A weak ferromagnetic (FM) ground state at low temperature in few-layered van der Waals (vdW) magnetic Ni1-xCoxPS3 nanosheets containing sulfur vacancies (Sv) was discovered by a research team led by Prof. HE Jun from National Center for Nanoscience and Technology (NCNST) of the Chinese Academy of Sciences (CAS), in collaboration with Prof. JIN Song from the University of Wisconsin-Madison. This work was published in Science Advances on Oct 22, 2021 (DOI: 10.1126/sciadv.abj4086).
Transition metal phosphorus trichalcogenides (MPX3, X= S or Se; M = Mn, Fe, Co, Ni, etc.), as the representatives of two-dimensional (2D) vdW magnetic materials, have gained wide attention in various fields, including superconductivity, optoelectronics, and catalysis. In particular, NiPS3 exhibits intriguing quantum properties owing to the intrinsic strong charge-spin correlation effects. It is an antiferromagnetic (AFM) material with a model Hamiltonian of the XXZ type.
In this study, researchers found that the existence of crystal defects in chemically synthesized Ni1-xCoxPS3 nanosheets, i.e., sulfur vacancies (Sv), could suppress the strong intralayer antiferromagnetic exchange interaction (J3) in NiPS3. The Co substitution decreases the formation energy of Sv during the synthesis process.
Also, the conversion synthesis process for the Ni1-xCoxPS3 nanosheets are necessary to promote the formation of Sv. While the sulfur vacancies do not seem to exist in sufficient quantity in chemical vapor transport grown single crystal. The presence of Sv in Ni1-xCoxPS3 nanosheets led to the suppression of long-range AFM correlations while other competing ferromagnetic exchange interactions dominate at low temperatures, creating a magnetically frustrated system.
As a consequence, the magnetic field required to tune this defect mediated ferromagnetic state (< 300 oersted) is much lower than the value needed to tune a typical vdW antiferromagnet (> several thousand oersted), which made these nanosheets more appealing for spintronic applications.
Theoretically, in correlated NiPS3, the half-filled Ni eg orbitals coupled with half-filled S 3p orbitals, which mediates the electron hooping between neighboring Ni sites through superexchange interaction. Owing to the negative charge transfer energy, the S ligand transfers one electron to the half-filled eg Ni 3d orbital to form a d9L ground state, namely negative charge transfer (NCT) state. NCT state also dominates between antiferromagnetically aligned neighboring Ni atoms. In this case, the presence of Sv could affect the electronic correlation and then tune the magnetic ordering in correlated NiPS3.
These findings provide a less explored route for controlling competing correlated states and magnetic ordering by defect engineering in 2D vdW magnets.
Sulfur vacancies effectively suppress the strong intralayer antiferromagnetic correlation, giving rise to a weak ferromagnetism in Ni1-xCoxPS3 NS (imaged by WANG F. et.al)
National Center for Nanoscience and Technology