Magnetic Proximity Research Articles

Efficient spin generation in graphene by magnetic proximity effect upon absorption of far-IR radiation

The magnetic proximity effect is significant for atomically thin layers of two-dimensional materials. In this paper, we study the mechanisms of photogeneration spin-polarized carriers in graphene on a magnetic insulator. The magnetic proximity effect and lowered symmetry at the interface enhance the spin response of graphene in the alternating electric field of the incident light. The first leads to spin splitting of the linear spectrum of Dirac electrons. The second increases the role of the spin-orbit interaction. The main mechanisms of photogenerated spin polarization have been considered, including spin flip intersubband and interband transitions, and their contribution to the absorption coefficient of graphene. Keywords: Dirac electrons, graphene, electric dipole spin resonance, spin-orbit coupling, spin generation.

Tailoring anomalous Hall effect by spin–orbit coupling in epitaxial Au/Fe4N bilayers

Anomalous Hall effect (AHE) is one of the most fascinating topics in condensed matter physics related to spin–orbit coupling (SOC). In this paper, we report on the AHE of high-quality epitaxial Au/Fe4N bilayer films, which were grown by a plasma-assisted molecular beam epitaxy system. A scaling involving multiple competing scattering mechanisms and a shunting model were adopted to analyze the AHE in detail. Compared with Fe4N single layers and Cu/Fe4N bilayers, the AHE of Au/Fe4N bilayers is dramatically modified by strong SOC of the Au layer. Analysis has shown that aside from extra scatterings from Au atoms that diffused from an Au layer to a Fe4N layer, both spin Hall effect of Au and magnetic proximity effect near the Au/Fe4N interface contribute to the modification of the AHE. Variation of coercivity with the change of current, which could be attributed to spin–orbit torque, implies that the measured AHE is a combination of the AHE of Fe4N and strong SOC of Au.

Stacking and tuning effects on magneto-electronic and electric contact features for arsenene/Fe3GeTe2 van der Waals heterostructure

Recently, experimentally available Fe3GeTe2 (FGT) monolayer has attracted tremendous research interest due to its long-range ferromagnetic (FM) order. Here, we systematically study the magneto-electronic and electric contact properties of FGT-based van der Waals (vdW) heterostructures integrated by arsenene (As) with multiple stacking patterns. The low binding energy (−117.69 to −52.69 meV atom−1) proves their highly geometric stability, and the high magnetized energy (91.61–213.61 meV/unit cell) and magnetic exchange energy (64.53–84.43 meV/unit cell) provide a strong evidence for magnetism stability. Particularly, the magnetic proximity exchange effect as well as spin transfer and spin rearrangement can enhance the FM coupling in heterostructures, so that the highest Curie temperature T C = 173.93 K is achieved, being 19.73% higher than isolated FGT. Meanwhile, the arsenene is magnetized as well. In particular, metal-induced gap states appear in band gap of original As monolayer. Besides, metal-semiconductor contact performance is improved in constructed heterostructures by lowering electronic Schottky barrier height to nearly obtain Ohmic contact. Magnetic exchange energy rises further to improve magnetic stability with the interlayer spacing narrowed, and T C is increased up to 184.80 K, an increase of 27.21% compared to FGT monolayer. Being attributed to the proximity exchange effect strengthened with a shrunk interlayer spacing, and the electric contact behavior is also boosted further by this compressive strain to realize high-performance electric junction. Our findings provide a new route to extend the applications of FGT by constructing suitable vdW heterostructures.

Asymmetric magnetic proximity interactions in MoSe2/CrBr3 van der Waals heterostructures.

Magnetic proximity interactions between atomically thin semiconductors and two-dimensional magnets provide a means to manipulate spin and valley degrees of freedom in non-magnetic monolayers, without using applied magnetic fields1-3. In such van der Waals heterostructures, magnetic proximity interactions originate in the nanometre-scale coupling between spin-dependent electronic wavefunctions in the two materials, and typically their overall effect is regarded as an effective magnetic field acting on the semiconductor monolayer4-8. Here we demonstrate that magnetic proximity interactions in van der Waals heterostructures can in fact be markedly asymmetric. Valley-resolved reflection spectroscopy of MoSe2/CrBr3 van der Waals structures reveals strikingly different energy shifts in the K and K' valleys of the MoSe2 due to ferromagnetism in the CrBr3 layer. Density functional calculations indicate that valley-asymmetric magnetic proximity interactions depend sensitively on the spin-dependent hybridization of overlapping bands and as such are likely a general feature of hybrid van der Waals structures. These studies suggest routes to control specific spin and valley states in monolayer semiconductors9,10.

