In recent years, 2D magnetic materials have gained widespread attention from researchers in basic research and potential application fields due to their rich crystal structure and unique physical properties. The discovery of 2D magnetic materials not only provides an ideal platform for exploring bizarre physical phenomena, but also provides a solid foundation for the development of the next generation of spintronic devices. However, the current types of 2D magnetic materials are quite limited, and most of them have poor environmental stability, small size and thickness controllability, and the preparation process is complex, which greatly limits their research and development and application in the development of the next generation of spin electronic devices. Therefore, exploring new 2D magnetic materials with better stability and simple and controllable synthesis methods is of great significance and value for further research and development in this field.

Recently, team members of Professor Yao Yugui, Professor Zhou Jiadong of the School of Physics of Beijing Institute of Technology and Professor Liu Zheng of Nanyang Technological University, cooperated with, Professor Lin Junhao of Southern University of Science and Technology and Professor Li Xingji of Harbin Institute of Technology to make important progress in new 2D magnetic materials. The results were published on April 25 in Nature Electronics, a top international academic journal, under the title of "Phase engineering of Cr₅Te₈with colossal anomalous Hall effect".

Figure 1. Phased synthesis and basic characterization of non-layered 2D magnetic Cr₅Te₈

The tunable structure of Cr₅Te₈phases stems from the difference in the formation energy of its trigonal phase and the monoclinic phase. The trigonal Cr₅Te₈belongs to the P-3m1 space group, while the monoclinic Cr₅Te₈belongs to the C2/m space group (Figure 1a-b). Therefore, the growth temperature in the CVD process is critical to achieving a phase transition. At higher temperatures, the trigonal Cr₅Te₈will become the dominant phase, while lower temperatures will be more conducive to the formation of a monoclinic phase. In addition, considering that the concentration of Te in the trigonal phase is slightly higher than that in the monoclinic phase (61.5-62.0 at. Te vs 59.6-61.5 at. % Te), the higher temperature is also conducive to the evaporation of the Te precursor, ensuring that a sufficient amount of Te reacts and enters the Cr-Te matrix to obtain the trigonal Phase Cr₅Te₈nanosheet. It can be seen from the optical and atomic force microscopy images that Cr₅Te₈nanosheets with different phase structures also exhibit different morphological characteristics due to different lattice symmetries, with the trigonal phases being mostly hexagonal and the monoclinic phase mostly being parallelograms (Figure 1c-f). The characterization results of XPS and RMCD preliminarily confirmed the composition and magnetism of nanosheets. Both the trigonal and the single-oblique Cr₅Te₈nanosheet exhibit rectangular hysteresis loops at low temperatures, clearly indicating their ferromagnetic properties with vertical magnetic anisotropy. In addition, at the same thickness, the monoclinic phase exhibits greater coercive forces and Curie temperatures, indicating a strong phase-dependent magnetic sequence in Cr₅Te₈crystals (Figure 1h-i).

Figure 2. STEM characterization of trigonal phase and monoclinic phase Cr₅Te₈nanosheets

The crystal structure of the trigonal Cr₅Te₈and monoclinic Cr₅Te₈is equivalent to the introduction of 1/4 of the Cr atom between the layers with CrTe₂as the backbone. The difference is that the distribution of Cr in the adjacent two intercalations in the trigonal phase is exactly the same, while the Cr in the adjacent two intercalations in the monoclinic phase is alternately arranged, which produces structural distortion while staggering, inducing the trigonal phase to change into a monoclinic phase (Figure 1a-b). Figures 2b and f show that the components of the different phase structures are evenly distributed, and the composition of the two is not much different. In order to distinguish between the two phase structures, the research team conducted an in-depth study using selective electron diffraction and high-resolution HAADF-STEM imaging. Due to the difference in the distribution of interpolated Cr in the two phase structures, different hyperlatifiers are generated in electron diffraction, and the trigonal phase Cr₅Te₈with triple symmetry and the double-symmetric monoclinic phase Cr₅Te₈can be clearly distinguished according to the diffraction image (Figures 2a and e). Further, high-resolution HAADF-STEM imaging clearly shows the difference in lining between the two phase structures at the atomic scale. The atomic lining distributions in different directions (Figures 2d and h) are fully consistent with the position of the interpolated Cr atoms in the structural model and the corresponding QSTEM simulation results, thus confirming that the CVD method can achieve the regulation of the Cr₅Te₈phase structure.

