Room‐Temperature Ferromagnetism in MoTe₂ by Post‐Growth Incorporation of Vanadium Impurities

Abstract

potentially to exploit them in spintronics applications. Such potential applications of magnetic vdW materials have recently sparked considerable interest in investigating magnetism of bulk ferromagnetic materials thinned to a single layer [1][2][3][4] or the emergence of ferromagnetism of paramagnetic bulk materials at the monolayer limit. [5,6] In addition, defect-and dopant-induced ferromagnetism has been predicted theoretically. [7][8][9][10][11][12] Recent experimental results suggest that 1T-2H phase boundaries in MoSe 2 , [13][14][15] edges in WS 2 or MoS 2 , [16][17][18][19] adsorbate induced defect states, [20] or substitutional doping of transition metals in 2D materials [21][22][23] can result in defect-or dopant-induced ferromagnetic ordering in these systems. Recently, long-range magnetic order has also been observed in both MoTe 2 and MoSe 2 , which has been suggested to be induced by intrinsic ferromagnetic defects, such as Mo antisites, that is, Mo atoms at chalcogen lattice sites. [24] This indicates that in these materials even dilute defects can cause long-range ferromagnetic ordering, making them promising diluted magnetic semiconductor (DMS) materials. Here, we investigate a new doping mechanism for 2H-MoTe 2 that enables altering the monolayer or surface layer of a MoTe 2 crystal with transition metal impurities. We have recently demonstrated that 2H-MoTe 2 and MoSe 2 can be modified by incorporation of transition metals into the host's interstitial site. [25] We have shown experimentally and by density functional theory (DFT) calculations that excess Mo atoms on MoTe 2 are energetically favored at interstitial sites as compared to adsorbed atoms at the surface. At elevated temperatures, these excess transition metal atoms are mobile and can undergo site-exchange with lattice Mo atoms. For high enough mobility (temperature), the interstitials rearrange into 1D Mo-rich crystal modifications, known as mirror twin grain boundaries. [26][27][28] These grain boundaries have shown no ferromagnetic properties. We demonstrate here, that this doping mechanism can also be expanded to other transition metals, in particular with the goal of inducing magnetism into MoTe 2 . Titanium or vanadium has been used as ferromagnetic dopants in diluted semiconductor systems [29,30] and thus, in this study we explore if V can be introduced into MoTe 2 interstitial Figure 4. Magnetization measurements of MoTe 2 with different V concentrations. a) M-H hysteresis loops taken at 10 K for MoTe 2 with 0.2, 0.3, and 0.8% of V coverage. The variation of the linear diamagnetic background is a consequence of different substrate thicknesses. b) Variation in magnetization saturation with the V concentration. c) Temperature dependences of H C and M S for the 0.8% V-doped sample. The error bars reflect the experimental uncertainty related to background noise. www.advancedsciencenews.com

Monolayer Vanadium‐Doped Tungsten Disulfide: A Room‐Temperature Dilute Magnetic Semiconductor

“…The magnetic properties of pristine and V-doped WS 2 monolayer samples were measured by a vibrating sample magnetometer. To exclude unwanted effects on the magnetization versus magnetic field (M-H) loops that can arise from the SiO 2 substrate and the double-sided carbon tape used to hold the sample (see Figure S8, Supporting Information) and from subtracting diamagnetic backgrounds (see Figure S9, Supporting Information), [33,34] Figure 2e-h presents smoothed as-measured M-H loops at 300 K, and we deduced the saturation magnetization (M S ) and coercive field (H C ) directly from these loops. The pristine WS 2 sample exhibits a weak ferromagnetic signal, which we tentatively ascribe to undercoordinated sulfur atoms at crystallographic defects (e.g., edges), [35] on a diamagnetic background.…”

“…The ferromagnetism observed in V-doped samples is too strong to originate from edge effects, and its dependence on vanadium concentration suggests an origin in local magnetic moments associated with unpaired electrons in vanadium d orbitals. [16,18,33] The 2 at% WS 2 sample shows large, clear hysteresis loops at all temperatures from which M S and H C (Figure 2i) are extracted and plotted as a function of temperature (the M-H loops are close to square when rotated to account for the diamagnetic background). We performed magnetic measurements on three V:WS 2 samples for each concentration and obtained reproducible results ( Figure S10, Supporting Information).…”

Enabling room temperature ferromagnetism in monolayer MoS2 via in situ iron-doping

“…Although being constrained by solubility and chemical stability, transition metal elements can be doped to some extent into monolayer TMDs, but ferromagnetism was not demonstrated [17][18][19] . Nevertheless, enhanced doping concentration of 2% Mn in Mn: MoS 2 monolayers grown on a graphene substrate 17 and 1% rhenium (Re) in Re:MoS 2 monolayer exhibited suppression of defect-bound emission at low temperatures, demonstrating the feasibility of realizing in situ doping of TMD monolayers 20,21 . Employing a direct vapor-phase method, ferromagnetism was reported for Fe:SnS 2 monolayer with a T C of 31 K and a saturation magnetization of 3.5 emu g −1 .…”

“…However, in this approach, mechanical exfoliation is required after growing bulk crystals, which is not scalable. A recent study shows post-growth incorporation of V into MoTe 2 monolayer with a small saturation magnetization of 0.6 µemu cm −2 , giving rise to room-temperature ferromagnetism, but only in the form of small, randomly distributed clusters that are not uniform 21 . Another technique is based on ion implantation and the idea of co-doping, giving rise to very high saturation magnetization values of 60.62 emu g −1 , but ferromagnetism is limited to a T C around 10 K (ref.…”

Molecular Beam Epitaxy and High-Pressure Studies of van der Waals Magnets

“…A few years later, scientists began studying the effects of depositing magnetic impurities into non-magnetic 2D semiconductors during MBE growth [71,82,83]. There has been many theoretical predictions about magnetic doping of Mn, Cr, and V into TMDCs such as MoS 2 [84], which is a motivating factor for attempting these studies.…”