Proximity Exchange Research Articles

Exploring magnetic stability and valley splitting on CrI3/SiC van der Waals heterostructure

The CrI3 monolayer fabricated from recent experiment has attracted much attention due to its long-range FM order. However, the very low Curie temperature Tc (~45 K) severely limits its realistic application. Herein, we construct the vdW heterostructure for CrI3 with SiC momolayer to explore improvement of magnetic properties. The detailed calculations show that the FM coupling of CrI3 layer in heterostructure can be enhanced with Tc up to 62.3 K, which is attributed to both superexchange interaction and proximity exchange effect. Besides, intrinsically nonmagnetic SiC layer is also magnetized and holds a significant magnetic exchange splitting of valley. More importantly, the magnetic exchange energy is found to be steadily increased with the interlayer gap reduced, making Tc up to 128.8 K, and the external electric field enhances the heterostructure FM coupling as well, with Tc reaching 93.14 K. This is because the superexchange interaction and proximity exchange effect are intensified by physical field coupling on the heterostructure. We also observe that the magnetic exchange splitting of valley causes the noticeable half-metal feature upon the electric–magnetic coupling. Overall, our findings provide new strategies for improving magnetic stability of two-dimensional magnetic materials and imprinting magnetism into other nonmagnetic materials.

Spin filtering induced by a magnetic insulator stripe on graphene

Proximity exchange interaction between graphene electrons and nearby magnetic insulators paves the way to create spin-polarised currents for spintronics applications. Different ferro- and ferrimagnetic insulators, such as europium chalcogenides, yttrium iron garnet and cobalt ferrite, have been proposed in the literature to induce magnetic correlations in graphene. We theoretically study electronic transport properties of graphene in close proximity to a strip of a magnetic insulator, when the system is connected to nonmagnetic source and drain leads. To this end, we describe graphene electrons by means of an effective Hamiltonian whose model parameters are extracted from first-principle calculations. We compare the spin-polarization of the electron current calculated for a number of different magnetic insulators, aiming at elucidating the effects of the various model parameters on the efficiency of the device. In particular, we demonstrate that the polarization of the electric current across the device can be tuned by the source–drain voltage. We conclude that the heterostructures based on europium chalcogenides are ideal candidates to achieve high polarisation at low temperature.

Nano-chevron quantum dot for spin-qubit applications

We study the theoretical properties of a parabolic hBN/MoS2/hBN heterostructure quantum dot potential generated via electrostatic gates and its interaction with a cobalt nano chevron. We demonstrate that such an example system can undergo electric dipole spin resonance for a single electron isolated to the K' valley within the MoS2 monolayer, and such a system can achieve pi-rotation times of approximately 5.5 ns under the influence of a 20.89 GHz driving field. Our proposed system requires operating conditions easily achievable with current experimental methods and would allow for the all-electrical control of a spin-qubit within an MoS2 device. Our results show that such a system is experimentally feasible and would have comparable properties to that of more traditional silicon based spin-qubits. Furthermore, the design of the device can be applied to other material systems beyond MoS2 and cobalt. In theory, the proposed structure could make use of any 2D material that experiences strong proximity exchange interactions with other magnetic materials, which makes our proposed design highly general.

Bandgap opening and magnetic anisotropy switching by uniaxial strain in graphene/CrI3 heterojunction

Two-dimensional van der Waals heterojunctions are significant building blocks for optoelectronic and spintronic devices. By first-principle calculations, we study the influence of uniaxial strain on the bandgap and magnetism of graphene/CrI3 heterojunction. The results show that the bandgap size of graphene in heterojunction can reach up to 0.5 eV, and it increases more obvious at the compressive strain relative to the isolated graphene resulting from stronger interlayer proximity exchange. Interestingly, we find that the proximity exchange can induce charge accumulation like a dumbbell between the nearest neighbor C–C atoms along the armchair direction. Unexpectedly, the uniaxial strain induces the magnetic anisotropy switching from out-of-plane to in-plane, while this change does not occur in isolated CrI3. Our findings can provide theoretical guidance for the application of graphene/CrI3 materials.

