Spin Filter Research Articles

The first polarized neutron diffraction experiment at the time-of-flight single crystal neutron diffractometer SENJU at J-PARC

The 3He spin filter has been developed to use pulsed neutrons effectively at J-PARC MLF. In this study, the 3He spin filter was installed in the single crystal neutron diffractometer (BL18 SENJU) at J-PARC MLF as a polarizer of the incident beam, and Half-polarized neutron diffraction experiments were performed. The 3He spin filter was used the ”drop-in cell” method with cells that had a relaxation time of 150 hours in the past experiments. The sample was a single crystal of Heusler crystal Cu2MnAl and the sample magnetic field was 0.3 T. The intensities of (111) Bragg peaks of Cu2MnAl were drastically changed with the polarity of the incident neutron beam, and the flipping ratio of 21.7 at 3.6 Å was observed. The experimental setup and the performance of the 3He spin filter used in this experiment are described in detail in this paper.

Magnetic and transport properties of two-dimensional ferromagnet VSe2 with Se vacancies

As a room-temperature van der Waals intrinsic ferromagnetic semiconductor, monolayer VSe2 was studied extensively for both fundamental research and potential low-dimensional semiconductor spintronic applications. Defects are inevitably encountered and can affect the electronic, magnetic, and transport properties during the experimental synthesis of VSe2 monolayer. However, the study of defects in VSe2 monolayer is still lacking at the atomic level. Here, the electronic, magnetic, and transport properties of VSe2 monolayer with Se vacancies are investigated by first-principles density-functional method. With the direction of easy magnetization axes unchanged, introducing Se vacancies results in the decrease of magnetic anisotropy energy and Curie temperature, which can be further increased by applying in-plane biaxial strain. After Se vacancies are introduced, the conductivity of the system decreases, while the larger doping range is obtained for pure spin-polarized current. The transition from semiconductor to half-metal with 100% spin polarization can be realized by introducing Se bivacancy in VSe2 monolayer, which is highly promising for next-generation spin filter, spin injection and spin transport nanodevices.PACS: 61.72.jd; 73.20.At; 75.50.Pp; 75.70.Ak

Stacking- and strain-dependent magnetism in Janus CrSTe bilayer

Stacking order and strain are the key component in tuning magnetic property of two-dimensional (2D) van der Waals magnetic materials. In this work, we investigated the crystal structure stability of a 2D ferromagnetic Janus chromium dichalcogenide CrSTe bilayer in AB- and AC-stacking orders and found that AB-stacking CrSTe bilayers, which have a smaller layer spacing and a Curie temperature near room temperature, are more stable than AC-stacking one. The magnetic ground states, exchange coupling constant, and Curie temperature of the AB-stacking CrSTe bilayer can be tuned by strain. It is found that the magnetic ground states of the AB- and AC-stacking CrSTe bilayers are ferromagnetic and interlayer antiferromagnetic within a certain strain range, respectively, indicating that the CrSTe bilayers are expected to be used in the double spin filter. Our results demonstrated that the 2D Janus CrSTe bilayer has the potential in the application of spintronic devices with stable performance and low-power consumption at room temperature.

Large magnetoresistance and temperature-driven spin filter effect in spin valve based on half Heusler alloy.

High spin-injection-efficiency (SIE) and thermal spin-filter-effect (SFE) from a magnetic material to a barrier material are crucial to the high performance of a spintronic device and a spin caloritronic device, respectively. By performing a nonequilibrium Green's function combined with first-principles calculations, we study the voltage-driven and temperature-driven spin transport properties of a half Heusler alloy RuCrAs based spin valve with different atom-terminated interfaces. The spin valve with a CrAs-top (or Ru-top) interface structure has an ultrahigh equilibrium magnetoresistance (MR) ratio of ∼1.56 × 109% (or ∼5.14 × 108%), ∼100% SIE, a large MR ratio, and high spin current intensity under bias voltage, suggesting that it has a great potential application in spintronic devices. The spin valve with the CrAs-top (or CrAs-bri) interface structure has a perfect SFE due to its very high spin polarization of temperature-driven currents, and it is useful in spin caloritronic devices.