Proximity induced longitudinal and transverse thermoelectric response in graphene-ferromagnetic CrBr3 vdW heterostructure

The integration of longitudinal and transverse thermoelectric (TE) fosters various new opportunities in tuning the charge transport behaviour and opens a platform for efficient thermopower devices. The presence of asymmetric electronic structure supposed to accomplish large thermopower and electronic figure of merit. Herein, we investigate magnetic proximity coupled longitudinal and transverse TE behaviour in heterostructure of monolayer semimetal, graphene and a monolayer ferromagnet, CrBr3 under the framework of ab initio-based calculations and employed constant relaxation time approximation (CRTA).The integrated density of states is elevated and asymmetric near Fermi energy region due to seamless proximity integration, depicting mixed character of graphene and CrBr3. The asymmetric nature of electronic structure significantly affects the Seebeck coefficients (S) and electrical conductivity (σ/τ) of heterostructure. The consistent step-like conductance spectrum influences interfacial polarization due to agile proximity integration. The magnitude of Seebeck coefficient (S) is found to be 653 µV K−1 near Fermi level. The heterostructure observes higher electrical conductivity and power factor in n-type region of the order of 106 S m−1 and 1020 cm−3 at room temperature. The dimensionless electronic figure of merit (zT e) advocates the heterostructure system to be an ideal TE material. Alongside longitudinal TE, we also find the heterostructure system is sensitive to anomalous Nernst effect (ANE) (transverse TE) with oscillatory nature. The Seebeck and ANE shows high degree of tunability with applied external electric field. The synergistic existence of Seebeck and ANE due to proximity integration in van der Waals atomic crystal at room temperature will provide realistic approach to experimentally fabricate and develop real-time thermopower devices.

Exchange bias and interface-related effects in two-dimensional van der Waals magnetic heterostructures: Open questions and perspectives

The exchange bias (EB) effect is known as a fundamentally and technologically important magnetic property of a magnetic bilayer film. It is manifested as a horizontal shift in a magnetic hysteresis loop of a film subject to cooling in the presence of a magnetic field. The EB effect in van der Waals (vdW) heterostructures offers a novel approach for tuning the magnetic properties of the newly discovered single-layer magnets, as well as adds a new impetus to magnetic vdW heterostructures. Indeed, intriguing EB effects have recently been reported in a variety of low-dimensional vdW magnetic systems ranging from a weakly interlayer-coupled vdW magnet (e.g., Fe3GeTe2) to a bilayer composed of two different magnetic vdW materials (e.g., Fe3GeTe2/CrCl3, Fe3GeTe2/FePS3, Fe3GeTe2/MnPS3, Fe3GeTe2/CrSe, Fe3GeTe2/CrOCl, Fe3GeTe2/CoPc, Fe5GeTe2/FePS3), to bilayers of two different vdW defective magnets (e.g., VSe2/MoS2), or to metallic ferromagnet/vdW defective magnet interfaces (e.g., Fe/MoS2). Despite their huge potential in spintronic device applications, the physical origins of the observed EB effects have remained elusive to researchers. We present here a critical review of the EB effect and associated phenomena such as magnetic proximity (MP) in various vdW heterostructure systems and propose approaches to addressing some of the emerging fundamental questions.

Tunable valley band and exciton splitting by interlayer orbital hybridization

Magnetic proximity effect has been demonstrated to be an effective routine to introduce valley splitting in two-dimensional van der Waals heterostructures. However, the control of its strength and the induced valley splitting remains challenging. In this work, taking heterobilayers combining monolayer MSe2 (M = Mo or W) with room-temperature ferromagnetic VSe2 as examples, we demonstrate that the valley splitting for both band edges and excitons can be modulated by the tuning of the interlayer orbital hybridization, achieved by inclusion of different amounts of exact Hartree exchange potential via hybrid functionals. Besides, we show such tuning of orbital hybridization could be experimentally realized by external strain and electric field. The calculations suggest that large valley band splitting about 30 meV and valley exciton splitting over 150 meV can be induced in monolayer MSe2. Our work reveals a way to control proximity effects and provides some guidance for the design of optoelectronic and valleytronic devices.

