image: With increasing intercalation ratios, the intercalated structures of Fe1+xSe2 undergo the disordered–ordered–half-ordered–ordered transition, accompanied by the regulation of the magnetic and electrical properties.
Credit: ©Science China Press
Intercalating native metal atoms (self-intercalation) into the van der Waals (vdW) gaps of transition metal dichalcogenides (TMD) has been considered as an effective method to create novel 2D materials. It can also induce newfangled phenomena while maintaining the outstanding characteristics of 2D parent structure. However, the general relationship between self-intercalated concentration and atomic structure remains largely unexplored, but it is of significant importance for designing new materials and predicting their structures. Moreover, it is also extremely desirable to reveal the influence of intercalated structures on the physicochemical properties to further improve their performance. Last but not least, previous studies mostly focused on V/Nb/Ta-based and Cr-based self-intercalated TMD, whose Curie temperatures are still below room temperature and magnetic properties are far from satisfactory for extending the applications.
Recently, scientists from Peking University published a research paper titled "The evolution of chemical ordering and property in Fe1+xSe2 upon intercalation ratios" in National Science Review. They revealed the general intercalation rule for the effect of the self-intercalation ratio on atomic ordering. To confirm this intercalation rule, the researchers synthesized a family of Fe1+xSe2 nanoflakes with controllable intercalation ratios, including Fe1.18Se2 (disordered), Fe1.25Se2 Fe1.5Se2 (ordered), Fe1.6Se2 (half-ordered), and Fe1.75Se2 (ordered) via a space confinement-assisted chemical potential regulation strategy. Aberration-corrected STEM characterizations declared the self-intercalation structures and verified the intercalation rule. Notably, the study introduced an innovative structure of “half-ordered intercalation”, providing a new class of intriguing materials that may be discovered in other intercalated TMD.
Furthermore, this research showed that intercalation can significantly regulate magnetic and electrical properties (including Curie temperatures, spin directions, magnetic domains, spin gap, and MR). Disordered Fe1.18Se2 are nonmagnetic, while all orderly intercalated samples show room-temperature magnetic ordering, which is induced by the charge transfer of intercalated Fe atoms. The magnetic structures transition from single- to multi-domain states by increasing the intercalation ratio. Strikingly, intercalation can induce the generation of room-temperature magnetic half-metals. Fe1.5Se2 shows a crossover from negative MR to positive MR below the saturation fields with decreasing temperatures, while Fe1.6Se2 keeps positive MR owing to the increased spin-up gap caused by more intercalated Fe atoms.
This work achieves the controllable synthesis of a new class of materials with remarkable properties, including obvious room-temperature magnetism, spin-reorientation, and unique half-metallic behavior, which shows great promise in magnetic tunnel junctions and other spintronic devices. These findings provide a classic paradigm for structural regulation of magnetic and electric properties.