Spin-polarized Current Research Articles

Néel Vector Auto-Oscillations and Reorientations Induced by Spin-Polarized Electric Currents in Antiferromagnetic Mn2Au Nanolayer

The electrical excitation of spin dynamics in antiferromagnets is important for the development of high-speed spintronic devices with a low power consumption. Here, we present a theoretical study of the spin dynamics generated in the Ni/Cu/Mn2Au/Cu/Ni nanostructure by a perpendicular-to-plane electric current. Our investigation includes atomistic simulations of spin reorientations in a few monolayer antiferromagnetic Mn2Au film and numerical calculations of nonequilibrium spin accumulation in the nanostructure comprising ferromagnetic Ni polarizers with antiparallel magnetizations. This combined approach enables us to quantify the dynamics of Mn magnetic moments induced by the spin-transfer torque created by the spin-polarized charge flow. The calculations show that the direct electric current with the density exceeding some temperature-dependent threshold value gives rise to steady-state auto-oscillations of the Néel vector in the [Formula: see text]-oriented Mn2Au nanolayer. As the precession of Mn magnetic moments occurs around an out-of-plane direction strongly deflected from their initial in-plane orientations, the emergence of auto-oscillations should be regarded as the realization of a dynamic spin reorientation transition in the antiferromagnetic crystal. Remarkably, the precession frequency rises with increasing current density J and exceeds 1[Formula: see text]THz at [Formula: see text][Formula: see text]A[Formula: see text]m[Formula: see text], which makes the described antiferromagnetic spin transfer nano-oscillator attractive for device applications. Furthermore, our simulations demonstrate that a picosecond current pulse injected into the Ni/Cu/Mn2Au/Cu/Ni nanostructure can induce a precessional switching of the Néel vector. Depending on the current density and pulse duration, the Néel vector rotates by either 90° or 180° and attains stable orientation along the corresponding [Formula: see text] easy axis in the plane of the Mn2Au nanolayer. This feature shows that the Ni/Cu/Mn2Au/Cu/Ni nanostructure could be used as a nonvolatile memory cell with the electrical writing and readout.

Dynamics of converting skyrmion bags with different topological degrees into skyrmions in synthetic antiferromagnetic nanotracks

Skyrmion bags are spin textures with any integer topological degree, which can be driven by spin-polarized currents and generate multiple skyrmions when passing through racetracks with special geometries. We have proposed three nanotrack configurations with different narrow channels on synthetic antiferromagnetic racetracks and investigated the dynamic process of current-induced conversion of skyrmion bags into skyrmions. We have found that when skyrmion bags enter narrow channels, they can be converted into magnetic domains, while when the driving force from spin-transfer torque is strong enough, the magnetic domains can break free from the pinning at the ends of channels and form skyrmions. Both the number of channels and driving current density affect the number of generated skyrmions. As the number of channels rises, magnetic domains split at the junctions of channels, forming more magnetic domains and producing more skyrmions. Furthermore, the number of generated skyrmions is also related to the quantity, arrangement, and interaction forces of inner antiskyrmions. When the number of channels remains constant, the number of antiskyrmions only affects the transition of skyrmion bags to magnetic domains and does not affect the movement of magnetic domains or the transition of magnetic domains to skyrmions. The maximum of generated skyrmions in nanotracks with triple channels reaches 9. Dzyaloshinskii–Moriya interaction and anisotropy may play an important role in the structural stability of skyrmion bags, which can affect the splitting behavior of skyrmion bags. This work is beneficial for the design of artificial synapses and the application of neuromorphic computing based on skyrmion bags.

Spin polarization and transport of hybrid interface states in π-stacked magnetic molecular junctions

Based on the first-principles method, the spin polarization and transport properties of the hybrid interface states (HIS) in π-stacked three-layer triphenylene molecular junction are studied theoretically. By rotating the torsion angle between two neighbor layers, the π-π interaction strength is adjusted. The results demonstrate that the HIS, generated by strong pz-d hybridization between molecules and ferromagnetic electrodes, can easily transfer to the central layer via a strong π-π interaction with similar spin polarization. A weak π-π interaction will block the transfer of the HIS, where the discrete molecular orbital states dominate the central molecule. However, an enhancement of the spin polarization for the molecular frontier orbital states is observed. The transport calculation shows that efficient transport of the HIS is realized in the π-stacked system, where the largest current appears at a strong π-π interaction but a significant current spin polarization appears at a weak π-π interaction. This work deepens our understanding of the role of the HIS in π-stacked organic systems.

