Spin-dependent Transport Research Articles

Exploring anisotropic phases and spin transport in perovskite heterostructures: Insights into 3d/5d interfaces for antiferromagnetic spintronics

Transition metal oxides (TMOs) demonstrate a broad spectrum of properties encompassing electronic correlations, anisotropic transport, magnetism, and optical behavior. The anisotropy arises from both intrinsic crystal symmetry and extrinsic factors like epitaxial strain and structural asymmetry at TMO interfaces. Weiss and Neel's work has elucidated anisotropic magnetic behavior in antiferromagnetic (AFM) materials. AFM TMOs exhibit unique magnetotransport behavior, including weak antilocalization (WAL) and anisotropic magnetoresistance (AMR). Understanding the magnetic structure and band topology in AFM perovskites and their interfaces enables the tailored design of materials for spintronics and energy conversion. In few interfaces lacking inversion symmetry, Rashba spin–orbit coupling (SOC) induces WAL, a quantum correction in conductivity in a two-dimensional electronic system. Electron accumulation and charge transfer across 3d, 5d transition metal-based perovskite interfaces affect WAL and AMR, as observed in 3d/3d and 3d/5d AFM heterostructures, respectively. Advancements in spintronics rely on exploring spin-dependent transport anisotropy. This review focuses on various scattering mechanisms, categorized as extrinsic and intrinsic, in anisotropic transport, particularly in 3d/5d AFM superlattices. The WAL scattering mechanism depends on both intrinsic factors related to Rashba SOC-induced band topology and extrinsic sources like spin impurities and lattice ions. Moreover, the investigation into AMR mechanisms involves the application of impurity-based extrinsic scattering models, which are aligned with the Rashba and Dresselhauss models on Fermi surfaces. This exploration specifically targets the interface of two-band insulators, exemplified by LaAlO3/SrTiO3 and LaVO3/KTaO3. Furthermore, this model achieves comprehensive coverage, extending its applicability to 3d/5d AFM heterostructures like LaMnO3/SrIrO3 and CaMnO3/CaIrO3. Additionally, the intrinsic scattering mechanism tied to Berry phase effects related to band topology is studied, focusing on the CaMnO3/CaIrO3 superlattice. Despite manipulation challenges stemming from reduced stray fields, AFM materials show potential in interface physics and applications within the realm of spintronics.

Controllable spin transport in a zigzag silicene nanoribbon embedding multiple rectangular quantum dots

Based on the recursive Green-function method together with Landauer–Büttiker formalism, the spin-dependent transport properties of electrons in a zigzag silicene nanoribbon embedding multiple rectangular quantum dots (QDs) are investigated. According to an analysis of the energy band under the periodically distributed electric field and exchange ferromagnetic field, the parallel exchange field induced by the ferromagnetic insulators eliminates the spin degeneracy, which leads to spin-polarized transport in the proposed structure. By tuning a periodic electric field, we found the relationship between the number of QDs and the splitting peak for conductance in the anti-parallel exchange field. We discover the population of electrons near QDs by calculating the local density of states. The effect of the geometry of periodic QDs on the shift of resonance peak is evaluated. The spin polarization is further explored for various configurations of electric field and exchange field in order to manipulate the spin filtering more effectively. The results provide an avenue to design a controllable spin bandpass filter with the modulation of electric field and exchange field.

Mechanism for Electrostatically Generated Magnetoresistance in Chiral Systems without Spin-Dependent Transport.

Significant attention has been drawn to electronic transport in chiral materials coupled to ferromagnets in the chirality-induced spin selectivity (CISS) effect. A large magnetoresistance (MR) is usually observed, which is widely interpreted to originate from spin (dependent) transport. However, there are severe discrepancies between the experimental results and the theoretical interpretations, most notably the apparent failure of the Onsager reciprocity relations in the linear response regime. We provide an alternative mechanism for the two terminal MR in chiral systems coupled to a ferromagnet. For this, we point out that it was observed experimentally that the electrostatic contact potential of chiral materials on a ferromagnet depends on the magnetization direction and chirality. The mechanism that we provide causes the transport barrier to be modified by the magnetization direction, already in equilibrium, in the absence of a bias current. This strongly alters the charge transport through and over the barrier, not requiring spin transport. This provides a mechanism that allows the linear response resistance to be sensitive to the magnetization direction and also explains the failure of the Onsager reciprocity relations. We propose experimental configurations to confirm our alternative mechanism for MR.