Spin dependent molecular junction with graphene electrodes as a thermoelectric nanodevice

Using Green's function method, spin-resolved thermoelectric quantum transport is examined in a molecular junction composed of a phenalene molecule connected to two external graphene leads with and without magnetic exchange potential application on the scattering region. Two different configurations of the system are considered: perpendicular and parallel leads. Theoretical results show that the application of the external exchange potential separates the Seebeck coefficients of different spin states and dramatically increases the coefficients. Furthermore, the temperature gradient between the left and right parts of the system generates a current of nanoAmpere order of magnitude even in the absence of a bias voltage. With the exchange potential, the current is spin-resolved and increased up to 50 times in comparison to the absence of the exchange potential. According to the results, the system acts as a spin filter at some specific chemical potential. Moreover, the current dramatically increases in parallel configuration compared to the perpendicular arrangement. Consequently, the proposed molecular device, with adjustable parameters and a magnificent Seebeck coefficient, can be a promising alternative to ordinary thermoelectric structures in the design of the new generation of thermal spintronic devices.

Spin polarization through axially chiral linkers: Length dependence and correlation with the dissymmetry factor.

The chiral-induced spin selectivity (CISS) effect relates to the spin-selective electron transport through chiral molecules; therefore, the chiral molecules act as spin filters. In past studies, correlation was found between the magnitude of the spin filtering and the intensity of the circular dichroism (CD) spectrum (the first Compton peak) of the molecules. Since the intensity of the CD peak relates to both the magnitude of the electric and magnetic dipole transitions, it was not clear which of these properties correlate with the CISS effect. This work aims at addressing this question. By studying the spin-dependent conduction and the CD spectra of the thiol-functionalized enantiopure binaphthalene (BINAP) and ternaphthalene (TERNAP), we found that both BINAP and TERNAP exhibit a similar spin polarization of 50%, despite the first Compton peak in TERNAP being almost twice as intense as the peak in BINAP. These results can be explained by the similar values of their anisotropy (or dissymmetry) factor, gabs , which is proportional to the magnetic transition dipole moment. Hence, we concluded that the CISS effect is proportional to the transition dipole moment in chiral molecules, namely, to the dissymmetry factor.

Spin filters based on two-dimensional materials Co2Si and Cu2Si

Spintronic devices have several advantages compared with conventional electronic devices, including non-volatility, faster data processing speed, higher integration densities, less electric power consumption and so on. However, we still face challenges for efficiently generating and injecting pure spin polarized current. In this work, we utilize two kinds of two-dimensional materials Co2Si and Cu2Si with both lattice match and band match to construct devices and then research their spin filter efficiency. The spin filter efficiency can be improved effectively either by an appropriate gate voltage at Co2Si region, or by series connection. In both cases the filter efficiencies are much larger than two-dimensional prepared Fe3GeTe2 spin valve and ferromagnetic metallic chairlike O-graphene-H. Also at a quite small bias, we obtain a comparable spin polarized current as those obtained in Fe3GeTe2 spin valve and O-graphene-H obtained at a much larger bias.

Chiral Single‐Molecule Potentiometers Based on Stapled ortho‐ Oligo(phenylene)ethynylenes

AbstractWe report on the chemical design of chiral molecular junctions with stress‐dependent conductance, whose helicity is maintained during the stretching of a single molecule junction due to the stapling of both ends of the inner helix. In the reported compounds, different conductive pathways are observed, with clearly different conductance values and plateau‐length distributions, attributed to different conformations of the helical structures. The large chiro‐optical responses and the potential use of these molecules as unimolecular spin filters have been theoretically proved using state‐of‐the‐art Density Functional Theory (DFT) calculations, including a fully ab‐initio estimation of the CISS‐originating spin polarization which is done, for the first time, for a realistic molecular system.

Chiral Single-Molecule Potentiometers Based on Stapled ortho- Oligo(phenylene)ethynylenes.

