Researchers discover ferromagnetism induced by defects in correlated 2D materials

Researchers discover ferromagnetism induced by defects in correlated 2D materials
Fig. 1. Structural characterization of Ni1−xCoxPS3 (0 ≤ x < 0.5) NS.(A) Crystal structure model of bilayer Ni1−xCoxPS3 with one pair of neighboring octahedral coordination units (bottom). (B) Powder x-ray diffraction (PXRD) patterns of various Ni1−xCoxPS3 NS samples in comparison with the standard monoclinic NiPS3 (PDF #33-0952) and CoPS3 (PDF #78-0498). The broad peak at 2θ ~26o in all PXRD patterns comes from the carbon cloth. a.u., arbitrary units. (C) Energy-dispersive spectroscopy (EDS) mapping and (D) the corresponding spectrum of a Ni0.68Co0.32PS3 NS show uniform distribution of constituent elements. (E) HAADF-STEM image of a Ni0.68Co0.32PS3 nanosheet collected from the Ni0.68Co0.32PS3 NS sample on carbon cloth shown in the inset SEM image. (F) SAED pattern of the Ni0.68Co0.32PS3 nanosheet along the [001] zone axis. (G) Atomic force microscopy image of a Ni0.68Co0.32PS3 NS transferred onto Si/SiO2 substrate, showing a thickness ~5.6 nm. (G) Raman spectra of various Ni1−xCoxPS3 (0 ≤ x < 0.5) NS. Credit: DOI: 10.1126/sciadv.abj4086

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.

Transition metal phosphorus trichalcogenides (MPX3, X= S or Se; M = Mn, Fe, Co, Ni, etc.), as the representatives of two-dimensional (2D) vdW , 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, and the Co substitution decreases the formation energy of Sv during the synthesis process.

Besides, they found that the conversion synthesis process for the Ni1-xCoxPS3 nanosheets are necessary to promote the formation of Sv. Sv 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 , 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.

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More information: Fengmei Wang et al, Defect-mediated ferromagnetism in correlated two-dimensional transition metal phosphorus trisulfides, Science Advances (2021). DOI: 10.1126/sciadv.abj4086
Journal information: Science Advances

Citation: Researchers discover ferromagnetism induced by defects in correlated 2D materials (2021, October 25) retrieved 30 November 2021 from
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