Quantum Oscillations of Gilbert Damping in Ferromagnetic/Graphene Bilayer Systems

We study the spin dynamics of a ferromagnetic insulator on which graphene is placed. We show that the Gilbert damping is enhanced by the proximity exchange coupling at the interface. The modulation of the Gilbert damping constant is proportional to the product of the spin-up and spin-down densities of states of graphene. Consequently, the Gilbert damping constant in a strong magnetic field oscillates as a function of the external magnetic field that originates from the Landau level structure of graphene. We find that a measurement of the oscillation period enables the strength of the exchange coupling constant to be determined. The results demonstrate in theory that the ferromagnetic resonance measurements may be used to detect the spin resolved electronic structure of the adjacent materials, which is critically important for future spin device evaluations.

Giant proximity exchange and valley splitting in transition metal dichalcogenide/ hBN /(Co, Ni) heterostructures

We investigate the proximity-induced exchange coupling in transition-metal dichalcogenides (TMDCs), originating from spin injector geometries composed of hexagonal boron-nitride (hBN) and ferromagnetic (FM) cobalt (Co) or nickel (Ni), from first-principles. We employ a minimal tight-binding Hamiltonian that captures the low energy bands of the TMDCs around K and K' valleys, to extract orbital, spin-orbit, and exchange parameters. The TMDC/hBN/FM heterostructure calculations show that due to the hBN buffer layer, the band structure of the TMDC is preserved, with an additional proximity-induced exchange splitting in the bands. We extract proximity exchange parameters in the 1--10 meV range, depending on the FM. The combination of proximity-induced exchange and intrinsic spin-orbit coupling (SOC) of the TMDCs, leads to a valley polarization, translating into magnetic exchange fields of tens of Tesla. The extracted parameters are useful for subsequent exciton calculations of TMDCs in the presence of a hBN/FM spin injector. Our calculated absorption spectra show large splittings for the exciton peaks; in the case of MoS$_2$/hBN/Co we find a value of about 8 meV, corresponding to about 50 Tesla external magnetic field in bare TMDCs. The reason lies in the band structure, where a hybridization with Co $d$ orbitals causes a giant valence band exchange splitting of more than 10 meV. Structures with Ni do not show any $d$ level hybridization features, but still sizeable proximity exchange and exciton peak splittings of around 2 meV are present in the TMDCs.

Layer-resolved magnetic proximity effect in van der Waals heterostructures.

Magnetic proximity effects are integral to manipulating spintronic1,2, superconducting3,4, excitonic5 and topological phenomena6-8 in heterostructures. These effects are highly sensitive to the interfacial electronic properties, such as electron wavefunction overlap and band alignment. The recent emergence of magnetic two-dimensional materials opens new possibilities for exploring proximity effects in van der Waals heterostructures9-12. In particular, atomically thin CrI3 exhibits layered antiferromagnetism, in which adjacent ferromagnetic monolayers are antiferromagnetically coupled9. Here we report a layer-resolved magnetic proximity effect in heterostructures formed by monolayer WSe2 and bi/trilayer CrI3. By controlling the individual layer magnetization in CrI3 with a magnetic field, we show that the spin-dependent charge transfer between WSe2 and CrI3 is dominated by the interfacial CrI3 layer, while the proximity exchange field is highly sensitive to the layered magnetic structure as a whole. In combination with reflective magnetic circular dichroism measurements, these properties allow the use of monolayer WSe2 as a spatially sensitive magnetic sensor to map out layered antiferromagnetic domain structures at zero magnetic field as well as antiferromagnetic/ferromagnetic domains at finite magnetic fields. Our work reveals a way to control proximity effects and probe interfacial magnetic order via van der Waals engineering13.

Excitonic magneto-optical Kerr effect in two-dimensional transition metal dichalcogenides induced by spin proximity

In this paper we develop the excitonic theory of Kerr rotation angle in a two-dimensional (2D) transition metal dichalcogenide at zero magnetic field. The finite Kerr angle is induced by the interplay between spin-orbit splitting and proximity exchange coupling due to the presence of a ferromagnet. We compare the excitonic effect with the single particle theory approach. We show that the excitonic properties of the 2D material lead to a dramatic change in the frequency dependence of the optical response function. We also find that the excitonic corrections enhance the optical response by a factor of two in the case of MoS2 in proximity to a Cobalt thin film.

