Direction Of Spin Polarization Research Articles

Unusual Spin Polarization in the Chirality-Induced Spin Selectivity.

Chirality-induced spin selectivity (CISS) refers to the fact that electrons get spin polarized after passing through chiral molecules in a nanoscale transport device or in photoemission experiments. In CISS, chiral molecules are commonly believed to be a spin filter through which one favored spin transmits and the opposite spin gets reflected; that is, transmitted and reflected electrons exhibit opposite spin polarization. In this work, we point out that such a spin filter scenario contradicts the principle that equilibrium spin current must vanish. Instead, we find that both transmitted and reflected electrons present the same type of spin polarization, which is actually ubiquitous for a two-terminal device. More accurately, chiral molecules play the role of a spin polarizer rather than a spin filter. The direction of spin polarization is determined by the molecule chirality and the electron incident direction. And the magnitude of spin polarization relies on local spin-orbit coupling in the device. Our work brings a deeper understanding on CISS and interprets recent experiments, for example, the CISS-driven anomalous Hall effect.

Novel magnetic ordering in LiYbO2 probed by muon spin relaxation

The stretched diamond lattice material LiYbO2 has recently been reported to exhibit two magnetic transitions ($T_{N1} = 1.1 K$, $T_{N2} = 0.45 K$) via specific heat, magnetization, and neutron scattering measurements [Bordelon et al., Phys. Rev. B 103, 014420 (2021)]. Here we report complementary magnetic measurements down to T = 0.28 K via the local probe technique of muon spin relaxation. While we observe a rapid increase in the zero-field muon depolarization rate at $T_{N1}$, we do not observe any spontaneous muon precession for $T < T_{N1}$, which is typically associated with long-range magnetic ordering. The depolarization rate in the ordered state shows a surprising sensitivity to magnetic fields applied along the initial spin polarization direction. Using a simple one-dimensional model, we show that these results are consistent with the unusual random-phase bipartite incommensurate magnetic structure proposed by Bordelon et al. for the intermediate temperature range $T_{N2} < T < T_{N1}$. We also find evidence for temperature-independent magnetic fluctuations persisting to our lowest temperatures, but no obvious signature of the transition or spontaneous muon precession at and below TN2, respectively. This result is suggestive of quantum dynamics within a highly degenerate ground state.

Spin-selective tunneling from nanowires of the candidate topological Kondo insulator SmB6.

Incorporating relativistic physics into quantum tunneling can lead to exotic behavior such as perfect transmission through Klein tunneling. Here, we probed the tunneling properties of spin-momentum-locked relativistic fermions by designing and implementing a tunneling geometry that uses nanowires of the topological Kondo insulator candidate samarium hexaboride. The nanowires are attached to the end of scanning tunneling microscope tips and used to image the bicollinear stripe spin order in the antiferromagnet Fe1.03Te with a Neel temperature of about 50 kelvin. The antiferromagnetic stripes become invisible above 10 kelvin concomitant with the suppression of the topological surface states in the tip. We further demonstrate that the direction of spin polarization is tied to the tunneling direction. Our technique establishes samarium hexaboride nanowires as ideal conduits for spin-polarized currents.

Strong spin filtering by silicon nanoparticles with adsorbed bismuth atom clusters: Role of symmetry and electrostatic control of the direction of spin polarization

We present a theoretical study, based on density functional theory and tight-binding modeling, of the electronic structure and spin transport properties of silicon nanoparticles with adsorbed bismuth atoms. We find the bismuth atoms to form clusters separated by quantum tunnel barriers. We predict strong spin filtering by these nanostructures in the high conductance regime when the source and drain leads are connected to the same bismuth cluster and in the low conductance regime when the source and drain leads are connected to different bismuth clusters. We relate the spin filtering to a symmetry obeyed by the spin transmission probability matrix of the system. We also predict that for such systems the direction of the spin polarization in the drain lead can be tuned through large angles and even reversed electrostatically simply by varying the voltage applied to a gate. Realization of these silicon-bismuth nanostructures in the laboratory is feasible. We expect the predicted spin filtering to be experimentally accessible and potentially relevant for device applications.

