Researchers discover ferromagnetism induced by defects in correlated 2D materials

A weak ferromagnetic (FM) ground state at low temperature in few-layered van der Waals (vdW) magnetic Ni_1-xCo_xPS₃ nanosheets containing sulfur vacancies (S_v) 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.

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 Ni_1-xCo_xPS₃ nanosheets, i.e., sulfur vacancies (S_v), could suppress the strong intralayer antiferromagnetic exchange interaction (J3) in NiPS₃, and the Co substitution decreases the formation energy of S_v during the synthesis process.

Besides, they found that the conversion synthesis process for the Ni_1-xCo_xPS₃ nanosheets are necessary to promote the formation of S_v. S_v do not seem to exist in sufficient quantity in chemical vapor transport grown single crystal. The presence of S_v in Ni_1-xCo_xPS₃ 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 e_g 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 e_g 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 S_v 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.