First-principle study of spin transport property in L10-FePd(001)/graphene heterojunction

In our previous work, we synthesized a metal/2D material heterointerface consisting of L10-ordered iron–palladium (FePd) and graphene (Gr) called FePd(001)/Gr. This system has been explored by both experimental measurements and theoretical calculations. In this study, we focus on a heterojunction composed of FePd and multilayer graphene referred to as FePd(001)/m-Gr/FePd(001), where m represents the number of graphene layers. We perform first-principles calculations to predict their spin-dependent transport properties. The quantitative calculations of spin-resolved conductance and magnetoresistance (MR) ratio (150%–200%) suggest that the proposed structure can function as a magnetic tunnel junction in spintronics applications. We also find that an increase in m not only reduces conductance but also changes transport properties from the tunneling behavior to the graphite π-band-like behavior. Additionally, we investigate the spin-transfer torque-induced magnetization switching behavior of our junction structures using micromagnetic simulations. Furthermore, we examine the impact of lateral displacements (sliding) at the interface and find that the spin transport properties remain robust despite these changes; this is the advantage of two-dimensional material hetero-interfaces over traditional insulating barrier layers such as MgO.

Electronic transport of an extended Ising chain with competing thermal fluctuation and magnetic ordering

In this study, we investigate the behavior of spin-dependent electronic transport in a magnetic nanowire with thermal random oriented moments, connected to two similar (anti-) ferromagnetic leads. The model is based on Green’s function technique within the nearest neighbor tight-binding approach in the framework of canonical ensemble. We analyze the electronic transmission and its variance in different ordered and disordered regimes at a finite temperature in an external magnetic field. The results indicate that at nonzero temperatures, thermal fluctuation of moment orientations decreases the system’s electronic conductance, leading to a smooth conductance-energy relationship. Depending on temperature and external magnetic field values as well as Ising exchange interaction strength, the system can transit to an ordered phase. Additionally, the larger length of the interacting part affects the transmission coefficient and amplifies its variance. This study sheds light on the intricate interplay between temperature, magnetic field, and moment orientations in the context of spin-dependent electronic transport through magnetic nanowires, offering valuable insights into the usage of such systems.

Ballistic transport and spin-dependent anomalous quantum tunneling in Rashba–Zeeman and bilayer graphene hybrid structures

In this work, we have studied the spin-dependent ballistic transport and anomalous quantum tunneling in bilayer graphene horizontally placed in between two Rashba–Zeeman (RZ) leads under external electric biasing. We investigated the transmission and conductance for the proposed system using scattering matrix formalism and the Landauer–Büttiker formula considering a double delta-like barrier under a set of experimentally viable parameters. We found that the transmission characteristics are notably different for up- and down-spin incoming electrons depending upon the strength of magnetization. Moreover, the transmission of up- and down-spin electrons is found to be magnetization orientation dependent. The maximum tunneling conductance can be achieved by tuning biasing energy and magnetization strength and choosing a material with suitable Rashba spin–orbit coupling (RSOC). This astonishing property of our system can be utilized in fabricating devices, such as spin filters. We found that the Fano factor of our system is 0.4 under strong magnetization conditions, while it reduces to 0.3 under low magnetization conditions. Moreover, we also noticed that the transmission and conductance significantly depend on the Rashba–Zeeman effect. Therefore, considering a suitable RZ material, the tunneling of the electrons can be tuned and controlled. Our result suggests that considering suitable strength and orientation of magnetization with moderate RSOC, one can obtain a different transmission probability for spin species under suitable biasing energy. These results indicate the suitability of the proposed system in fabrication of spintronic devices, such as spin filter, spin transistor, etc.