On the origins of life’s homochirality: Inducing enantiomeric excess with spin-polarized electrons

Life as we know it is homochiral, but the origins of biological homochirality on early Earth remain elusive. Shallow closed-basin lakes are a plausible prebiotic environment on early Earth, and most are expected to have significant sedimentary magnetite deposits. We hypothesize that ultraviolet (200- to 300-nm) irradiation of magnetite deposits could generate hydrated spin-polarized electrons sufficient to induce enantioselective prebiotic chemistry. Such electrons are potent reducing agents that drive reduction reactions where the spin polarization direction can enantioselectively alter the reaction kinetics. Our estimate of this chiral bias is based on the strong effective spin-orbit coupling observed in the chiral-induced spin selectivity (CISS) effect, as applied to energy differences in reduction reactions for different isomers. In the original CISS experiments, spin-selective electron transmission through a monolayer of double-strand DNA molecules is observed at room temperature-indicating a strong coupling between molecular chirality and electron spin. We propose that the chiral symmetry breaking due to the CISS effect, when applied to reduction chemistry, can induce enantioselective synthesis on the prebiotic Earth and thus facilitate the homochiral assembly of life's building blocks.

Bipolar Magnetic Molecules for Spin‐Polarized Electric Current in Molecular Junctions

AbstractElectrical control of spin transport at single molecule level is highly desired for molecular nanospintronics. By exploiting magnetic bistability of spin crossover complexes, the magnitude of spin polarization can be modulated. However, efficiently controlling the direction of spin polarization at single molecule level is still challenging. Here, we propose a general method to flip the transport electron spin simply by electrical gating in single molecule devices based on bipolar magnetic molecules (BMMs), of which the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) come from different spin channels. Thus, when the device's Fermi level is reversibly adjusted approaching either HOMO or LUMO by changing the polarity of the applied voltage gate, a 100 % spin polarized current with switchable spin direction is achievable. The proposed method is verified by the calculated electronic and transport properties of 9 potential transition metal coordination BMMs.

Bipolar magnetic semiconductors and half-metals in transition metals doped SnSe2 zigzag nanoribbons

The magnetic and electronic properties of transition metals (TM) (TM = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni) doped SnSe2 zigzag nanoribbons (ZNRs) are investigated by employing density functional theory. When an Sn atom on edge of ZNR is substituted by Sc atom, the ZNR behaves as bipolar magnetic semiconductors with 100% spin polarization of valence band and conduction band carriers, where the spin-polarization direction can easily be switched by an external electric field. The Cr-doped ZNR is a spin-gapless semiconductor in which the valence band and conduction band in the spin-up channel touch each other at Fermi energy. The others either are usual magnetic semiconductors or nonmagnetic semiconductors. When two Sn atoms on the edge are substituted by Cr atoms, it is observed that the SnSe2 nanoribbon exhibits half-metallic behavior. Either one Cr dopant or two Cr dopants on edge, the nanoribbons have stable 100% spin polarization of conduction carriers at a wide energy range. These findings demonstrate that the Sc-, Cr-, and 2Cr-doped SnSe2 zigzag nanoribbons are excellent candidates for spintronics.

Bipolar Magnetic Molecules for Spin-Polarized Electric Current in Molecular Junctions.

Electrical control of spin transport at single molecule level is highly desired for molecular nanospintronics. By exploiting magnetic bistability of spin crossover complexes, the magnitude of spin polarization can be modulated. However, efficiently controlling the direction of spin polarization at single molecule level is still challenging. Here, we propose a general method to flip the transport electron spin simply by electrical gating in single molecule devices based on bipolar magnetic molecules (BMMs), of which the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) come from different spin channels. Thus, when the device's Fermi level is reversibly adjusted approaching either HOMO or LUMO by changing the polarity of the applied voltage gate, a 100 % spin polarized current with switchable spin direction is achievable. The proposed method is verified by the calculated electronic and transport properties of 9 potential transition metal coordination BMMs.

