News Resource: Advanced Research Institute of Multidisciplinary Science

Editor: News Agency of BIT

Translator: Wang Yuxuan, News Agency of BIT

In recent years, Intrinsic two-dimensional magnetic topological insulator has received attention widely, especially when many novel quantum phenomena were observed in MnBi₂Te₄material, such as Chern Insulator and Axion insulator, etc., providing new chances for future design of information electronic devices. Compared with MnBi₂Te₄material, MnSb₂Te₄material has larger preparation window. Previous studies have found that the substitution concentration of Mn (manganese) and Sb (antimony) atoms would vary with different growth conditions, changing the magnetic ground state of the material. Therefore, in the two-dimensional limit, revealing the magnetic state of ferromagnetic and antiferromagnetic MnSb₂Te₄materials in the parameter space of layer number, temperature and external magnetic field, is of great significance to further study its topological properties.

Professor Huang Yuan from BIT Advanced Research Institute of Multidisciplinary Science, cooperated with Researcher Ye Yu from School of Physics, Peking University, Professor Lei Hechang from Department of Physics, Renmin University of China and others, carried out the study of MnSb₂Te₄material under the two-dimensional limit for the first time. Using the unique cleavage method developed by Professor Huang Yuan in recent years, the joint research team thinned the bulk MnSb₂Te₄of antiferromagnetic phase and ferromagnetic phase respectively, obtained few-SL (septuple layer) samples with different layers (FIG.1), and studied the magnetic evolution of the samples by reflected magnetic circular dichroism (RMCD) spectrum.

FIG.1. (a) atomic structure of MnSb₂Te₄; (b ~ e) characterization of few layer samples and magnetic evolution of four-layer antiferromagnetic phase and ferromagnetic phase MnSb₂Te₄.

In the study of antiferromagnetic phase (FIG.2), the samples of 1 to 9 layers show typical A-type antiferromagnetic behavior, reflecting obvious odd-even layer-number oscillation effect. In odd-SL flakes, hysteresis loops appear near the zero field due to the uncompensated net magnetic moment of a single layer, and with the increase of magnetic field, all except single-SL samples undergo a spin-flop process, reached the magnetic saturation state at last. In even-SL flakes, the hysteresis loop near the zero field disappears because no uncompensated net magnetic moment exists. Samples with more than two layers, with the increase of magnetic field, would first experience a surface "spin-flop" process to reach a collinear M2 state (the magnetic moment being the static magnetic moment of two MnSb₂Te₄layers), that is, the magnetic moment of the surface layer whose magnetic moment is antiparallel to the external magnetic field would flip from antiparallel to parallel to the magnetic field, then experience the bulk "spin-flop" process, and finally reach the magnetic saturation state. The joint research team obtained the anisotropy energy and interlayer interaction energy of the material by fitting the experimental data through one-dimensional linear chain model, theoretically got the magnetic evolution process of antiferromagnetic samples with different layer numbers, and discovered the ratio of anisotropic energy to interaction energy of antiferromagnetic phase MnSb₂Te₄is slightly larger than that in MnBi₂Te₄, which explains the reason why no stable M2 state is observed in MnBi₂Te₄, providing a material system for the study of quantum transport properties in collinear M2 state.

FIG.2. Layer-number dependent magnetic evolution behavior in antiferromagnetic phase MnSb₂Te₄.

In the study of ferromagnetic phase (FIG.3.), there is no obvious dependence between the magnetic evolution of few-SL samples and the number of SL. Compared with the thick-SL samples, a larger coercivity field is observed in the few-SL samples, which may be caused by the increase of magnetic anisotropy and the decrease of shielding effect. Large anisotropy properties can make few-SL samples maintaining single domain behaviors in the whole measurement temperature range below Curie temperature; With the increase of temperature, the single domain behaviors of thick-SL samples is difficult to keep, and there would be obvious labyrinth domains. The domain size measured at zero field is about 625 nm.

FIG.3. Magnetic evolution behavior of few-SL ferromagnetic phase MnSb₂Te₄and labyrinth domains in thick-SL samples.

On January 3, 2022, relevant research results were published online in the top international journal Physical Review Letters titled Layer-Number-Dependent Antiferromagnetic and Ferromagnetic Behavior in MnSb₂Te₄(Physical Review Letters, impact Factor: 9.161). Huang Yuan, Ye Yu and Lei Hechang are co-corresponding authors. Collaborators also include Professor Liu Liwei, Professor Wang Yeliang from BIT and Researcher Gao Peng, Professor Yang Jinbo from Peking University.

The above research work was supported by the National Key R&D Program of China, the National Natural Science Foundation of China, the Beijing Natural Science Foundation, and the Electron Microscopy Laboratory of Peking University and others.

Paper Link: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.017201

Author profile attached:

Huang Yuan, professor and doctoral supervisor of Advanced Research Institute of Multidisciplinary Science of BIT. His main research fields focus on the preparation, characterization, device processing and physical property measurement/regulation of two-dimensional materials. He has published more than 70 SCI papers on Nature Physics, Nature Communications, Physical Review Letters, etc., including more than 30 articles in which he is the first author (including co-first) or corresponding authors, with a total of more than 3700 citations. He presided over the National key R & D plan of China (Youth Project), the Excellent Young Scientists Fund and general projects of the National Natural Science Foundation of China, and the Chongqing Science Fund for Distinguished Young Scholars. He was selected as a member of Youth Innovation Promotion Association of Chinese Academy of Sciences in 2019, won the honorary title of "China's top ten new scientific and technological figures" by the China Association for Science and Technology in 2020, and won the second prize of the Creative Founder Award-Innovation Award of the China Association of Inventions in 2021 (ranking first). He served as reviewer of Physical Review Letters, Nature Communications, Advanced Functional Materials, ACS Nano, Nano Letters, Chinese Physics and other well-known journals. He also served as young editorial board member of Physics, Chinese Physics Letters, InfoMat and Materials.