Bias-induced magnetic to nonmagnetic transition in polyacene junctions

By means of the first-principles method, the bias effect on the magnetism of polyacene (n-acene) connected to gold electrodes is investigated. A magnetic to nonmagnetic transition for the polyacene (n > 6) is observed when the bias exceeds a critical value. The mechanism is explored as the bias-induced variation of electronic localization, which leads to the exchange of dominant mechanism for molecular magnetism from Columbic interaction between electrons to electron hopping rate. A significant enhancement of the differential conductance and suppression of current spin polarization for the molecular device are also obtained accompanied by the transition of molecular magnetism. This work proposes a feasible way to manipulate the magnetism of polyacene via electric method and reveals the relation between molecular magnetism and its conductance.

Real-Space Topology-Engineering of Skyrmionic Spin Textures in a van der Waals Ferromagnet Fe3GaTe2.

Realizing magnetic skyrmions in two-dimensional (2D) van der Waals (vdW) ferromagnets offers unparalleled prospects for future spintronic applications. The room-temperature ferromagnet Fe3GaTe2 provides an ideal platform for tailoring these magnetic solitons. Here, skyrmions of distinct topological charges are artificially introduced and engineered by using magnetic force microscopy (MFM). The skyrmion lattice is realized by a specific field-cooling process and can be further erased and painted via delicate manipulation of the tip stray field. The skyrmion lattice with opposite topological charges (S = ±1) can be tailored at the target regions to form topological skyrmion junctions (TSJs) with specific configurations. The delicate interplay of TSJs and spin-polarized device current were finally investigated via the in situ transport measurements, alongside the topological stability of TSJs. Our results demonstrate that Fe3GaTe2 not only serves as a potential building block for skyrmion-based spintronic devices, but also presents prospects for Fe3GaTe2-based heterostructures with the engineered topological spin textures.

Magnetic and electronic properties of 1D hybrid nanoobjects composed of alternating polycyclic hydrocarbon regions and double carbon chains

It has been proposed recently that 1D hybrid nanoobjects consisting of alternating double carbon chains and polycyclic carbon regions can be obtained from graphene nanoribbons of alternating width by electron irradiation. Here, based on density functional theory calculations, we show that magnetic and electronic properties of such nanoobjects can be changed dramatically by modifying the chain length and edge structure of polycyclic regions and this opens wide possibilities for spintronic applications. Nanoobjects composed of polycyclic regions with dangling bonds and even chains are found to behave as magnetic semiconductors that can generate spin-polarized currents. Band gaps of nanoobjects with odd chains change considerably upon switching between magnetic states making them promising for magnetic tunnel junctions. We also demonstrate that use of a hybrid exchange–correlation functional is important to properly describe stability of magnetic states, band gaps and synergistic effects of nanoobject components leading, for example, to magnetism in even chains.

First-principles investigation of electronic, magnetic, and optical properties of FeMnSb/GaZ (Z = As or P) interfaces

We systematically investigated the electronic, magnetic, and optical properties of the FeMnSb half-Heusler alloy, its (001) surface, and its interfaces with GaAs and GaP semiconductors by using first-principles calculations based on density functional theory. The bulk FeMnSb reveals its half-metallic ferrimagnetism with 100% spin-polarization. Extending this study to the (001) surface, we uncover distinct electronic and magnetic behaviors for Fe- and MnSb-terminated surfaces, the MnSb-terminated surface preserving the half-metallicity observed in the bulk material. Our evaluation of the interfaces exhibits positive work of separation, indicating that these interfaces are energetically favorable. The density of states analysis reveals that all interfaces exhibit a metallic nature. High spin-polarization values, particularly 97.316% for the Fe-Ga interface, suggest a substantial degree of spin-polarized current. Notably, the absorption coefficient peaks shift from the ultraviolet (UV) region in the bulk alloy to the visible region at the (001) surfaces. However, at the FeMnSb/GaAs and FeMnSb/GaP interfaces, the highest absorption peaks revert to the UV region, highlighting strong interfacial coupling effects. These results suggest the tunability of FeMnSb optical properties via surface termination and interface engineering, making it a promising candidate for spintronic and optoelectronic applications.