We report on the chemical design of chiral molecular junctions with stress-dependent conductance, whose helicity is maintained during the stretching of a single molecule junction due to the stapling of both ends of the inner helix. In the reported compounds, different conductive pathways are observed, with clearly different conductance values and plateau-length distributions, attributed to different conformations of the helical structures. The large chiro-optical responses and the potential use of these molecules as unimolecular spin filters have been theoretically proved using state-of-the-art Density Functional Theory (DFT) calculations, including a fully ab-initio estimation of the CISS-originating spin polarization which is done, for the first time, for a realistic molecular system.

Effect of Residual Carbon on Spin‐Polarized Coupling at a Graphene/Ferromagnet Interface

AbstractVertical stacks of graphene and ferromagnetic layers are predicted to be efficient spin filters, while the experimentally observed figures of merit systematically remain below the theoretical predictions. According to general consensus, a vaguely defined interface contamination is found responsible for this discrepancy. Here, it is demonstrated how the spin‐polarized electronic structure of single‐layer graphene supported on a ferromagnetic cobalt substrate is affected by the presence of an interfacial carbidic buffer layer, formed by residual carbon present in the Co substrate. It is found that the Co‐C hybridized single‐spin state near the Fermi level disappears upon thermal segregation of bulk carbon at the graphene–Co interface, which determines the electronic decoupling of graphene from the ferromagnetic support and consequently, the suppression of net spin polarization. These observations are shown to be independent of the graphene azimuthal orientation with respect to the high symmetry directions of the substrate. The findings provide clear evidence that the realization of highly polarized spin currents in graphene/ferromagnet heterostacks depends on careful control of the graphene growth process in order to eliminate interfacial carbon.

Spin-dependent tunnelling time in phosphorene superlattice

ABSTRACT We study theoretically the Hartman effect and spin-dependent group delay time in a strained phosphorene superlattice in the presence of Rashba spin–orbit interaction (RSOI). It is found that the total group delay time depends on the spin degree of freedom. Thus, spin filtering can be obtained in the time domain by means of the phosphorene superlattice. The total group delay time can be efficiently controlled by the angles of incidence of electrons, strength of RSOI and number of superlattice barriers. Particularly, the magnitude of the total group delay time can be tuned via the direction and strength of strain. Therefore, in phosphorene superlattice the total group delay time can be efficiently adjusted mechanically. Moreover, in a strained phosphorene superlattice the superluminal tunnelling or Hartman effect can be observed even for a normal incident.

Stacking Order, Perfect Spin Polarization, and Giant Magnetoresistance in Zigzag Graphene/ h - BN Heterobilayer Nanoribbons

In a van der Waals heterostructure of two-dimensional materials, electronic properties are tunable by means of stacking orders. Here, we study the spin-dependent quantum transport in the bilayer of ferromagnetic zigzag-edged graphene/hexagonal-$\mathrm{BN}$ nanoribbons (ZGr-BNNRs) using density-functional theory combined with the Keldysh nonequilibrium Green's-function method. We reveal a strong odd-even effect of transport across the ZGr-BNNRs and a giant magnetoresistance value observed only in even-width ZGr-BNNRs. More interestingly, this value can be optimized by engineering stacking orders, yielding the perfect spin polarization efficiency of 100% and the magnetoresistance value of over ${10}^{4}$ in even-width ZGr-BNNRs. Our results provide a route to design and fabricate high-performance spin filters and magnetic storage devices.

First principles study of coherent electron/spin transport across metallothionein: A cadmium-binding protein

Metallothioneins (MTs) play an important role in controlling the concentration of heavy metals in the body by forming metal-thiolate clusters. In addition, the proteins can scavenge free radicals, to exhibit an antioxidant effect. To clarify the precise physiological function of these proteins, it is necessary to examine the non-equilibrium state of electron/spin transport in the molecule. In this study, coherent spin transport in MTs connected to metal electrodes has been calculated to examine the behavior of the molecule toward spin flow (radicals) in the body. The estimated values of conductance were found to be very low, while the spin polarization appears to derive considerable spin filtering. Furthermore, spin polarization did not localize on cadmium but on the sulfur atoms of cadmium-thiolate clusters. These results confirmed that cysteine plays an important role in the scavenging of free radicals even when forming the cadmium-thiolate cluster.Significance Statement.The purpose of this study is to better understand how spin polarization (induced radical) flows in metallothionein, even if it forms cadmium-thiolate clusters. This is important because it can be known where the radical is stored and processed in the body. Our results provide a guide to examine non-equilibrium electron transient in the metallothionein by observing the single-molecule electric conduction with scanning tunneling microscope.

