| 1. |
- Brondin, Carlo Alberto, et al.
(author)
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Tailoring Magnetic Anisotropy in Ultrathin Cobalt by Surface Carbon Chemistry
- 2024
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In: Advanced Electronic Materials. - 2199-160X. ; 10:4
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Journal article (peer-reviewed)abstract
- The ability to manipulate magnetic anisotropy is essential for magnetic sensing and storage tools. Surface carbon species offer cost-effective alternatives to metal-oxide and noble metal capping layers, inducing perpendicular magnetic anisotropy in ultrathin ferromagnetic films. Here, the different mechanisms by which the magnetism in a few-layer-thick Co thin film is modified upon adsorption of carbon monoxide (CO), dispersed carbon, and graphene are elucidated. Using X-ray microscopy with chemical and magnetic sensitivity, the in-plane to out-of-plane spin reorientation transition in cobalt is monitored during the accumulation of surface carbon up to the formation of graphene. Complementary magneto-optical measurements show weak perpendicular magnetic anisotropy (PMA) at room temperature for dispersed carbon on Co, while graphene-covered cobalt exhibits a significant out-of-plane coercive field. Density-functional theory (DFT) calculations show that going from CO/Co to C/Co and to graphene/Co, the magnetocrystalline and magnetostatic anisotropies combined promote out-of-plane magnetization. Anisotropy energies weakly depend on carbidic species coverage. Instead, the evolution of the carbon chemical state from carbidic to graphitic is accompanied by an exponential increase in the characteristic domain size, controlled by the magnetic anisotropy energy. Beyond providing a basic understanding of the carbon-ferromagnet interfaces, this study presents a sustainable approach to tailor magnetic anisotropy in ultrathin ferromagnetic films. Magnetic properties of Co ultrathin films are shown to undergo dramatic changes upon surface carbon accumulation. Chemical transformation from molecular carbon monoxide to surface carbide and to a graphene layer progressively enhances the perpendicular magnetic anisotropy of Co. Calculations reveal that magnetocrystalline and magnetostatic contributions play distinctly different roles for the different carbon species.image
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| 2. |
- Ershadrad, Soheil, et al.
(author)
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Unusual Magnetic Features in Two-Dimensional Fe5GeTe2 Induced by Structural Reconstructions
- 2022
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In: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 13:22, s. 4877-4883
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Journal article (peer-reviewed)abstract
- Recent experiments on Fe5GeTe2 suggested the presence of a symmetry breaking of its conventional crystal structure. Here, using density functional theory calculations, we elucidate that the stabilization of the (root 3 X root 3)R30 degrees supercell structure is caused by the swapping of Fe atoms occurring in the monolayer limit. The swapping to the vicinity of Te atoms is facilitated by the spontaneous occurrence of Fe vacancy and its low diffusion barrier. Our calculated magnetic exchange parameters show the simultaneous presence of ferromagnetic and antiferromagnetic exchange among a particular type of Fe atom. The Fe sublattice projected magnetization obtained from Monte Carlo simulations dearly demonstrates an exotic temperature-dependent behavior of this Fe type along with a large canting angle at T = 0 K, indicating the presence of a complex noncollinear magnetic order. We propose that the low-temperature crystal structure results from the swapping between two sublattices of Fe, giving rise to peculiar magnetization obtained in experiments.
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| 3. |
- Ghosh, Sukanya, et al.