Figure 3. 2D Cr₅Te₈magnetic transport measurement

In order to confirm and analyze the ferromagnetism of the 2D Cr₅Te₈, the research team selected materials of different crystal phases and different thicknesses to prepare the Hall device. The trilateral phase and monoclinic phase Cr₅Te₈longitudinal resistance with a thickness of 6 nm exhibit a significant phase transition process during cooling, and the peak position of the first derivative of the resistance-temperature curve indicates that the Curie temperatures of the trigonal phase and the monoclinic phase Cr₅Te₈sample are 125 K and 150 K, respectively (Figure 3b). Below the Curie temperature, the Hall resistance of the trigonal phase and the monoclinic phase Cr₅Te₈under the action of the external magnetic field, and the obvious stagnation phenomenon is presented, which once again proves the spontaneous magnetization and ferromagnetic long-range order outside the 2D Cr₅Te₈surface (Figures 3c and d). By analyzing the variation relationship of the orthodontic field with temperature, the Curie transition temperature of different crystal phases and different thicknesses of Cr₅Te₈was reconfirmed (Figures 3e and f).

Figure 4. Anomalous Hall effect of 2D Cr₅Te₈

Through the abnormal Hall test results, the research team further extracted the anomalous Hall angle of the 2D Cr₅Te₈, of which the 2D Cr₅Te₈Hall angle of the trigonal phase is 2.7%, and the monoclinic phase is 5%, which is higher than the conventional ferromagnetic material and much higher than the abnormal Hall angle of the block Cr₅Te₈(Figure 4a). By analyzing the generation mechanism of the anomalous Hall effect of 2D Cr₅Te₈by scale relationship, it is found that the anomalous Hall conductance of Cr₅Te₈samples of different thicknesses and different crystal phases is proportional to the square of its longitudinal conductance, indicating that the main contribution of the anomalous Hall effect generation mechanism is the skew scattering mechanism and the intrinsic mechanism (Figure 4b), in which the exponential factor contributed by the skew scattering mechanism is two orders of magnitude higher than that of the traditional ferromagnetic materials iron and nickel. The higher skew scattering index factor and the quadratic scaling relationship between the anomalous Hall conductance and the longitudinal conductance make it easier for the 2D Cr₅Te₈to achieve larger anomalous Hall conductances within a reasonable longitudinal conductance range, such as when the longitudinal conductivity is 2× 10⁵Ω⁻¹cm⁻¹, the 2D Cr₅Te₈can achieve an anomalous Hall angle of 45 degrees, while the traditional ferromagnetic material requires a conductivity greater than 10⁸Ω⁻¹cm⁻¹to obtain the same anomalous Hall angle (Figure 4c).

Figure 5. Differences in the electronic structure and magnetic sequence of ternary and monoclinic Cr₅Te₈

Finally, the research team studied the ground-state electronic structure and magnetic sequence of the Cr₅Te₈material system based on the first principles method. The trigonal and single-oblique Cr₅Te₈are both Stoner ferromagnetic metal systems, with Stoner parameters of 1.01 and 1.14, respectively, and the monoclinic phase has stronger Stoner ferromagnetic coupling characteristics. By analyzing magnetic anisotropy, it can be seen that both structures are susceptible to magnetization in the out-of-plane direction. Relying on structural aberrations of the CrTe octahedron, the monoclinic Cr₅Te₈exhibits distinct non-collinear magnetic features (Figure 5d-e). This work obtains Cr₅Te₈, a 2D magnetic material with a large anomalous Hall angle, which is expected to promote the realization of the quantum anomalous Hall effect at the 2D limit, which is particularly important for the new generation of Hall devices, especially quantum anomalous Hall devices.

The work was supported by the National Natural Science Foundation of China, overseas high-level talents and the Beijing Institute of Technology Innovation Fund and other related projects.

Link to the paper: https://www.nature.com/articles/s41928-022-00754-6