Proximity exchange coupling in a Fe/MgO/Si tunnel contact detected by the inverted Hanle effect

A ferromagnet is shown to exert an exchange field on the spin accumulation localized in the semiconductor interface region of a Fe/MgO/Si tunnel contact. The proximity exchange coupling across the MgO modifies spin precession of the spin accumulation and thereby produces detectable signatures in the inverted Hanle effect: exchange-induced shifts of the inverted Hanle curves, hysteresis, and discontinuities at the coercive field of the ferromagnet, at which the exchange field is reversed. The proximity exchange field is locked antiparallel to the magnetization of the ferromagnet, has values up to 380 Oe, and can be controlled by a bias voltage across the tunnel contact.

Spin-dependent electronic lenses based on hybrid graphene nanostructures

We study electronic transport in graphene/ferromagnetic insulator hybrid devices. The system comprises an armchair graphene nanoribbon with a lens-shaped EuO ferromagnetic insulator layer deposited on top of it. When the device supports a large number of propagating modes, the proximity exchange interaction of electrons with the magnetic ions of the ferromagnetic insulator results in electrons being spatially localised at different spots depending on their spin. We found the spin-dependent electron focusing is robust under moderate edge disorder. A spin-polarised electric current can be generated by placing a third contact in the proper place. This opens the possibility to use these effects for fabricating tunable sources of polarized electrons.

Proximity exchange effects in MoSe2 and WSe2 heterostructures with CrI3 : Twist angle, layer, and gate dependence

Proximity effects in two-dimensional (2D) van der Waals heterostructures offer controllable ways to tailor the electronic band structure of adjacent materials. Exchange proximity in particular is important for making materials magnetic without hosting magnetic ions. Such synthetic magnets could be used for studying magnetotransport in high-mobility 2D materials, or magneto-optics in highly absorptive nominally nonmagnetic semiconductors. Using first-principles calculations, we show that the proximity exchange in monolayer MoSe$_2$ and WSe$_2$ due to ferromagnetic monolayer CrI$_3$ can be tuned (even qualitatively) by twisting and gating. Remarkably, the proximity exchange remains the same when using antiferromagnetic CrI$_3$ bilayer, paving the way for optical and electrical detection of layered antiferromagnets. Interestingly, the proximity exchange is opposite to the exchange of the adjacent antiferromagnetic layer. Finally, we show that the exchange proximity is confined to the layer adjacent to CrI$_3$, and that adding a separating hBN barrier drastically reduces the proximity effect. We complement our it ab initio results with tight-binding modeling and solve the Bethe-Salpeter equation to provide experimentally verifiable optical signatures (in the exciton spectra) of the proximity exchange effects.

Klein tunnelling and Hartman effect in graphene junctions with proximity exchange field

Tunnelling of electrons in graphene-based junctions is studied theoretically. Graphene is assumed to be deposited either directly on a ferromagnetic insulator or on a few atomic layers of boron nitride which separate graphene from a metallic ferromagnetic substrate. Such junctions can be formed by appropriate external gating of the corresponding system. To describe low-energy electronic states near the Dirac points, certain effective Hamiltonians available in the relevant literature are used. These Hamiltonians include staggered potential and exchange interaction due to ferromagnetic substrates. Tunnelling in the systems under consideration is then spin-dependent. The main focus is on Klein tunnelling and also on the group delay and the associated Hartman effect. The impact of a gap induced in the spectrum at the Dirac points on tunnelling is analysed in detail.

