiskyrmions, and antiferromagnetic vortex-antivortex pairs, the specific results are shown in Figure 3. This series of topological magnetic structures can all appear without the assistance of an external field. In particular, isolated antiferromagnetic antiskyrmions with a diameter of only 6.8 nm appeared in the monolayer MnBr2. This small-scale isolated topological magnetic structure is promising for next-generation memory-computing spintronics devices with low power consumption and high density.

In summary, the Ningbo Institute of Materials team proposed and demonstrated the existence of anisotropic DM interactions protected by crystal symmetry in a symmetric monolayer AX2, and revealed many novel ferromagnetic/antiferrous interactions in this material system. Magnetic topology magnetic structure. This class of materials can make spintronic devices based on topological magnetic structures have simpler structures and higher operating efficiency, and provide a new idea and method for the construction of DM interactions and topological chiral magnetic structures protected by crystal symmetry. and material reserves.

This work was completed by Dr. Cui Qirui and others under the joint guidance of Researcher Cui Ping and Researcher Yang Hongxin. The research work has been supported by the “From 0 to 1” original innovation project of the Basic Frontier Scientific Research Program of the Chinese Academy of Sciences (ZDBS-LY-7021), the General Project of the National Natural Science Foundation of China (11874059, 12174405), the Natural Science Foundation of Zhejiang Province (LR19A040002), Zhejiang Province Supported by the Leading Soldier Program (2022C01053), the Ningbo Major Science and Technology Project (2021000215), and the Open Project of the Beijing National Laboratory for Condensed Matter Physics (2021000123).

Fig.1 DM interaction in AX2 system calculated based on supercellular short-spin wave DMI algorithm

Fig.2 DM interaction in AX2 system calculated by DMI algorithm based on generalized Bloch theorem of spin helical states

Fig. 3 Topological magnetic structure in real space simulated based on magnetic interactions calculated from first-principles calculations

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