Molecular rectification induced by magnetization alignment in organic-ferromagnetic devices.

Spin-dependent transport in ferromagnet/organic-ferromagnet/ferromagnet junctions is investigated theoretically under different alignment of magnetization orientations. The results demonstrate a significant current rectification at low bias voltages, and the rectifying direction relies on the relative magnetization orientation in each component. The orbital analysis demonstrates two underlying mechanisms for the rectification, the slight structural asymmetry of the molecule from spin radicals and distinct spin match between conducting electrons and the magnetic molecule upon the reversal of bias. The latter is responsible for the strong low-bias rectification and relies on the magnetization alignment. The effects of parameter strength, temperature and size on the rectification are discussed. This work explores a new route to achieve high-performance molecular rectifiers operating at low bias with controlled rectifying direction.

Spin-dependent transport and spin transfer torque in a borophene-based spin valve.

This article presents a theoretical analysis of spin-dependent transport and spin-transfer torque in a borophene-based ferromagnetic/normal/ferromagnetic junction. This study focuses on borophene nanoribbons (BNRs) as a basis for spin valve numerical calculations for the investigation of conduction in both configurations where the magnetization vectors of the leads are parallel or antiparallel to each other (P and AP configurations, respectively), magnetoresistance (MR), and spin transfer torque (STT). The Landauer formalism and non-equilibrium Green's function (NEGF) approaches are used to derive the spin-dependent tunneling currents in the Magnetic Tunnel Junction (MTJ). The results of the calculations for a zigzag BNR show that the conductance is always larger than e2/h for the P configuration of lead magnetizations. For the AP configuration, the conductance becomes zero in specific energy ranges. A perfect MR plateau is found for the junction in the absence of disorder, which proves to be an excellent spin valve candidate. The variations of STT with Fermi energy and the relative angle between the magnetizations of two electrodes are studied for different strengths of ferromagnetic magnetization. The STT per unit bias voltage, as a function of Fermi energy, decreases significantly near the Dirac point energy. A sinusoidal oscillatory pattern can be evidently observed in the STT at unit bias voltage V versus the angle between the magnetizations of two electrodes, which amplifies as M increases. Borophene has unique properties, including low density and high hardness, heat resistance, and electrical conductance, which make it a promising candidate for spintronics. This article provides a comprehensive analysis of the spin-dependent properties of borophene and its potential applications in spintronics.

Two-dimensional half-metals MSi2N4 (M = Al, Ga, In, Tl) with intrinsic p-type ferromagnetism and ultrawide bandgaps.

Intrinsic half-metallic nanomaterials with 100% spin polarization are highly demanded for next-generation spintronic devices. Here, by using first-principles calculations, we have designed a class of new two-dimensional (2D) p-type half-metals, MSi2N4 (M = Al, Ga, In and Tl), which show high mechanical, thermal and dynamic stabilities. MSi2N4 not only have ultrawide electronic bandgaps for spin-up channels in the range of 4.05 to 6.82 eV but also have large half-metallic gaps in the range of 0.75 to 1.47 eV, which are large enough to prevent the spin-flip transition. The calculated magnetic moment is 1 μB per cell, resulting from polarized N1-px/py orbitals. Moreover, MSi2N4 possess robust long-range ferromagnetic orderings with Curie temperatures in the range of 35-140 K, originating from the interplay of N1-M-N1 superexchange interactions. Furthermore, spin dependent electronic transport calculations reveal 100% spin polarization. Our results highlight new promising 2D ferromagnetic half-metals toward future spintronic applications.

Efficient spin filtering through Fe4GeTe2-based van der Waals heterostructures.