Enantiomer discrimination in absorption spectroscopy and in voltammetry: highlighting fascinating similarities and connections

Absorption spectroscopy and voltammetry, of known analogies and connections, share even more fascinating similarities and connections at a higher complexity level, when “upgrading” them with the ability to discriminate between enantiomers by chiral selector implementation. In both techniques either “molecular” selectors or “electromagnetic” ones (L- versus R-circularly polarized light components for spectroscopy, α - versus β-spin electrons for voltammetry) can be considered; moreover, external magnetic field application can replace a truly chiral actor. A tentative schematization is provided. Analogies and connections also concern molecular features of the enantiodiscrimination actors. In both techniques outstanding performances are obtained with inherently chiral molecules, in which a conjugated backbone with tailored torsion is source of chirality as well as spectroscopic and electrochemical activity, in an attractive three-fold interconnection. Their outstanding effects can be justified by a combination of chemical and electromagnetic properties (excellent potential molecular spin filters), a fascinating challenge for future developments.

Magnetically-controlled spin filter and spin-polarized oscillations in a four-quantum-dot system

Spin polarization through a four-quantum-dot system is theoretically studied by considering the effects of Zeeman magnetic field, extrinsic Rashba spin–orbit interaction and external magnetic field. We analyze how the resonance and anti-resonance bands are formed by means of the mathematical formula. By adjusting the external magnetic field, the spin polarization can be converted between −100% and 100%, which suggests a physical scheme of an effective magnetically-controlled spin filter. The combined effect of external magnetic field and extrinsic Rashba spin–orbit interaction leads to spin-polarized oscillations. The introduction of the Zeeman magnetic field makes the system easier to be applied as a spin filter. This study provides theoretical insights into the realization of quantum computing, quantum information transmission and development of nanoscale quantum devices.

First-principles study on switching performance and spin filtering efficiency of dimethyldihydropyrene/cyclophanediene single-molecule devices with zigzag graphene nanoribbon electrodes

Molecular devices with high switching performance and/or the perfect spin filtering effect have always been the pursuit with the development of molecular electronics. Here, by using the nonequilibrium Green’s function method in combination with the density functional theory, the switching performance and spin filtering properties of dimethyldihydropyrene (DHP)/cyclophanediene (CPD) photoswitchable molecule connected by carbon atomic chains (CACs) to two zigzag graphene nanoribbon electrodes have been theoretically investigated. The results show that DHP is more conductive than CPD and therefore an evident switching effect is demonstrated, and the switching ratio (RON/OFF) can reach 4.5 x 103. It is further revealed that the RON/OFF of DHP /CPD closely depends on the length of CACs. More specifically, the RON/OFF values of DHP /CPD with odd-numbered CACs are larger than those with even-numbered CACs. More interestingly, a high or even perfect spin filtering effect can be obtained in these investigated DHP /CPD single-molecule devices. Our study is helpful for future design of single-molecule switches and spin filters and provides a way to optimize their performance by means of varying the length of bridging CACs.

Room-temperature electric field control of spin filtering efficiency for enhanced modulation of optical spin polarization in a defect-functional 0D-2D hybrid nanostructure.

In order to accomplish spin-based photoelectric information processing, it is necessary to modulate electron spin polarization in III-V semiconductor quantum dots (QDs) using an electric field. However, there is a principal limitation to the spin polarization degree and its control range, as the electron spin polarization is rapidly lost during injection into the QDs at room temperature (RT). Here, electric field control of optical spin polarization in the range of 15-40% is demonstrated at RT using InAs QDs tunnel-coupled with a defect-functional GaNAs quantum well (QW) spin filter. This compares with an electric field control of 1-4% for InAs QDs tunnel-coupled with an InGaAs QW. Transient polarization in the range of 30-60% is also obtained in the ultrafast time domain of less than 100 ps, the degree of polarization depending on the electric field. The enhanced polarization control is achieved by tuning the amplified spin polarization of electrons tunnel-injected from the GaNAs QW into QDs via the electric-field-dependent spin-filtering efficiency of GaNAs. These findings will provide a new way to extensively modulate the electron spin polarization in opto-semiconductors, by electric-field-induced on/off switching of spin amplification.