Giant gate-controlled odd-parity magnetoresistance in one-dimensional channels with a magnetic proximity effect

According to Onsager’s principle, electrical resistance R of general conductors behaves as an even function of external magnetic field B. Only in special circumstances, which involve time reversal symmetry (TRS) broken by ferromagnetism, the odd component of R against B is observed. This unusual phenomenon, called odd-parity magnetoresistance (OMR), was hitherto subtle (< 2%) and hard to control by external means. Here, we report a giant OMR as large as 27% in edge transport channels of an InAs quantum well, which is magnetized by a proximity effect from an underlying ferromagnetic semiconductor (Ga,Fe)Sb layer. Combining experimental results and theoretical analysis using the linearized Boltzmann’s equation, we found that simultaneous breaking of both the TRS by the magnetic proximity effect (MPE) and spatial inversion symmetry (SIS) in the one-dimensional (1D) InAs edge channels is the origin of this giant OMR. We also demonstrated the ability to turn on and off the OMR using electrical gating of either TRS or SIS in the edge channels. These findings provide a deep insight into the 1D semiconducting system with a strong magnetic coupling.

Atomic Force Manipulation of Single Magnetic Nanoparticles for Spin-Based Electronics.

Magnetic nanoparticles (MNPs) are instrumental for fabrication of tailored nanomagnetic structures, especially where top-down lithographic patterning is not feasible. Here, we demonstrate precise and controllable manipulation of individual magnetite MNPs using the tip of an atomic force microscope. We verify our approach by placing a single MNP with a diameter of 50 nm on top of a 100 nm Hall bar fabricated in a quasi-two-dimensional electron gas (q2DEG) at the oxide interface between LaAlO3 and SrTiO3 (LAO/STO). A hysteresis loop due to the magnetic hysteresis properties of the magnetite MNPs was observed in the Hall resistance. Further, the effective coercivity of the Hall resistance hysteresis loop could be changed upon field cooling at different angles of the cooling field with respect to the measuring field. The effect is associated with the alignment of the MNP magnetic moment along the easy axis closest to the external field direction across the Verwey transition in magnetite. Our results can facilitate experimental realization of magnetic proximity devices using single MNPs and two-dimensional materials for spin-based nanoelectronics.

Reversal of anomalous Hall conductivity by perpendicular electric field in 2D WSe2/VSe2 heterostructure

Anomalous Hall conductivity (AHC) and valley polarization are attracting tremendous interest in spintronics and valleytronics technologies. Here, we investigate the possibility of the electric field induced switching of the AHC and magnetic proximity effect induced valley polarization in the two-dimensional (2D) WSe2/1T-VSe2 heterostructure. Due to the small total energy difference, two stackings could happen (C-I and C-II). The WSe2 layer has a valley polarization of -19 meV in the C-II stacking, and it is further increased up to -28 meV under electric fields. Also, we obtain an AHC of 75 (80) S/cm in the C-I (II) stacking. We find a sign change from positive AHC to negative value under the electric field in hole doping of the C-II stacking. We attribute this reversal of the AHC to the electric field dependent Berry curvature variation. Our finding suggests that the electric field induced AHC switching can be possible in the 2D heterostructure.

2D Magnetic Semiconductor Fe3GeTe2 with Few and Single Layers with a Greatly Enhanced Intrinsic Exchange Bias by Liquid-Phase Exfoliation.

A 2D van der Waals (vdW) magnet can get rid of the constraints of lattice matching and compatibility and then create a variety of vdW heterostructures, which provides a opportunity for spintronic devices. However, the ability to reliably exfoliate large, high-quality vdW ferromagnetic Fe3GeTe2 (FGT) nanoflakes in scaled-up production is severely limited. Herein, an efficient and stable three-stage sonication-assisted liquid-phase exfoliation was developed for mass preparation of high-structural-integrity few- and single-layer FGT nanoflakes with a greatly enhanced intrinsic exchange bias. The three stages include slicing crystals, weakening interlayer vdW forces, and using ultrasonic cavitation. The highest yield of FGT nanoflakes is 22.3 wt % with single layers accounting for 6%. The size is controllable, and several micrometers, tens of micrometers, and a maximum of 103 μm are available. The 200 mg level output has overcome the limitations of mechanical exfoliation and molecular beam epitaxy in economically amplificated production. An intrinsic exchange bias is observed in the restacked nanoflakes due to the magnetic proximity on the interface of the FGT/natural surface oxide layer. The material reaches 578 Oe (2 K) and 2300 Oe after further oxidation, at least 250% higher than other precisely tailored vdW magnetic heterostructures. In addition, the unusual semiconductivity of the liquid-phase exfoliated FGT nanoflakes is reported. This work skillfully utilizes oxidation to enhance the potential of FGT for large-scale spintronics, optoelectronics, efficient data storage, and various extended applications, and it is beneficial for exfoliating other promising magnetic vdW materials.

