Room-Temperature Ferromagnetism in Two-Dimensional Janus Titanium Chalcogenides★

doi:10.6023/A23050194

Abstract

Design of two-dimensional (2D) ferromagnetic materials with room-temperature magnetism is basic cornerstone for developing nanoscale spintronics. However, the experimental realized room-temperature ferromagnets are very limited and the design of room-temperature ferromagnets based on non-magnetic materials is rarely reported. Motivated by recently synthesized Janus MoSSe monolayer, we have predicted that Janus TiXY (X=S, Se and Te; Y=H and F) monolayers are intrinsic 2D ferromagnetic materials with above room-temperature long-range spin ordering and diverse electronic structures based on systematically first-principles calculations. The six Janus TiXY monolayers tend to adopt T-phase symmetry. The dynamically and thermally stability of lattice are confirmed by phonon spectrum calculations and molecular dynamics simulations. Band structure calculations at HSE06 functional level demonstrate that the monolayer TiTeH and TiTeF are bipolar ferromagnetic semiconductors (BMS) with bandgap of 0.18 and 0.48 eV, respectively. Further, the spin direction-controlled band crossing to band gaped transition and tunable spin splitting at Γ-point are observed in TiTeH monolayer after flipping the spin orientation along from out-of-plane to in-plane direction owing to giant magneto band-structure effect. The TiSH, TiSeH, TeSF, and TiSeF monolayers are half-metallic ferromagnets (HMF) with a large spin gap of 2.67, 1.73, 3.11, and 2.27 eV, respectively. Especially, the estimated Curie temperature (T_c) of TiXH and TiXF based on Monte Carlo simulations are range from 339~401 K and 341~497 K, respectively, which are higher than room-temperature. Similar to bilayer CrI₃, we also find a stacking configuration dependent interlayer magnetic coupling ground state in bilayer TiSF. Moreover, taking TiSH and TiSF as prototypes, the intrinsic electronic properties can be maintained after the formation of heterojunctions by applying a protective coating graphene layer. The intrinsic room-temperature ferromagnetism together with diverse electronic structures, making the Janus TiXY monolayer a promising candidate for realistic spintronic applications. Further, constructing Janus monolayers based on nonmagnetic materials opens up new pathway for designing novel 2D ferromagnets.

Key words: Janus monolayer, first-principles, room-temperature ferromagnetism, electronic structures, heterojunction

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Kai Zhang, Xiaojun Wu. Room-Temperature Ferromagnetism in Two-Dimensional Janus Titanium Chalcogenides^★[J]. Acta Chimica Sinica, 2023, 81(9): 1142-1147.

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Fig. & Tab. 6

Figure 1. Structures of the (a) T-phase and (b) H-phase Janus TiXY monolayers. Isosurface plots of deformation electronic density of (c) TiSH and (d) TiSF monolayers with charge depletion and accumulation regions colored red and blue, respectively. The isovalue are equal to 0.03 and 0.05 e/Bohr3 for TiSH and TiSF, respectively. The top and side views are displayed in left and right panels

TiXY	ΔE/eV	a/nm	G.S	M/µ_B	E_f/eV
TiSH	–0.319	0.322	HMF^a	0.84	–0.233
TiSeH	–0.351	0.329	HMF^a	0.87	–0.244
TiTeH	–0.223	0.336	BMS^b	0.88	–0.487
TiSF	–0.147	0.328	HMF^a	0.92	–2.138
TiSeF	–0.117	0.335	HMF^a	1.00	–2.126
TiTeF	–0.107	0.349	BMS^b	1.11	–2.258

Table 1. The calculated energy difference (ΔE=ET–EH) between T-phase and H-phase monolayer TiXY per Ti atom, optimized lattice constant a, ground state electronic configuration (G.S), magnetic moment M, and formation energy per Ti atom Ef

Figure 2. The phonon spectra of (a) TiSH and (b) TiSF monolayers. The evolution of total energy and temperature during the simulations and the structure snapshot at 300 K after 5 ps simulation are shown in (c) and (d), the top and side views are displayed in left and right panels, respectively

Figure 3. (a~d) The spin resolved band structures and projected density of states of TiSH, TiSF, TiTeH, and TiTeF monolayers, the majority and minority spin are displayed in red and blue colors.

Figure 4. The spin charge density distribution for (a) TiSH and (b) TiSF monolayers, the isovalue is 0.01 e/Bohr3. The simulated magnetic moment (M) and specific heat (Cv) with respect to temperature for (c) TiSH and (d) TiSF monolayers

Figure 5. The band structures of Janus TiTeH monolayer with spin direction along (a) out-of-plane and (b) in-plane. The red and blue circles highlight the band crossing become band gaped and spin splitting when spin direction changed, the red arrows represent the spin direction

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具有室温铁磁性的二维Janus钛硫属化物^★