Spin-polarized thermoelectrical currents induced in Aharonov-Bohm rings

We propose an interferometric device that generates spin-polarized electric currents based on an Aharonov-Bohm ring geometry. The device is driven by a temperature gradient and a magnetic flux without any voltage bias. Remarkably, no ferromagnetic contact is necessary once the two-path interference effects are present combined with a spin-orbit interaction. A large amplitude of spin current signal occurs where the charge current is small or even vanishing, thus easily creating pure spin currents by applying a magnetic field. Our proposal provides an ingredient of information processing devices in a generic caloritronic setup. Published by the American Physical Society 2024

Ultrafast manipulations of nanoscale skyrmioniums

The advancement of next-generation magnetic devices depends on fast manipulating magnetic microstructures at the nanoscale. A universal method is presented for rapid and reliable generating, controlling, and driving nano-scale skyrmioniums, through high-throughput micromagnetic simulations. Ultrafast switches are realized between skyrmionium and skyrmion states and rapidly change their polarities in monolayer magnetic nanodisks by perpendicular magnetic fields. The transition mechanism by alternating magnetic fields differs from that under steady magnetic fields. New skyrmionic textures, such as flower-like and windmill-like skyrmions, are discovered. Moreover, this nanoscale skyrmionium can move rapidly and stably in nanoribbons using weaker spin-polarized currents. Explicit discussions are held regarding the physical mechanisms involved in ultrafast manipulations of skyrmioniums. This work provides further physical insights into the manipulation and application of topological skyrmionic structures for developing low-power consumption and nanostorage devices.

Electronic and Transport Engineering of A-Type Antiferromagnets with Ferroelectric Sandwich Structure: Toward Multistate Nonvolatile Memory Applications.

Achieving higher-order multistates with mutual interstate switching at the nanoscale is essential for high-density storage devices; yet, it remains a significant challenge. Here, we demonstrate that integrating A-type antiferromagnetic semiconductors sandwiched between ferroelectric layers is an effective strategy to achieve high-performance multistate data storage. Taking the Sc2CO2/VSi2P4 bilayer (bi-VSi2P4)/Sc2CO2 van der Waals multiferroic heterostructure as an example, our first-principles calculations show that by switching the polarization direction of the upper and bottom ferroelectric Sc2CO2 layers, antiferromagnetic bi-VSi2P4 can exhibit four distinct states with different band structures. The intriguing band structure engineering stems from the polarization-field-induced band shift and interface charge transfer. Accordingly, the proposed Sc2CO2/bi-VSi2P4/Sc2CO2-based multiferroic device can achieve four different resistance states, accompanied by fully spin-polarized currents and giant tunneling electroresistance ratios. Our results propose a viable strategy for realizing nonvolatile electrical control of antiferromagnets at the nanoscale and provide insights into the development of advanced memories.

M3XSe4 (M = V, Cr; X = S, Te) monolayers: Intrinsic high-temperature ferromagnetic semiconductors and half metals

Creating low dimensional ferromagnetic (FM) semiconductors or half metals with strong FM orders is promising to meet the requirement for next-generation spintronics. However, most of the demonstrated FM semiconductors or half metals suffer from low Curie temperatures (TCs). Here, by first-principles calculations, we predict that the two-dimensional (2D) M3XSe4 (M = V, Cr; X = S, Te) monolayers are a type of intrinsic 2D ferromagnets with thermodynamical stability. Our results show that V3XSe4 (X = S, Te) monolayers are FM semiconductors with indirect bandgaps of 0.60 and 0.50 eV, respectively. Particularly, both structures are revealed to have high TCs of 387 and 770 K and suppress the application limit of room-temperature. In addition, Cr3XSe4 (X = S, Te) monolayers are FM half metals with 100% spin-polarized currents. Moreover, the electronic and magnetic properties of these M3XSe4 monolayers can be modulated by biaxial strains. V3TeSe4 monolayer can be tuned to be room temperature direct bandgap semiconductor under biaxial 1% tensile strain, and TC of V3SSe4 can be largely enhanced under compressive strains. Our results suggest that M3XSe4 monolayers are promising candidates for spintronic devices.