Gate-Tunable Proximity Effects in Graphene on Layered Magnetic Insulators.

The extreme versatility of van der Waals materials originates from their ability to exhibit new electronic properties when assembled in close proximity to dissimilar crystals. For example, although graphene is inherently nonmagnetic, recent work has reported a magnetic proximity effect in graphene interfaced with magnetic substrates, potentially enabling a pathway toward achieving a high-temperature quantum anomalous Hall effect. Here, we investigate heterostructures of graphene and chromium trihalide magnetic insulators (CrI3, CrBr3, and CrCl3). Surprisingly, we are unable to detect a magnetic exchange field in the graphene but instead discover proximity effects featuring unprecedented gate tunability. The graphene becomes highly hole-doped due to charge transfer from the neighboring magnetic insulator and further exhibits a variety of atypical gate-dependent transport features. The charge transfer can additionally be altered upon switching the magnetic states of the nearest CrI3 layers. Our results provide a roadmap for exploiting proximity effects arising in graphene coupled to magnetic insulators.

State-of-the-Art Pulsed-High-Magnetic-Field Magnetometries Based on X-ray Magnetic Circular Dichroism and Proximity Detector Oscillator

State-of-the-Art Pulsed-High-Magnetic-Field Magnetometries Based on X-ray Magnetic Circular Dichroism and Proximity Detector Oscillator

Observation of nonlinear planar Hall effect in magnetic-insulator–topological-insulator heterostructures

Interfacing topological insulators (TIs) with magnetic insulators (MIs) has been widely used to study the interaction between topological surface states and magnetism. Previous transport studies typically interpret the suppression of weak antilocalization or appearance of the anomalous Hall effect as signatures of magnetic proximity effect (MPE) imposed to TIs. Here, we report the observation of nonlinear planar Hall effect (NPHE) in Bi2Se3 films grown on MI thulium and yttrium iron garnet (TmIG and YIG) substrates, which is an order of magnitude larger than that in Bi2Se3 grown on nonmagnetic gadolinium gallium garnet (GGG) substrate. The nonlinear Hall resistance in TmIG/Bi2Se3 depends linearly on the external magnetic field, while that in YIG/Bi2Se3 exhibits an extra hysteresis loop around zero field. The magnitude of the NPHE is found to scale inversely with carrier density. We speculate the observed NPHE is related to the MPE-induced exchange gap opening and out-of-plane spin textures in the TI surface states, which may be used as an alternative transport signature of the MPE in MI/TI heterostructures.

Gate-Tunable Anomalous Hall Effect in Stacked van der Waals Ferromagnetic Insulator-Topological Insulator Heterostructures.

The search of novel topological states, such as the quantum anomalous Hall insulator and chiral Majorana fermions, has motivated different schemes to introduce magnetism into topological insulators. A promising scheme is using the magnetic proximity effect (MPE), where a ferromagnetic insulator magnetizes the topological insulator. Most of these heterostructures are synthesized by growth techniques which prevent mixing many of the available ferromagnetic and topological insulators due to difference in growth conditions. Here, we demonstrate that MPE can be obtained in heterostructures stacked via the dry transfer of flakes of van der Waals ferromagnetic and topological insulators (Cr2Ge2Te6/BiSbTeSe2), as evidenced in the observation of an anomalous Hall effect (AHE). Furthermore, devices made from these heterostructures allow modulation of the AHE when controlling the carrier density via electrostatic gating. These results show that simple mechanical transfer of magnetic van der Waals materials provides another possible avenue to magnetize topological insulators by MPE.

Exchange bias toggling in amine-ended single-molecule magnetic junctions by contact geometry

The molecular scale magnetic proximity effect is proposed in single-molecule magnetic junctions (SMMJs) consisting of a dissociated amine-ended 1,4-benzenediamine (BDA) molecule coupled to two ferromagnetic Co electrodes. Our self-developed JunPy + Landau-Lifshitz-Gilbert simulation combined with first-principles calculation is employed to investigate the role of contact geometry in the magnetotransport properties of SMMJs with the choice of top, bridge, and hollow contact sites. The strong spinterface effect gives rise to distinct angular dependence of equilibrium field-like spin torque (FLST), asymmetric magnetic hysteresis loop and tunable exchange bias. From the analytical derivation of nonequilibrium Keldysh formalism, we believe that a promising way forward is to activate the multi-reflection process via the so-called molecular spinterface that will allow us to conquer as-yet unexplored magnetotransport properties of organic-based spintronics.

