Recently, a joint research team composed of Professor He Jun from the School of Physics and Technology of Wuhan University and Professor Jin Song from the University of Wisconsin-Madison in the United States developed a two-dimensional transition metal phosphorous sulfide (MPX3, M is a metal element, X is S, Se). Significant progress has been made in research, and experiments have found that low-temperature ferromagnetic properties can be introduced in the layered Ni1-xCoxPS3 material with intrinsic strong correlation antiferromagnetic van der Waals through S defect regulation. Related research results were published in the journal Science Advances with the title "Defect-Mediated Ferromagnetism in Correlated Two-Dimensional Transition Metal Phosphorus Trisulfides" ("Defect-Mediated Ferromagnetism in Correlated Two-Dimensional Transition Metal Phosphorus Trisulfides") , eabj4086 (2021)).
The two-dimensional magnetic system provides a new research platform for studying the basic physical problems of magnetism and regulating low-dimensional magnetism, and has important application prospects in optoelectronic devices and spintronic devices. Professor Jun He's research group has long been committed to the research on the controllable preparation of two-dimensional magnetic materials and the application of new principle devices. Has accumulated rich experience in the field of two-dimensional magnetic research, and can control the preparation of atomic-level thickness of MPX3, CrSe, Cr2S3, Cr2Te3, etc. (Nano Lett. 20, 5, 3130 (2020); Nature Commun. 10, 4133 (2019) ；Adv. Mater. 31, 1900056 (2019); Nano Lett. 19, 2154(2019); Adv. Mater. 30, 1707433 (2018); Adv. Funct. Mater. 28, 1800548 (2018)), and was invited Published reviews of relevant materials on Science (Science 370, 525-526 (2020)).
NiPS3 is a kind of XXZ antiferromagnetic van der Waals layered material with intrinsic strong correlation. After solving the XXZ Heisenberg spin model of the system, it is found that the center Ni atom passes through the metal atom of the third nearest neighbor. The S atom has a strong TNN antiferromagnetic (AFM) super-exchange function. In this work, the research team synthesized Co-substituted NiPS3 (ie Ni1-xCoxPS3) nanosheets for the first time, and found that Ni1-xCoxPS3 nanosheets contain S vacancies (Sv) through X-ray absorption spectroscopy and electron spin resonance spectroscopy analysis. However, the substitution of Co atoms for Ni atoms is beneficial to the formation of S vacancies. The formation energy of a single Sv (1.38 eV) in Ni1-xCoxPS3 (0<x<0.5) is much smaller than the 2.19 eV in NiPS3. At the same time, the superconducting quantum magnetic measurement system SQUID was used to test the magnetic properties of nanosheets with different compositions (x), and the results showed that the presence of Sv would cause them to have weak ferromagnetic properties at low temperatures (< 100 K). This allows the use of low magnetic field strength (< 300 Oe) to effectively suppress the strong antiferromagnetic exchange interaction in the system to achieve weak ferromagnetism. Combined with theoretical calculations, it is found that Sv in Ni1-xCoxPS3 can change the local lattice configuration and electronic structure, thereby breaking the strong third-neighbor AFM exchange effect mediated by non-magnetic S atoms, and finally making Ni1-xCoxPS3 (0≤ x<0.5 ) Nanosheets exhibit weak ferromagnetic properties at low temperatures, which also illustrates the controllability of NiPS3 antiferromagnets. This work expounds the physical mechanism of defect engineering regulating magnetic exchange, and provides new ideas for the study of two-dimensional magnetic material systems.
Figure (A) Schematic diagram of the exchange between adjacent Ni atoms connected by S atoms in NiPS3 and its zig-zag type antiferromagnetic properties; (B-C) M-T and M-H curves of different Ni1-xCoxPS3 nanosheets and single crystals
This research work was supported by the National Key Research and Development Program of the Ministry of Science and Technology and the Key Support Projects of the National Natural Science Foundation of China. Professor Jun He and Professor Jin Song are the co-corresponding authors, and Wuhan University is the co-signing and corresponding author unit of the paper.
Original link: https://www.science.org/doi/10.1126/sciadv.abj4086
