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Register a new userIntrinsic magnetism in monolayer transition metal trihalides: a comparative study
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Two dimensional magnetic materials, with tunable electronic properties could lead to new spintronic, magnetic and magneto-optic applications. Here, we explore intrinsic magnetic ordering in two dimensional monolayers of transition metal tri-halides (MX$_3$, M = V, Cr, Mn, Fe and Ni, and X = F, Cl, Br and I), using density functional theory. We find that other than FeX$_3$ family which has an anti-ferromagnetic ground state, rest of the trihalides are ferromagnetic. Amongst these the VX$_3$ and NiX$_3$ family are found to have the highest magnetic transition temperature, beyond the room temperature. In terms of electronic properties, the tri-halides of Mn and Ni are either half metals or Dirac half metals, while the tri-halides of V, Fe and Cr are insulators. Among all the trihalides studied in this paper, we find the existence of very clean spin polarized Dirac half metallic state in MnF$_3$, MnCl$_3$, MnBr$_3$, NiF$_3$ and NiCl$_3$. These spin polarized Dirac half metals will be immensely useful for spin-current generation and other spintronic applications.
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The realization of multiferroicity in 2D nanomaterials is crucially important for designing advanced nanoelectronic devices such as non-volatile multistate data storage. In this work, the coexistence of ferromagnetism and ferroelectricity is reported in monolayer SnTe system by transition metal (TM) doping. Based on first-principles calculations, the spontaneous spin polarization could be realized by TM doping in ferroelectric SnTe monolayer. In addition to in-plane ferroelectric polarization, the out-of-plane ferroelectric polarization emerges in Mn (Fe)-doped SnTe monolayer due to the internal displacement of TM from the surface. Interestingly, the crystalline field centered on TM and interaction between the dopant and Te gradually enhanced with the increment of atomic number of doping elements, which explains why the formation energy decreases. The realization of multiferroics in SnTe monolayer could provide theoretical guidance for experimental preparation of low-dimensional multiferroic materials.
Materials composed of two dimensional layers bonded to one another through weak van der Waals interactions often exhibit strongly anisotropic behaviors and can be cleaved into very thin specimens and sometimes into monolayer crystals. Interest in such materials is driven by the study of low dimensional physics and the design of functional heterostructures. Binary compounds with the compositions MX2 and MX3 where M is a metal cation and X is a halogen anion often form such structures. Magnetism can be incorporated by choosing a transition metal with a partially filled d-shell for M, enabling ferroic responses for enhanced functionality. Here a brief overview of binary transition metal dihalides and trihalides is given, summarizing their crystallographic properties and long-range-ordered magnetic structures, focusing on those materials with layered crystal structures and partially filled d-shells required for combining low dimensionality and cleavability with magnetism.
Two-dimensional (2D) intrinsic ferromagnetic semiconductors are expected to stand out in the spintronic field. Recently, the monolayer VI$_{3}$ has been experimentally synthesized but the weak ferromagnetism and low Curie temperature ($T_C$) limit its potential application. Here we report that the Janus structure can elevate the $T_C$ to 240 K. And it is discussed that the reason for high $T_C$ in Janus structure originates from the lower virtual exchange gap between $t_{2g}$ and $e_{g}$ states of nearest-neighbor V atoms. Besides, $T_C$ can be further substantially enhanced by tensile strain due to the increasing ferromagnetism driven by rapidly quenched direct exchange interaction. Our work supports a feasible approach to enhance Curie temperature of monolayer VI$_{3}$ and unveils novel stable intrinsic FM semiconductors for realistic applications in spintronics.
Grain boundaries (GBs) are structural imperfections that typically degrade the performance of materials. Here we show that dislocations and GBs in two-dimensional (2D) metal dichalcogenides MX2 (M = Mo, W; X = S, Se) can actually improve the material by giving it a qualitatively new physical property: magnetism. The dislocations studied all have a substantial magnetic moment of ~1 Bohr magneton. In contrast, dislocations in other well-studied 2D materials are typically non-magnetic. GBs composed of pentagon-heptagon pairs interact ferromagnetically and transition from semiconductor to half-metal or metal as a function of tilt angle and/or doping level. When the tilt angle exceeds 47{deg} the structural energetics favor square-octagon pairs and the GB becomes an antiferromagnetic semiconductor. These exceptional magnetic properties arise from an interplay of dislocation-induced localized states, doping, and locally unbalanced stoichiometry. Purposeful engineering of topological GBs may be able to convert MX2 into a promising 2D magnetic semiconductor.
The optical properties of atomically thin transition metal dichalcogenide (TMDC) semiconductors are shaped by the emergence of correlated many-body complexes due to strong Coulomb interaction. Exceptional electron-hole exchange predestines TMDCs to study fundamental and applied properties of Coulomb complexes such as valley depolarization of excitons and fine-structure splitting of trions. Biexcitons in these materials are less understood and it has been established only recently that they are spectrally located between exciton and trion. Here we show that biexcitons in monolayer TMDCs exhibit a distinct fine structure on the order of meV due to electron-hole exchange. Ultrafast pump-probe experiments on monolayer WSe$_2$ reveal decisive biexciton signatures and a fine structure in excellent agreement with a microscopic theory. We provide a pathway to access biexciton spectra with unprecedented accuracy, which is valuable beyond the class of TMDCs, and to understand even higher Coulomb complexes under the influence of electron-hole exchange.
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