On December 26th, Proceedings of the National Academy of Sciences of the United States of America ("Proceedings of the American Academy of Sciences") published the latest research results of the Chinese Academy of Sciences Ningbo Institute of Materials Technology and Engineering in the field of MAX phase new materials creation " Multi-elemental single-atom-thick A layers in nanolaminated V2 (Sn, A) C (A = Fe, Co, Ni, Mn) for tailoring magnetic properties ”(DOI: 10.1073 / pnas.1916256117).
Laminated magnetic materials have attracted much attention due to their unique structure and potential applications in the field of spintronics. For example, the giant magnetoresistance effect found in magnetic layered materials has completely changed data storage and magnetic recording. At present, researchers are committed to In order to find more new magnetic materials to meet the application of materials in different service environments. The MAX phase is a type of nano-layered transition metal compound with a hexagonal lattice structure, the molecular formula is Mn + 1AXn, (where M is generally a metal of the former transition group, A is mainly an element of the main group of 13-15, X is carbon or / and Nitrogen, n more values 1-3), as shown in Figure 1. Judging from the bonding characteristics of the MAX phase lattice, the overlap of electron clouds between M and X atoms determines the strong ionic and covalent bonds, while the overlap of electron clouds between M and A atoms is weak, which is The physical and chemical properties of element A have some help. Therefore, if a magnetic element is introduced at the A site in the MAX phase, with the unique nano-layered structure, high stability, and adjustable anisotropy, it is expected to be used as a functional material in spintronic devices. However, previous studies generally believed that post-transition metal elements such as Fe, Co, Ni, and Mn with 3d electrons should exist in the M-atomic lattice site of the MAX phase material, and a magnetic element was synthesized to occupy the A site of the two-dimensional monoatomic layer in the MAX phase. It is considered to be a great challenge.
Recently, the Advanced Energy Materials Engineering Laboratory of the Ningbo Institute of Materials Science and Technology, Chinese Academy of Sciences has adopted the synthesis strategy of alloy-controlled reaction path, and successfully introduced the magnetic elements Fe / Co / Ni / Mn into the A phase of MAX phase. Theoretical analysis shows that V2SnC is the only thermodynamically stable ternary layered MAX phase in the V-Sn-C system, which can be in phase equilibrium with Sn metal and vanadium carbide. In the case of adding a magnetic element, the V2 (AxSny) C phase can be in phase equilibrium with the VC1-x and AxSny alloy phases, that is, the VC1-x and intermediate liquid AxSny are transformed into V2 (AxSny) C. Compared with the ternary V-Sn-C system, Fe and other magnetic elements have a higher chemical affinity for Sn, so it takes precedence over the V metal combination to form Fe-Sn alloy, which will facilitate the nucleation of VC1-x phase at low temperature and Suppresses the formation of competitive phase of V-Sn alloy. The liquid AxSny alloy and VC1-x nanocrystals further form the V2 (AxSn1-x) C phase through the peritectic reaction. The research team further confirmed that all magnetic elements are distributed in the A-site monoatomic layer through the scanning electron microscope Z-contrast imaging technique and atom-resolved energy spectrum analysis technique, and the M-site is only a single element of vanadium (Figure 2). The introduction of a magnetic element with an outer layer of d electrons at the A site will provide great imagination for the expansion of the function of the MAX phase material.
Co, Ni, Mn and other magnetic elements have similar atomic radii and electronegativity to Fe, so these elements can enter the A-site lattice at the same time as Sn metal atoms in any ratio or combination. Experimental studies have also shown that in addition to the synthesis of four MAX phases containing a single magnetic element at the A site of V2 (AxSn1-x) C (A = Fe, Co, Ni, or Mn), binary / The ternary / quaternary magnetic elements (A = FeCo, FeNi, FeMn, CoNi, CoMn, NiMn, FeCoNi, FeCoMn, FeNiMn, CoNiMn or FeCoNiMn) successfully synthesize 15 new nano-layered magnetic MAX phase materials. Multi-component magnetic elements were introduced at the same time, and a single-atomic layer high-entropy MAX phase material was successfully synthesized (Figure 3). The discovery of the high-entropy state of the A-site magnetic element indicates that the MAX phase is very inclusive and has the potential for multi-element control. This is for the control of the MA chemical bond of the MAX phase material, the lattice defect formation energy, and the A-site element stripping chemistry And microscopic mechanisms such as c atom plane dislocation slip will have a profound impact.
In addition, the research team also carried out preliminary magnetic research on the newly synthesized MAX phase. The results show that the MAX phase exhibits a "S-shaped" hysteresis loop at low temperatures, and its saturation magnetization (Ms) increases with temperature. High and gradually decrease, indicating that V2 (AxSn1-x) C MAX phase material is a typical soft magnetic material. Interestingly, the magnetic properties of the V2 (AxSn1-x) MAX phase show a strong dependence on various element combinations. Therefore, the magnetic properties can be adjusted by adjusting the composition and content of the A-site magnetic element. The above preliminary results show that the MAX phase can serve as a good single-layer atomic structure template, and it is expected to further improve the magnetic and Curie temperature of the MAX phase by enhancing the interlayer electronic coupling of the A-site atoms and realize the application in functional devices. .
The microstructural characterization and material calculation of this study were obtained by Dr. Jun Lu of Lin Xueping University in Sweden and Professor Joseph S. Francisco of the University of Pennsylvania in the United States. They were also awarded by the Ningbo Top Talent Program (Advanced Energy Materials Cross-Innovation Team), PIFI International of Chinese Academy of Sciences Funded by cooperative projects and the National Natural Science Foundation of China. At present, the laboratory has applied for two Chinese invention patents (CN201910068169.3, CN201810930369.0) around the magnetic MAX phase.