Electrically tunable exchange splitting in bilayer graphene on monolayer Cr2X2Te6 with X = Ge, Si, and Sn

We investigate the electronic band structure and the proximity exchange effect in bilayer graphene (BLG) on a family of ferromagnetic multilayers Cr2X2Te6, X = Ge, Si, and Sn, with first principles methods. In each case the intrinsic electric field of the heterostructure induces an orbital gap on the order of 10 meV in the graphene bilayer. The proximity exchange is strongly band-dependent. For example, in the case of Cr2Ge2Te6, the low energy valence band of BLG has exchange splitting of 8 meV, while the low energy conduction band’s splitting is 30 times less (0.3 meV). This striking discrepancy stems from the layer-dependent hybridization with the ferromagnetic substrate. Remarkably, applying a vertical electric field of a few V nm–1 reverses the exchange, allowing us to effectively turn ON and OFF proximity magnetism in BLG. Such a field-effect should be generic for van der Waals bilayers on ferromagnetic insulators, opening new possibilities for spin-based devices.

Proximity exchange induced gap opening and topological feature in graphene/1T′-MX2 (M = Mo,W; X = S,Se,Te) Dirac heterostructures

By using first-principles calculations, we demonstrate the influence of proximity effect on the band structures of heterostructures formed by graphene stacking on a two dimensional (2D) topological insulator (TI) 1T′-MX2. The interlayer distance d between graphene and TI decreases with the enhancement of the intrinsic lattice anisotropy of 1T′-MX2, which determines different strength of the interlayer proximity interaction. The bandgap can be opened by the proximity exchange. The weak anisotropic symmetry of heterostructure (large d) only results in a small band gap (∼50 meV) in graphene/MoTe2. However, a large energy gap (up to ∼200 meV) can be obtained in graphene/MoS2, which is attributed to the inter-intralayer charge transfer due to the strong proximity interaction of the hetero-interface (small d). In addition, the 1T′-MX2 of heterostructure still possesses the topological feature of Z2 = 1, since the graphene has a negligible effect on the band structure of the system.

The Ukraine conflict and the European media: A comparative study of newspapers in 13 European countries

The crisis in Ukraine was one of the dominant topics in international news coverage of 2014 and the following years. Representing a conflict along the lines of an East-Western confrontation unprecedented since the end of the Cold War, the news reporting in different European countries with different historical backgrounds is an essential research topic. This article presents findings of a content analysis examining coverage of the conflict in the first half of 2014 in newspapers from a diverse set of 13 countries: Albania, Czech Republic, Germany, Latvia, the Netherlands, Poland, Portugal, Romania, Serbia, Switzerland and the United Kingdom, as well as Ukraine and Russia. Drawing on prior literature on news values, key events, and news cycles in foreign coverage, this study maps the evolution of the conflict in the course of four key events and identifies specific characteristics of the coverage in different newspapers. The results show that attention for the conflict varies considerably across the countries, which might be traced back to different degrees of geographical and cultural proximity, domestication, and economic exchange, as well as lack of editorial resources especially in Eastern Europe. Russia dominated the news agenda in all newspapers under study with a constant stream of conflict news. Contradicting prior literature, media sought to contextualise the events, and meta-coverage of the media’s role in the crisis emerged as a relevant topic in many countries with a developed media system.

Magnetic proximity effect in Pt/CoFe2O4 bilayers

We observe the magnetic proximity effect (MPE) in Pt/CoFe2O4 bilayers grown by molecular beam epitaxy. This is revealed through angle-dependent magnetoresistance measurements at 5 K, which isolate the contributions of induced ferromagnetism (i.e. anisotropic magnetoresistance) and spin Hall effect (i.e. spin Hall magnetoresistance) in the Pt layer. The observation of induced ferromagnetism in Pt via AMR is further supported by density functional theory calculations and various control measurements including insertion of a Cu spacer layer to suppress the induced ferromagnetism. In addition, anomalous Hall effect measurements show an out-of-plane magnetic hysteresis loop of the induced ferromagnetic phase with larger coercivity and larger remanence than the bulk CoFe2O4. By demonstrating MPE in Pt/CoFe2O4, these results establish the spinel ferrite family as a promising material for MPE and spin manipulation via proximity exchange fields.