Valley-dependent Klein tunneling and spin-valley filter in silicene with magnetic barrier

We have theoretically studied spin- and valley-transport properties in a silicene quantum structure with a square magnetic barrier modulated by off-resonant circularly polarized light (OCPL). The result shows that in the magnetic quantum structure, valley and spin filtering can be realized simultaneously with appropriate OCPL at small incident angles. More interestingly, at normal incidence, Klein tunneling is observed in the K′ valley. Besides, it is also found that valley polarization can be inverted by reversing the sign of the staggered potential or the chirality of OCPL. Nevertheless, the direction of spin polarization can only be modulated by the chirality of OCPL. These findings provide the potential applications for constructing novel type of silicene devices.

Observation of Spin Splitting Torque in a Collinear Antiferromagnet RuO_{2}.

Current-induced spin torques provide efficient data writing approaches for magnetic memories. Recently, the spin splitting torque (SST) was theoretically predicted, which combines advantages of conventional spin transfer torque (STT) and spin-orbit torque (SOT) as well as enables controllable spin polarization. Here we provide the experimental evidence of SST in collinear antiferromagnet RuO_{2} films. The spin current direction is found to be correlated to the crystal orientation of RuO_{2} and the spin polarization direction is dependent on (parallel to) the Néel vector. These features are quite characteristic for the predicted SST. Our finding not only presents a new member for the spin torques besides traditional STT and SOT, but also proposes a promising spin source RuO_{2} for spintronics.

Dynamic properties of a ferromagnetic skyrmion in an in-plane magnetic field

The magnetic field is one of the widely used external stimuli to manipulate magnetic skyrmions. On the other hand, the skyrmion Hall effect can force the skyrmion to move perpendicular to the driving direction, which may lead to the annihilation of the skyrmion at the sample edge. Here, we study a skyrmion guide based on a nanotrack, where the skyrmion is controlled by the spin-transfer torque or spin–orbit torque in the presence of an in-plane field. The effects of spin-torque parameters, spin polarization directions, and magnetic field with an appropriate magnitude on the skyrmion motion are explored. Both static and dynamic properties of the skyrmion are demonstrated through micromagnetic simulations. Our results can be useful for the development of topological transport channels for spintronic devices.

Two-dimensional Dirac semiconductor and its material realization

We propose a new concept of two-dimensional (2D) Dirac semiconductor which is characterized by the emergence of fourfold degenerate band crossings near the band edge and provide a generic approach to realize this novel semiconductor in the community of material science. Based on the first-principle calculations and symmetry analysis, we discover recently synthesised triple-layer (TL)-BiOS2 is such Dirac semiconductor that features Dirac cone at X/Y point, protected by nonsymmorphic symmetry. Due to sandwich-like structure, each Dirac fermion in TL-BiOS2 can be regarded as a combination of two Weyl fermions with opposite chiralities, degenerate in momentum-energy space but separated in real space. Such Dirac semiconductor carries layer-dependent helical spin textures that never been reported before. Moreover, novel topological phase transitions are flexibly achieved in TL-BiOS2: (i) an vertical electric field can drive it into Weyl semiconductor with switchable spin polarization direction, (ii) an extensive strain is able to generate ferroelectric polarization and actuate it into Weyl nodal ring around X point and into another type of four-fold degenerate point at Y point. Our work extends the Dirac fermion into semiconductor systems and provides a promising avenue to integrate spintronics and optoelectronics in topological materials.

Current-Induced Spin Photocurrent in GaAs at Room Temperature.

Circularly polarized photocurrent, observed in p-doped bulk GaAs, varies nonlinearly with the applied bias voltage at room temperature. It has been explored that this phenomenon arises from the current-induced spin polarization in GaAs. In addition, we found that the current-induced spin polarization direction of p-doped bulk GaAs grown in the (001) direction lies in the sample plane and is perpendicular to the applied electric field, which is the same as that in GaAs quantum well. This research indicates that circularly polarized photocurrent is a new optical approach to investigate the current-induced spin polarization at room temperature.