Utilizing ab initio simulations, we study the spin-dependent electronic transport characteristics within Fe4GeTe2-based van der Waals heterostructures. The electronic density of states for both free-standing and device-configured Fe4GeTe2 (F4GT) confirms its ferromagnetic metallic nature and reveals a weak interface interaction between F4GT and PtTe2 electrodes, enabling efficient spin filtering. The ballistic transport through a double-layer F4GT with a ferromagnetic configuration sandwiched between two PtTe2 electrodes is predicted to exhibit an impressive spin polarization of 97% with spin-up electrons exhibiting higher transmission probability than spin-down electrons. Moreover, we investigate the spin transport properties of Fe4GeTe2/GaTe/Fe4GeTe2 van der Waals heterostructures sandwiched between PtTe2 electrodes to explore their potential as magnetic tunnel junctions in spintronic devices. The inclusion of monolayer GaTe as a 2D semiconducting spacer between F4GT layers results in a tunnel magnetoresistance of 487% at a low bias and decreases with increasing bias voltage. Overall, our findings underscore the potential of F4GT/GaTe/F4GT heterostructures in advancing spintronic devices based on van der Waals materials.

Spintronic performance of bent zigzag phosphorene nanoribbons: effects of mechanical deformation and gate voltage.

This study explores the spintronic properties of an innovative device incorporating in-plane bent zigzag phosphorene nanoribbons (ZPNRs). The device features ZPNRs with a channel length of 23.4 nm, bent into circular arcs with varying curvatures. We investigate the impact of mechanical deformation and gate voltage on the spin-dependent properties, including the density of states, transmission coefficients, and spin Seebeck coefficient (SSC). Our results demonstrate that the device exhibits a spin-semiconducting phase with tunable spin-splitting characteristics and spin-dependent transport properties, both of which are influenced by the curvature. An increase in the bending parameter markedly enhances spin splitting, leading to the SSC attaining values as high as 1.35 mV K-1. Moreover, the application of gate voltage further enhances both spin polarization and spin current. The significant impact of mechanical deformation and gate voltage on spintronic performance showcases the potential of bent ZPNRs for advanced applications.

Tunable multiple nonvolatile resistance states in a MnSe-based van der Waals multiferroic tunnel junction.

Two-dimensional (2D) van der Waals (vdW) multiferroic tunnel junctions (MFTJs) composed of a ferromagnetic metal and a ferroelectric barrier have controllable thickness and clean interface and can realize the coexistence of tunneling magnetoresistance (TMR) and tunneling electroresistance (TER). Therefore, they have enormous potential application in nonvolatile multistate memories. Here, using first principles combined with non-equilibrium Green's function method, we have systematically investigated the spin-dependent transport properties of Fe3GeTe2/MnSe/Fe3GeTe2 vdW MFTJs with various numbers of barrier layers. By controlling the polarization orientation of the ferroelectric barrier MnSe and the magnetization alignment of the ferromagnetic electrodes Fe3GeTe2, the MnSe-based MFTJs exhibit four nonvolatile resistance states, with the TMR (TER) becoming higher and reaching a maximum of 1.4 × 106% (4114%) as the MnSe layers increase from a bilayer to a tetralayer. Using asymmetric Cu and Fe3GeTe2 as the electrodes, the TER can be further improved from 349% to 618%. Moreover, there is a perfect spin filtering effect in these MFTJs. This work demonstrates the potential applications of MnSe-based devices in multistate nonvolatile memories and spin filters, which will stimulate experimental studies on layer-controllable spintronic devices.

Local symmetry-driven interfacial magnetization and electronic states in (ZnO)n/(w-FeO)n superlattices.