Dirac-electron-mediated magnetic proximity effect in topological insulator/magnetic insulator heterostructures

The possible realization of dissipationless chiral edge current in a topological insulator / magnetic insulator heterostructure is based on the condition that the magnetic proximity exchange coupling at the interface is dominated by the Dirac surface states of the topological insulator. Here we report a polarized neutron reflectometry observation of Dirac electrons mediated magnetic proximity effect in a bulk-insulating topological insulator (Bi$_{0.2}$Sb$_{0.8}$)$_{2}$Te$_{3}$ / magnetic insulator EuS heterostructure. We are able to maximize the proximity induced magnetism by applying an electrical back gate to tune the Fermi level of topological insulator to be close to the charge neutral point. A phenomenological model based on diamagnetic screening is developed to explain the suppressed proximity induced magnetism at high carrier density. Our work paves the way to utilize the magnetic proximity effect at the topological insulator/magnetic insulator hetero-interface for low-power spintronic applications.

Proximity Band Structure and Spin Textures on Both Sides of Topological-Insulator/Ferromagnetic-Metal Interface and Their Charge Transport Probes.

The control of recently observed spintronic effects in topological-insulator/ferromagnetic-metal (TI/FM) heterostructures is thwarted by the lack of understanding of band structure and spin textures around their interfaces. Here we combine density functional theory with Green's function techniques to obtain the spectral function at any plane passing through atoms of Bi2Se3 and Co or Cu layers comprising the interface. Instead of naively assumed Dirac cone gapped by the proximity exchange field spectral function, we find that the Rashba ferromagnetic model describes the spectral function on the surface of Bi2Se3 in contact with Co near the Fermi level EF0, where circular and snowflake-like constant energy contours coexist around which spin locks to momentum. The remnant of the Dirac cone is hybridized with evanescent wave functions from metallic layers and pushed, due to charge transfer from Co or Cu layers, a few tenths of an electron-volt below EF0 for both Bi2Se3/Co and Bi2Se3/Cu interfaces while hosting distorted helical spin texture wounding around a single circle. These features explain recent observation of sensitivity of spin-to-charge conversion signal at TI/Cu interface to tuning of EF0. Crucially for spin-orbit torque in TI/FM heterostructures, few monolayers of Co adjacent to Bi2Se3 host spectral functions very different from the bulk metal, as well as in-plane spin textures (despite Co magnetization being out-of-plane) due to proximity spin-orbit coupling in Co induced by Bi2Se3. We predict that out-of-plane tunneling anisotropic magnetoresistance in Cu/Bi2Se3/Co vertical heterostructure can serve as a sensitive probe of the type of spin texture residing at EF0.

Strong Modulation of Spin Currents in Bilayer Graphene by Static and Fluctuating Proximity Exchange Fields.

Two-dimensional materials provide a unique platform to explore the full potential of magnetic proximity-driven phenomena, which can be further used for applications in next-generation spintronic devices. Of particular interest is to understand and control spin currents in graphene by the magnetic exchange field of a nearby ferromagnetic material in graphene-ferromagnetic-insulator (FMI) heterostructures. Here, we present the experimental study showing the strong modulation of spin currents in graphene layers by controlling the direction of the exchange field due to FMI magnetization. Owing to clean interfaces, a strong magnetic exchange coupling leads to the experimental observation of complete spin modulation at low externally applied magnetic fields in short graphene channels. Additionally, we discover that the graphene spin current can be fully dephased by randomly fluctuating exchange fields. This is manifested as an unusually strong temperature dependence of the nonlocal spin signals in graphene, which is due to spin relaxation by thermally induced transverse fluctuations of the FMI magnetization.

Effect of intervalley interaction on band topology of commensurate graphene/EuO heterostructures

In a heterostructure of graphene and the ferromagnetic insulator EuO, the Eu atoms induce proximity exchange and inter-valley interactions in the graphene layer. Constrained by the lattice symmetries, and guided by ab initio calculations, a model Hamiltonian is constructed that describes the low-energy bands. Band parameters such as proximity induced exchange splitting, spin orbit coupling, and inter-valley interaction are determined. Calculations of the Chern number identify the conditions under which the hetero-structures exhibit topologically non-trivial bands that give rise to the quantum anomalous Hall effect with a Hall conductivity of $\sigma_{xy} = 2 e^2/h$.