High-Throughput Computational Screening for Bipolar Magnetic Semiconductors.

Searching ferromagnetic semiconductor materials with electrically controllable spin polarization is a long-term challenge for spintronics. Bipolar magnetic semiconductors (BMS), with valence and conduction band edges fully spin polarized in different spin directions, show great promise in this aspect because the carrier spin polarization direction can be easily tuned by voltage gate. Here, we propose a standard high-throughput computational screening scheme for searching BMS materials. The application of this scheme to the Materials Project database gives 11 intrinsic BMS materials (1 experimental and 10 theoretical) from nearly ~40000 structures. Among them, a room-temperature BMS Li2V3TeO8 (mp-771246) is discovered with a Curie temperature of 478 K. Moreover, the BMS feature can be maintained well when cutting the bulk Li2V3TeO8 into (001) nanofilms for realistic applications. This work provides a feasible solution for discovering novel intrinsic BMS materials from various crystal structure databases, paving the way for realizing electric-field controlled spintronics devices.

Rashba-split surface state and spin-dependent photon emission from Re(0001) at Γ¯

The unoccupied electronic structure of the Re(0001) surface is investigated by spin- and angle-resolved inverse photoemission, experimentally and theoretically. The work is focused on the states around the center of the surface Brillouin zone $\overline{\mathrm{\ensuremath{\Gamma}}}$, where a hole pocket of a surface state with Rashba-type spin splitting is detected. Furthermore, we observe spin-dependent photon emission from unpolarized surface and bulk states at $\overline{\mathrm{\ensuremath{\Gamma}}}$. The size and sign of the spin asymmetry depends on experimental parameters such as the direction of the electron spin polarization and the photon detection angle. Maximum (zero) spin asymmetry is detected if the electron spin polarization and the plane of photon emission are perpendicular (parallel). The effect is traced back to spin-orbit-induced hybridization of the involved states.

Two-dimensional bipolar magnetic semiconductors with high Curie-temperature and electrically controllable spin polarization realized in exfoliated Cr(pyrazine)2 monolayers

Exploring two-dimensional (2D) magnetic semiconductors with room temperature magnetic ordering and electrically controllable spin polarization is a highly desirable but challenging task for nanospintronics. Here, through first principles calculations, we propose to realize such a material by exfoliating the recently synthesized organometallic layered crystal Li$_{0.7}$[Cr(pyz)$_2$]Cl$_{0.7}$0.25$\cdot$(THF) (pyz = pyrazine, THF = tetrahydrofuran) [Science 370, 587 (2020)]. The feasibility of exfoliation is confirmed by the rather low exfoliation energy of 0.27 J/m$^2$, even smaller than that of graphite. In exfoliated Cr(pyz)$_2$ monolayer, each pyrazine ring grabs one electron from the Cr atom to become a radical anion, then a strong $d$-$p$ direct exchange magnetic interaction emerges between Cr cations and pyrazine radicals, resulting in room temperature ferrimagnetism with a Curie temperature of 342 K. Moreover, Cr(pyz)$_2$ monolayer is revealed to be an intrinsic bipolar magnetic semiconductor where electrical doping can induce half-metallic conduction with controllable spin-polarization direction.

Spectral features of magnetic domain walls on the surface of three-dimensional topological insulators

We present a theoretical investigation of electron states hosted by magnetic domain walls on the 3D topological insulator surface. The consideration includes the domain walls with distinct vectorial and spatial textures. The study is carried out on the basis of the Hamiltonian for quasi-relativistic fermions by using a continual approach and tight-binding calculations. We derive the spectral characteristics and spatial localization of the the one-dimensional low-energy states appearing at the domain walls. The antiphase domain walls are shown to generate the topologically protected chiral states with linear dispersion, the group velocity and spin-polarization direction of which depend on an easy axis orientation. In the case of an easy plane anisotropy, we predict a realization of a dispersionless state, flat band in the energy spectrum, that is spin-polarized along the surface normal. Modification of the surface states in the multi-domain case, which is approximated by a periodic set of domain walls, is described as well. We find that the magnetic domain walls with complex internal texture, such as N\'eel-like or Bloch-like walls, also host the topological states, although their spectrum and spin structure can be changed compared with the sharp wall case.