Superlattices constructed with the wide-band-gap semiconductor ZnO and magnetic oxide FeO, both in the wurtzite structure, have been investigated using spin-polarized first-principles calculations. The structural, electronic and magnetic properties of the (ZnO)n/(w-FeO)n superlattices were studied in great detail. Two different interfaces in the (ZnO)n/(w-FeO)n superlattices were identified and they showed very different magnetic and electronic properties. Local symmetry-driven interfacial magnetization and electronic states can arise from different Fe/Zn distributions at different interfaces or spin ordering of Fe in the superlattice. The local symmetry-driven interfacial magnetization and electronic states, originating either from different Fe/Zn distribution across interfaces I and II, or by spin ordering of Fe in the superlattice, can be identified. It was also found that, in the case of the ferromagnetic phase, the electrons are more delocalized for the majority spin but strongly localized for the minority spin, which resulted in interesting spin-dependent transport properties. Our results will pave the way for designing novel spin-dependent electronic devices through the construction of superlattices from semiconductors and multiferroics.

Spin Quantum Dot Light-Emitting Diodes Enabled by 2D Chiral Perovskite with Spin-Dependent Carrier Transport.

Chiral-induced spin selectivity (CISS) effect provides innovative approach to spintronics and quantum-based devices for chiral materials. Different from the conventional ferromagnetic devices, the application of CISS effect is potential to operate under room temperature and zero applied magnetic field. Low dimensional chiral perovskites by introducing chiral amines are beginning to show significant CISS effect for spin injection, but research on chiral perovskites is still in its infancy, especially on spin-light emitting diode (spin-LED) construction. Here, the spin-QLEDs enabled by 2D chiral perovskites as CISS layer for spin-dependent carrier injection and CdSe/ZnS quantum dots (QDs) as light emitting layer are reported. The regulation pattern of the chirality and thickness of chiral perovskites, which affects the circularly polarized electroluminescence (CP-EL) emission of spin-QLED, is discovered. Notably, the spin injection polarization of 2D chiral perovskites is higher than 80% and the CP-EL asymmetric factor (gCP-EL ) achieves up to 1.6×10-2 . Consequently, this work opens up a new and effective approach for high-performance spin-LEDs.

Modulation of spin Hall magnetoresistance in α-Fe2O3/Pt by uniaxial tensile strain

We demonstrate the modulation of spin Hall magnetoresistance (SMR) by uniaxial tensile strain at RT in heterostructures of antiferromagnetic insulating (0001)-oriented α-Fe2O3 (hematite) and Pt. The SMR signals consistent with previous reports are observed in α-Fe2O3/Pt without strain. Under tensile strains, we found that the magnetoresistance amplitude changes almost linearly with the strain amplitude. The observed magnetoresistance change is attributed to the strain modulation of SMR owing to the change in domain distribution via magnetoelastic couplings. The results provide a useful method for controlling the spin-dependent transport effect in antiferromagnetic-insulator/metal bilayers.

Spin-related thermoelectricity in a hybrid Aharonov–Bohm interferometer with the Rashba effect

In order to explore the interplay among heat, charge, and spin, we investigate the spin-related thermoelectricity of a hybrid double quantum dot Aharonov–Bohm interferometer with Rashba spin–orbit coupling (RSOC). Interestingly, the nonzero RSOC phase difference can induce significant spin-dependent thermoelectric transport. Not only can the strong violation of the Wiedemann-Franz law be obtained, but also a pure spin-Seebeck effect can be produced, which may serve as a pure spin-current generator. The influence of system parameters on thermoelectric coefficients (especially for spin counterparts) are analyzed in detail, and the underlying physics is further elucidated. Furthermore, the spin figure of merit can be much larger than the charge one, which, thus, provides a realistic possibility of engineering spin thermoelectric devices with high performance. These results obtained may be of interest for developing spin thermoelectric devices in the field of spin caloritronics.

