Recently, Professor Yan Wensheng’s research group from the National Synchrotron Radiation Laboratory of the University of Science and Technology of China, in cooperation with Associate Researcher Sun Zhihu, achieved room temperature ferromagnetism of two-dimensional graphene through the strategy of precise and controllable doping of magnetic metal atoms. They stably anchored Co atoms in the graphene lattice with the assistance of co-doping N atoms, thereby activating room-temperature intrinsic ferromagnetism in graphene. The research results were published in the recent issue of “Nature-Communication” under the title “Embedding atomic cobalt into graphene lattices to activate room-temperature ferromagnetism” (Nat. Commun. 2021, doi.org/10.1038/s41467-021-22122-2) .
Graphene is considered a promising material for next-generation spintronics applications due to its excellent properties such as high carrier mobility, long spin diffusion length, and weak spin-orbit coupling. How to induce stable room temperature ferromagnetism in intrinsically diamagnetic graphene is one of the primary problems in the fabrication of graphene-based spintronic devices. At present, researchers have tried various approaches to achieve ferromagnetic ordering in graphene (including utilizing vacancy defects, sp3 functionalization, chemical doping, surface adsorption, and constructing edge states, etc.), but the obtained magnetic moments are often relatively Weak and unstable, the ferromagnetic order cannot be maintained at room temperature.
Based on previous research experience on magnetic regulation of two-dimensional transition metal chalcogenides (Nat. Commun. 10, 1584; Angew Chem Int Ed, 60, 7251) and DFT material simulation design, the research group believes that precise and controllable magnetic transition metal (Fe , Co, Ni, etc.) doping is an effective solution to this problem. In order to overcome the huge potential barrier of embedding transition metal atoms into the graphene lattice, the research group used N element (3.5) with higher electronegativity than C element (2.5) for co-doping, using N atoms to construct anchor sites, The Co atoms are firmly bound in the graphene lattice, thereby providing a stable local magnetic moment, and through the orbital hybridization between Co-N-C, the ferromagnetic exchange is formed, and finally the room temperature ferromagnetism of graphene is realized.
The research group used a two-step impregnation-pyrolysis method to monodisperse Co atoms in the graphene lattice with the assistance of N atoms. The saturation magnetization of the sample was 0.11emu g-1 at room temperature, and the Curie temperature reached 400K. Through synchrotron radiation soft and hard X-ray spectroscopy techniques and a variety of X-ray spectroscopy analytical methods (real-space multiple scattering theory calculation, extended edge quantitative fitting, multi-configuration calculation and wavelet transform), the researchers confirmed the samples in the sample. Co is atomically dispersed in the graphene lattice in the form of planar quadrilateral CoN4 structural units, which excludes the possibility that the magnetism originates from the Co-related second phase. DFT electronic structure calculations further show that the CoN4-graphene system has a metallic band structure with a significantly enhanced density of states at the Fermi surface (according to the Stoner criterion, ensuring room temperature ferromagnetism), Co-3d and C/N-2p The orbital hybridization, as well as the π-electron spin polarization, suggest that the room temperature ferromagnetism in the CoN4-graphene system originates from the conduction electron-mediated RKKY-like long-range ferromagnetic exchange mechanism, and the Co-N4 building block is the main source of room temperature ferromagnetism.
This research was supported by the National Natural Science Foundation of China, the High-end User Cultivation Fund of Hefei University Science Center, and the China Postdoctoral Science Foundation.