Two-dimensional bipolar ferromagnetic semiconductors from layered antiferromagnets

Bipolar magnetic semiconductors (BMSs) are a class of compounds possessing different valence band maximum (VBM) and conduction band minimum (CBM) in terms of electron spins, which are related to but distinct from half-metals. They allow one to switch one spin current to another by applying external gate voltage or chemical doping, which can meet the requirement of bipolar manipulation in spintronic devices. Designing and searching BMSs are still a challenge. Here we predict that monolayer ${\mathrm{CrPS}}_{4}$ and ${\mathrm{CrPSe}}_{4}$ are bipolar ferromagnetic semiconductors with Curie temperature 58 K and 82 K, respectively. We predicted that they can be exfoliated from their A-type antiferromagnetic bulk form. Electric-field gating could drive ${\mathrm{CrPS}}_{4}$ and ${\mathrm{CrPSe}}_{4}$ into half-metals with reversible spin-polarization directions, where the bipolar doping is relatively easier to access for ${\mathrm{CrPSe}}_{4}$ in experiment than ${\mathrm{CrPS}}_{4}$ for its suitable electron affinity and ionic potential. Furthermore, we highlight the role played by the different magnitude between spin exchange splitting and crystal field splitting in the formation of BMSs. The results presented here may provide new clues in designing or searching BMSs.

A frustrated bimeronium: Static structure and dynamics

We show a topological spin texture called “bimeronium” in magnets with in-plane magnetization. It is a topological counterpart of skyrmionium in perpendicularly magnetized magnets and can be seen as a combination of two bimerons with opposite topological charges. We report the static structure and spin-orbit-torque-induced dynamics of an isolated bimeronium in a magnetic monolayer with frustrated exchange interactions. We study the anisotropy and magnetic field dependences of a static bimeronium. We also explore the bimeronium dynamics driven by the damping-like spin-orbit torque. We find that the bimeronium shows steady rotation when the spin polarization direction is parallel to the easy axis. Moreover, we demonstrate the annihilation of the bimeronium when the spin polarization direction is perpendicular to the easy axis. Our results are useful for understanding the fundamental properties of bimeronium structures and may offer an approach to build bimeronium-based spintronic devices.

Bipolar magnetic semiconductor materials based on 2D Fe2O3 lattice

Bipolar magnetic materials play an important role in spintronics. Its unique electronic structure allows the materials to easily regulate fully spin-polarized currents with different spin polarization directions by a gate voltage. It is predicted that 2D monolayer Fe2O3 is a new class of bipolar magnetic semiconductor (BMS) materials by first-principles calculations. The valence band (VB) and the conduction band (CB) have opposite spin polarizations near the Fermi level, and a direct band gap of 0.28 eV between the conduction band bottom (CBM) and the valence band top (VBM). At the same time, the two spin channels have relatively large spin-conserved gaps of 1.05 eV and 2.40 eV. The ground state of the Fe2O3 lattice is a ferromagnetic (FM) state. It has strong intrinsic spin polarization and has a magnetic moment of 10.0 μB per unit cell, and the spin polarization is mainly derived from the transition metal Fe atom. The Bader analysis find that the local magnetic moment of each Fe atom is 4.2 μB, and the local magnetic moment of each O atom is only 0.5 μB. The mechanism of magnetism could be understood by the direct exchange between the orbitals of Fe atom. The Curie temperature (TC) of the Fe2O3 calculated based on Monte Carlo (MC) simulation up to 110 K, which is much larger than the 45 K of CrI3. The characteristics of electrically-controlled polarization currents in bipolar magnetic semiconductor materials have broadened application prospects in the development of spintronics and the construction of bipolar magnetic electronic devices.