(author)
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Rashba-induced spin texture and spin-layer-locking effects in the antiferromagnetic CrI3 bilayer
- 2023
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In: Journal of Physics and Chemistry of Solids. - : Elsevier. - 0022-3697 .- 1879-2553. ; 173
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Journal article (peer-reviewed)abstract
- The antiferromagnetic (AFM) CrI3 bilayer is a particularly interesting representative of van der Waals 2D semiconductors, which are currently being studied for their magnetism and for their potential in spintronics. Using ab initio density-functional theory calculations, we investigate the spin texture in momentum space of the states of the (doubly degenerate) highest valence band of the AFM CrI3 bilayer with Cr-spin moments perpendicular to the layers. We find the existence, in the main central part of the Brillouin zone, of a Rashba in-plane spin texture of opposite signs on the two layers, resulting from the intrinsic local electric fields acting on each layer. To study the layer segregation of the wavefunctions, we apply a small electric field that splits the degenerate states according to their layer occupancy. We find that the wavefunctions of the highest valence band are layer-segregated, belonging to only one of the two layers with opposite in-plane spin textures, and the segregation occurs over nearly the whole Brillouin zone. The corresponding layer locking of the in-plane -canted spin is related to the separation in energy of the highest AFM band from the rest of the valence bands. We explain how the band interactions destroy the layer locking at the K, K ', and Gamma points. Furthermore, we compare the layer locking of the in-plane-canted spin in our AFM bilayer system with the hidden spin polarization in centrosymmetric nonmagnetic materials, pointing out the differences in segregation mechanisms and their consequences for the layer locking. We note that a similar Rashba effect with layer locking of in -plane-canted spin could occur in other van der Waals AFM bilayers with strong spin-orbit coupling and an isolated energy band.
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| 4. |
- Ghosh, Sukanya, et al.
(author)
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Structural distortion and dynamical electron correlation driven enhanced ferromagnetism in Ni-doped two-dimensional Fe5GeTe2 beyond room temperature
- 2024
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In: 2D Materials. - : Institute of Physics Publishing (IOPP). - 2053-1583. ; 11:3
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Journal article (peer-reviewed)abstract
- Achieving beyond room-temperature ferromagnetism in two-dimensional (2D) magnets is immensely desirable for spintronic applications. Fe5GeTe2 is an exceptional van der Waals metallic ferromagnet due to its tunable physical properties and relatively higher Curie temperature (TC) than other 2D magnets. Using density functional theory combined with dynamical electron correlation and Monte Carlo simulations, we find the TC of (Fe1-δNiδ) 5GeTe2 monolayer can increase up to ∼ 400 K at δ ∼ 0.20 (δ: fractional occupation). Two specific Fe sublattices are identified to be the most energetically preferred sites to host Ni. Exchange interactions between particular Fe pairs play a dominating role in controlling TC, influenced by the dopant-induced structural distortions. Dynamical electron correlation induces site- and orbital-specific quasi-particle mass of Fe-d states with varying Ni concentrations. This work provides fundamental insights into 2D magnetism as an interplay of structural and electronic aspects and would guide to tailoring exciting magnetic phenomena in similar systems.
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| 5. |
- Ghosh, Sukanya, et al.
(author)
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Unraveling effects of electron correlation in two-dimensional FenGeTe2 (n=3, 4, 5) by dynamical mean field theory
- 2023
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In: npj Computational Materials. - : Springer Nature. - 2057-3960. ; 9:1
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Journal article (peer-reviewed)abstract
- The FenGeTe2 systems are recently discovered two-dimensional van-der-Waals materials, exhibiting magnetism at room temperature. The sub-systems belonging to FenGeTe2 class are special because they show site-dependent magnetic behavior. We focus on the critical evaluation of magnetic properties and electron correlation effects in FenGeTe2 (n = 3, 4, 5) (FGT) systems performing first-principles calculations. Three different ab initio approaches have been used primarily, viz., (i) standard density functional theory (GGA), (ii) incorporating static electron correlation (GGA + U) and (iii) inclusion of dynamic electron correlation effect (GGA + DMFT). Our results show that GGA + DMFT is the more accurate technique to correctly reproduce the magnetic interactions, experimentally observed transition temperatures and electronic properties. The inaccurate values of magnetic moments, exchange interactions obtained from GGA + U make this method inapplicable for the FGT family. Correct determination of magnetic properties for this class of materials is important since they are promising candidates for spin transport and spintronic applications at room temperature.