Magnon excitation-induced spin memory loss in epitaxial L10-FePt/MgO/L10-FePt magnetic tunnel junctions

This work investigates the interplay between interfacial spin–orbit coupling (SOC) and magnon excitation-induced spin memory loss in epitaxial L10-FePt/MgO/L10-FePt magnetic tunnel junctions, which is crucial for advancing spintronic technologies. By employing systematic temperature-dependent transport measurements and inelastic electron tunneling spectroscopy, our study reveals that interfacial SOC at the Pt-terminated FePt/MgO interface significantly enhances magnon excitation during electron tunneling. This process results in a pronounced loss of spin memory in the spin-polarized current, diminishing the tunnel magnetoresistance ratio. Our findings provide critical insights into the mechanisms of spin memory loss, offering directions for optimizing spintronic device performance in the context of pronounced SOC environments.

Generation of out-of-plane polarized spin current by non-uniform oxygen octahedral tilt/rotation

The free-field switching of the perpendicular magnetization by the out-of-plane polarized spin current induced spin-orbit torque makes it a promising technology for developing high-density memory and logic devices. The materials intrinsically with low symmetry are generally utilized to generate the spin current with out-of-plane spin polarization. However, the generation of the out-of-plane polarized spin current by engineering the symmetry of materials has not yet been reported. Here, we demonstrate that paramagnetic CaRuO3 films are able to generate out-of-plane polarized spin current by engineering the crystal symmetry. The non-uniform oxygen octahedral tilt/rotation along film’s normal direction induced by oxygen octahedral coupling near interface breaks the screw-axis and glide-plane symmetries, which gives rise to a significant out-of-plane polarized spin current. This spin current can drive field-free spin-orbit torque switching of perpendicular magnetization with high efficiency. Our results offer a promising strategy based on crystal symmetry design to manipulate spin current and could have potential applications in advanced spintronic devices.

The electron resistance of a single skyrmion within ballistic approach

An alternative way of skyrmion quasi-particle detection is simulated at low voltage bias. The point contact (PC), attached to the strip with a Néel-type skyrmion, can detect it with a higher efficiency than a magnetic tunnel junction. The method is based on detecting the skyrmion via the ballistic magnetoresistance ratio (BRR). PC's resistance with skyrmion significantly differs from the one without it. BRR is estimated in the framework of the point contact model for two directions of spin-polarized current: perpendicular to the transport direction (case 1) and along one (case 2). Skyrmion's size is assumed to be around 3.6 nm in diameter—smaller, or comparable, to the mean free path of electrons, allowing it to utilize the ballistic transport approach. As a result, resistance values for the considered Néel type skyrmion within the related size are estimated as 157 Ω for case 1 and 452.2 Ω for case 2 with optimistic BRR 101.3% and 291.7%, respectively. BRR for case 2 is higher due to the spin-filtering effect. The method also has the potential to detect the skyrmion type, or other magnetic nano structures such as bimeron, domain wall (DW), etc.

Ferroelectricity-controlled magnetic ordering and spin photocurrent in NiCl2/GeS multiferroic heterostructures

Controlling magnetism solely through electrical means is indeed a significant challenge, yet holds great potential for advancing information technology. Herein, our investigation presents a promising avenue for electrically manipulating magnetic ordering within 2D van der Waals NiCl2/GeS heterostructures. These heterostructures, characterized by their unique magnetic-ferroelectric (FE) layer stacking, demonstrate spin-constrained photoelectric memory, enabling low-power electrical writing and non-destructive optical reading. The two orientations of the polarization in the GeS FE layer bring about changes in the ground state configuration, transitioning from ferromagnetic (FM) to antiferromagnetic (AFM) orderings within the NiCl2magnetic layer. Correspondingly, the light-induced charge transfer prompts either spin-polarized or unpolarized currents from the FM or AFM states, serving as distinct '1' or '0' states, and facilitating applications in logic processing and memory devices. This transition stems from the interplay of interfacial charge transfer mechanisms and the influence of the effective electric field (Eeff), bringing a non-volatile electric enhancement in the magnetic anisotropy energy within the NiCl2/GeS heterostructure. Overall, our study highlights the NiCl2/GeS heterostructure as an optimal candidate for realizing spin-dependent photoelectric memory, offering unprecedented opportunities for seamlessly integrating memory processing capabilities into a single device through the utilization of layered multiferroic heterostructures.

Evidence of an Off-Resonant Electronic Transport Mechanism in Helicenes.