First-principles study of two-dimensional half-metallic ferromagnetism in CrSiSe4 monolayer

Two-dimensional (2D) ferromagnetic (FM) half-metallic materials have attracted intensive attention due to their unique electronic and magnetic properties and potential applications in spintronic devices. In this study, we predicted a stable 2D half-metallic material monolayer CrSiSe4 using first-principles density functional theory. The structure, electronic and magnetic properties were systematically studied. The calculations show that the monolayer CrSiSe4 is a dynamically stable FM half-metallic material. The spin-dependent transport properties and the Curie temperature up to 239 K are demonstrated. The spin band gap of monolayer CrSiSe4 was about 0.83 eV by the the Heyd–Scuseria–Ernzerhof function calculation. The magnetic anisotropy energy of each Cr atom in the monolayer of CrSiSe4 is eV. When the applied biaxial tensile strain is greater than 2%, monolayer CrSiSe4 spin-up conduction band and valence band will show a band gap at the Fermi level, and the electronic properties change from a half-metal to a semiconductor. Thus, the monolayer CrSiSe4 can provide an ideal candidate material for exploring 2D magnetic and spintronics experiments.

Multilevel resistance states in van der Waals multiferroic tunnel junctions above room temperature

Multiferroic tunnel junctions (MFTJs) comprised of magnetic electrodes and a ferroelectric tunnel barrier have been emerging as promising candidates for nonvolatile memory applications. The recently discovered above room-temperature van der Waals (vdW) ferromagnet Fe3GaTe2 and ferroelectric α-In2Se3 provides possibilities for constructing room-temperature vdW MFTJs. In this work, by using first-principles calculations, we investigate the spin-dependent transport in vdW MFTJs with structure of Fe3GaTe2/bilayer α-In2Se3/Fe3GaTe2. We predict a giant tunneling magnetoresistance (TMR) high up to 10 000% by switching the magnetic alignments of Fe3GaTe2 and tunneling electroresistance (TER) exceeding 300% by controlling the ferroelectric configurations of bilayer α-In2Se3 in the proposed MFTJs. Furthermore, by introducing interface asymmetry and inserting monolayer of h-BN on one interface, the TMR and TER ratios of the MFTJs can be substantially enhanced. In consequence, the multi-level resistance states can be achieved by applying magnetic and electric field in asymmetric MFTJs. Our results highlight full vdW MFTJs for their potential applications in spintronic devices, particularly in the field of multilevel nonvolatile memories.

Double stranded antiferromagnetic helix as an efficient spin filter

We report a large degree of spin filtration in a double stranded (ds) antiferromagnetic helix (AFH) system. Generally spin channel separation is not possible when a magnetic system possesses a vanishing net magnetization. But we show that a highly polarized spin current can be obtained through the ds AFH when it is subjected to a transverse electric field. Describing the physical system within a tight-binding framework, spin-dependent transport phenomena are studied using the well known Green’s function formalism. The effects of chirality (left-handed and right-handed), electric field, electronic hopping (short-range and long-range) in each strand, system temperature, and size of the helix on spin filtration are critically investigated. Two different configurations of magnetic moments in the helix are taken into account, for a more general description. Suitably adjusting the physical parameters, almost cent percent spin polarization can be substantiated and that feature persists even for a broad parameter range. Our analysis may open up a new direction for achieving spin-selective electron transmission and designing of controlled spintronic devices using similar kinds of fascinating helical magnetic systems.

Floquet-engineered chiral-induced spin selectivity.

The control of electron spin, crucial to the stability of matter, offers new possibilities for manipulating the properties of molecules and materials with potential applications in spintronics and chemical reactions. Recent experiments have demonstrated that electron transmission through chiral molecules depends on the electron spin orientation, a phenomenon known as chiral-induced spin selectivity (CISS). In this study, we show that CISS can be observed in achiral systems driven by an external circularly polarized laser field in the framework of Floquet engineering. By using the Floquet theory for a time-periodically driven system to investigate spin-dependent electron transport in a two-terminal setup, we demonstrate that the spin polarization can approach unity if the light intensity is sufficiently strong, the rate of dephasing is sufficiently low, and the average chemical potential of the two leads is within an appropriate range of values, which is narrow because of the high frequency of the laser field. To obtain a broader range of energies for large spin polarization, a combination of chiral molecules and light-matter interactions is considered, and the spin polarization of electrons transported through a helical molecule driven by a laser field is evaluated.