Reversible switching performance of water droplet-driven triboelectric nanogenerators using a magnetocontrollable lubricant-infused surface for sustainable power generation

Triboelectric nanogenerators (TENGs) are attracting great attention as potential renewable power sources; in particular, water droplet-driven liquid–solid (LS) TENGs are highly useful due to their abundant sources in daily life. This study developed a novel approach for switching the LS triboelectrification by using a magnetocontrollable lubricant-infused surface (MCLIS). The basic units of the MCLIS-based TENG (MCLIS-TENG), that is, magnetocontrollable microwires, showed different alignment states, i.e., vertically standing or lying down, depending on the direction of the applied magnetic field. These reversible wetting states generated distinctive voltage outputs of ∼2 V (ON state) and < 0.5 V (OFF state), correspondingly. ON/OFF cycles revealed excellent reversibility and stability even after 90 cycles. The switching characteristics of the MCLIS-TENG were studied systematically by varying the Weber number, inclination angle, and lubricant thickness. The proposed device also demonstrated highly sustainable power generation by utilizing the switchable wetting states even under high humidity, where the performance of most LS-TENGs degraded due to surface wetting problems. In addition, the MCLIS-TENG based self-powered magnetic proximity sensor is proposed as an exemplary application to detect the magnetic field intensity and the location of sensing object. This work provides a new idea of magnetoresponsive triboelectric switching, widening the TENG usability in low-power-consumption applications such as wireless switches and self-powered sensors.

Magnetic proximity effect of YIG/PtSe2

The magnetic proximity effect has pronounced influence on the transport properties of bilayer system with a heavy metal and a magnetic insulator. Here we first review the magneto-transport properties of non-magnetic heavy metal/magnetic insulator bilayer structure and emphasize the role played by magnetic proximity effect. Since there are no current shunting and Joule heating effects within the insulating layer, it is relatively easier to understand the physical origin and impact of the magnetic proximity response for these bilayers. In the second part of this article, we study magneto-transport properties of YIG/PtSe2. YIG is known to have large band gap and low magnetization damping coefficient and is a frequently used magnetic insulator, while PtSe2 belongs to the family of two dimensional (2D) transition metal dichalcogenides in which spin–orbit coupling is strong. We found that the spontaneous magnetization in PtSe2 has been induced by the proximity of YIG, which results in a negative MR. Moreover, the AMR and spin Hall MR are both existed in YIG/PtSe2 system. The value of Anomalous Hall resistance can reach 1 Ω, which is much greater than that of the planar Hall effect due to the spontaneous magnetization in PtSe2.

Using Different Smartphone Sensors to Find the Speed of a Toy Car

There are many examples of using smartphones in physics experiments. Students and teachers can easily perform many physics experiments with the help of smartphone sensors, even experiments for which high-cost materials are needed. There are different types of such experiments in which various kinds of sensors are used. Many physical quantities can be reliably measured with smartphone sensors after some calibration. One of the physical quantities that can be determined with these sensors is object speed. For example, the angular velocities of rotating objects have been measured with a smartphone light as well as proximity, acceleration, and magnetic field sensors. The speeds of light-emitting objects traveling on a straight path and inclined plane have also been determined with a smartphone light sensor. Ball velocities have also been determined with a smartphone oscilloscope app. In this study, we determine the speed of a toy car over a certain distance using smartphone magnetic field, light, and proximity sensors simultaneously with just one experiment.

Enhanced Anomalous Hall Effect of Pt on an Antiferromagnetic Insulator with Fully Compensated Surface

We report a significantly enhanced anomalous Hall effect (AHE) of Pt on antiferromagnetic insulator thin film (3-unit-cell La0.7Sr0.3MnO3, abbreviated as LSMO), which is one order of magnitude larger than that of Pt on other ferromagnetic (e.g. Y3Fe5O12) and antiferromagnetic (e.g. Cr2O3) insulator thin films. Our experiments demonstrate that the antiferromagnetic La0.7Sr0.3MnO3 with fully compensated surface suppresses the positive anomalous Hall resistivity induced by the magnetic proximity effect and facilitates the negative anomalous Hall resistivity induced by the spin Hall effect. By changing the substrate’s temperature during Pt deposition, we observed that the diffusion of Mn atoms into Pt layer can further enhance the AHE. The anomalous Hall resistivity increases with increasing temperature and persists even well above the Neel temperature (T N) of LSMO. The Monte Carlo simulations manifest that the unusual rise of anomalous Hall resistivity above T N originates from the thermal induced magnetization in the antiferromagnetic insulator.