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| 6. |
- Gürbüz, Emel, et al.
(author)
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Spin-polarized two-dimensional electron/hole gas at the interface of nonmagnetic semiconducting half-Heusler compounds : Modified Slater-Pauling rule for half-metallicity at the interface
- 2023
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In: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 7:5
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Journal article (peer-reviewed)abstract
- Half-Heusler compounds with 18 valence electrons per unit cell are well-known nonmagnetic semiconductors. Employing first-principles electronic band-structure calculations, we study the interface properties of the half-Heusler heterojunctions based on FeVSb, CoTiSb, CoVSn, and NiTiSn compounds, which belong to this category of materials. Our results show that several of these heterojunction interfaces become not only metallic but also magnetic. The emergence of spin-polarization is accompanied by the formation of two-dimensional electron gas or hole gas at the interface. A qualitative analysis of the origin of the spin polarization at the interfaces suggests that strong correlations are responsible. For the cases of magnetic interfaces where half-metallicity is also present, we propose a modified Slater-Pauling rule similar to the one for bulk half-metallic half-Heusler compounds. Additionally, we calculate exchange parameters, Curie temperatures, and magnetic anisotropy energies for magnetic interfaces. Our study, along with recent experimental evidence confirming the presence of two-dimensional electron gas at CoTiSb/NiTiSn heterojunctions, may motivate future efforts to explore and realize device applications using these heterojunctions.
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| 7. |
- Nayak, Chumki, et al.
(author)
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Valley polarization and photocurrent generation in transition metal dichalcogenide alloy MoS 2 x Se 2(1- x )
- 2024
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In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 109:11
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Journal article (peer-reviewed)abstract
- Monolayer transition metal dichalcogenides (TMDCs) constitute the core group of materials in the emerging semiconductor technology of valleytronics. While the coupled spin -valley physics of pristine TMDC materials and their heterostructures has been extensively investigated, less attention has been given to TMDC alloys, which could be useful in optoelectronic applications due to the tunability of their band gaps. The experimental investigations presented herein focus on the exploration of the spin -valley physics of the monolayer and bilayer TMDC alloy MoS 2 x Se 2(1- x ) in terms of valley polarization and the generation as well as electrical control of a photocurrent utilizing the circular photogalvanic effect. Piezoelectric force microscopy provides evidence for an internal electric field perpendicular to the alloy layer, thus breaking the out -of -plane mirror symmetry. The experimental observation is supported by first -principles calculations based on the density functional theory. A comparison of the photocurrent device, based on the alloy material, is made with similar devices involving other TMDC materials.
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| 8. |
- Zhao, Bing, 1990, et al.
(author)
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A Room-Temperature Spin-Valve with van der Waals Ferromagnet Fe 5 GeTe 2 /Graphene Heterostructure
- 2023
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In: Advanced Materials. - : Wiley. - 0935-9648 .- 1521-4095. ; 35:16
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Journal article (peer-reviewed)abstract
- The discovery of van der Waals (vdW) magnets opened a new paradigm for condensed matter physics and spintronic technologies. However, the operations of active spintronic devices with vdW ferromagnets are limited to cryogenic temperatures, inhibiting their broader practical applications. Here, the robust room-temperature operation of lateral spin-valve devices using the vdW itinerant ferromagnet Fe5GeTe2 in heterostructures with graphene is demonstrated. The room-temperature spintronic properties of Fe5GeTe2 are measured at the interface with graphene with a negative spin polarization. Lateral spin-valve and spin-precession measurements provide unique insights by probing the Fe5GeTe2/graphene interface spintronic properties via spin-dynamics measurements, revealing multidirectional spin polarization. Density functional theory calculations in conjunction with Monte Carlo simulations reveal significantly canted Fe magnetic moments in Fe5GeTe2 along with the presence of negative spin polarization at the Fe5GeTe2/graphene interface. These findings open opportunities for vdW interface design and applications of vdW-magnet-based spintronic devices at ambient temperatures.
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