Helical molecules have been proposed as candidates for producing spin-polarized currents, even at room conditions, due to their chiral asymmetry. However, describing their transport mechanism in single molecular junctions is not straightforward. In this work, we show the synthesis of two novel kinds of dithia[11]helicenes to study their electronic transport in break junctions among a series of three helical molecules: dithia[n]helicenes, with n = 7, 9, and 11 molecular units. Our experimental measurements and clustering-based analysis demonstrate low conductance values that remain similar across different applied voltages and molecules. Additionally, we assess the length dependence of the conductance for each helicene, revealing an exponential decay characteristic of off-resonant transport. This behavior is primarily attributed to the misalignment between the energy levels of the molecule-electrodes system. The length dependence trend described above is supported by ab initio calculations, further confirming an off-resonant transport mechanism.

Kinetomagnetism of chirality and its applications

Chiral functionalities exhibited by systems lacking any mirror symmetry encompass natural optical activity, magnetochiral effect, diagonal current-induced magnetization, chirality-selective spin-polarized current of charged electrons or neutral neutrons, self-inductance, and chiral phonons. These phenomena are unified under the hypothesis of “kinetomagnetism of chirality,” which posits that any moving (charged or neutral) object in chiral systems induces magnetization in its direction of motion, consequently imparting chirality to the object due to this induced magnetization. We also found conjugate relationships among the kinetomagnetism of chirality, “linear magnetoelectricity,” and “electric field-induced directional nonreciprocity,” highlighting their interconnections with magnetic, electric, and toroidal orders. The concept of the kinetomagnetism of chirality will be an essential basis for the theoretical understanding of known chiral phenomena, such as natural optical activity or chiral phonons, and also the discovery of unexplored chiral functionalities.

Thermal and Coherent Spin Pumping by Noncollinear Antiferromagnets.

Antiferromagnets attract much interest because of their potential for spintronic applications and open fundamental physics questions, but especially noncollinear antiferromagnets remain relatively unexplored. Here, we formulate the thermal and coherent pumping of spins in noncollinear antiferromagnets|normal metal bilayers. We find that the spin current polarization is a vector with components along both the Néel vector and net magnetic moment. The spin mixing conductance for the coherent spin pumping is a tensor with elements depending on the degree of noncollinearity and interface spin configuration. We explain the controversial sign problem of the antiferromagnetic spin Seebeck effect by interface effects and suggest that interface engineering may enhance the spin pumping efficiency.

Spin–orbit torque effect in silicon-based sputtered Mn3Sn film

Abstract Noncollinear antiferromagnet Mn3Sn has shown remarkable efficiency in charge–spin conversion, a novel magnetic spin Hall effect, and a stable topological antiferromagnetic state, which has resulted in great interest from researchers in the field of spin–orbit torque. Current research has primarily focused on the spin–orbit torque effect of epitaxially grown noncollinear antiferromagnet Mn3Sn films. However, this method is not suitable for large-scale industrial preparation. In this study, amorphous Mn3Sn films and Mn3Sn/Py heterostructures were prepared using magnetron sputtering on silicon substrates. The spin-torque ferromagnetic resonance measurement demonstrated that only the conventional spin–orbit torque effect generated by in-plane polarized spin currents existed in the Mn3Sn/Py heterostructure, with a spin–orbit torque efficiency of 0.016. Additionally, we prepared the perpendicular magnetized Mn3Sn/CoTb heterostructure based on amorphous Mn3Sn film, where the spin–orbit torque driven perpendicular magnetization switching was achieved with a lower critical switching current density (3.9×107 A/cm2) compared to Ta/CoTb heterostructure. This research reveals the spin–orbit torque effect of amorphous Mn3Sn films and establishes a foundation for further advancement in the practical application of Mn3Sn materials in spintronic devices.

Spin torque-driven electron paramagnetic resonance of a single spin in a pentacene molecule.

Control over quantum systems is typically achieved by time-dependent electric or magnetic fields. Alternatively, electronic spins can be controlled by spin-polarized currents. Here, we demonstrate coherent driving of a single spin by a radiofrequency spin-polarized current injected from the tip of a scanning tunneling microscope into an organic molecule. With the excitation of electron paramagnetic resonance, we established dynamic control of single spins by spin torque using a local electric current. In addition, our work highlights the dissipative action of the spin-transfer torque, in contrast to the nondissipative action of the magnetic field, which allows for the manipulation of individual spins based on controlled decoherence.