Easily processable spin filters: exploring the chiral induced spin selectivity of bowl-shaped chiral subphthalocyanines.

High spin polarization (SP) in studies of chiral induced spin selectivity (CISS) is only observed when chiral molecules are properly organized. This is generally achieved by using anchoring groups or complex supramolecular polymers. A new class of spin filters based on bowl-shaped aromatics is reported, which form high-quality thin-films by simply spin-coating and displaying high spin filtering properties. In particular, we fabricate devices containing enantiopure tribromo-subphthalocyanines (SubPcs), and measure the CISS effect by means of magnetic conductive probe atomic force microscopy (mc-AFM). Circular dichroism and AFM experiments reveal that the resulting thin-film presents a well-ordered chiral structure. Remarkably, the resulting devices show SPs as high as ca. 50%, which are comparable to those obtained by using the current complex methodologies. These results boost the potential of bowl-shaped aromatics as easily processable spin filters, opening new frontiers toward realistic and efficient spintronic devices based on the CISS effect.

Spin-switching effect and giant magnetoresistance in quantum structure of monolayer MoS<sub>2</sub> nanoribbons with ferromagnetic electrode

Spintronics is a new type of electronics based on electron spin rather than charge as the information carrier, which can be stored and calculated by regulating and manipulating the spin. The discovery and application of the giant magnetoresistance effect opens the door to the application of electron spin properties. Realizing on-demand control of spin degree of freedom for spin-based devices is essential. The two-dimensional novel material, monolayer transition metal dichalcogenide (TMD) (MoS<sub>2</sub> is a typical example from the family of TMD materials), has become an excellent platform for studying spintronics due to its novel physical properties, such as direct band gap and strong spin-orbit coupling. Obtaining high spin polarization and achieving controllability of degrees of freedom are fundamental problems in spintronics. In this paper, we construct the monolayer zigzag MoS<sub>2</sub> nanoribbon quantum structure of electrically controlled ferromagnetic electrode to solve this problem. Based on the non-equilibrium Green’s function method, the regulation of the magnetic exchange field and electrostatic barrier on the spin transport in parallel configuration and anti-parallel configuration are studied. It is found that in the parallel structure, spin transport is obviously related to the magnetic exchange field, and 100% spin filtering can occur near the Fermi energy level to obtain pure spin current. When an additional electric field is applied to the middle region, the spin filtering effect is more significant. Therefore, the spin switching effect can be achieved by regulating the incident energy. In addition, it is also found that within a specific energy range, electrons in the parallel configuration are excited to participate in transport, while electrons in the anti-parallel structure are significantly inhibited. Consequently, a noticeable giant magnetoresistance effect can be obtained in this quantum structure. Moreover, it can be seen that the magnetic exchange field strength can effectively modulate the giant magnetoresistance effect. These results provide valuable theoretical references for the development of giant magnetoresistance devices and spin filters based on monolayer zigzag MoS<sub>2</sub> nanoribbons.

Self-assembled stereomutation with supramolecular chirality inversion.

Supramolecular chirality is involved not only in biological events such as gene communication, replication, and enzyme catalysis but also in artificial self-assembly systems and aggregated materials. The precise control of supramolecular chirality, and especially supramolecular chirality inversion (SMCI), would deepen the understanding of chiral transfer and regulation in both living systems and artificial self-assembly systems, providing efficient ways to construct advanced chiral materials with an optimum assembly pathway necessary for various functions. In this review, the fundamental principles of SMCI are comprehensively summarized, with a focus on the helical assemblies having opposite handedness or chiroptical properties of the compositions. Thereafter, various SMCI strategies that have been developed for chiral nanostructures and assembled materials are systematically reviewed, and the promising applications of SMCI, including chiroptical switches, chiral recognition, enantiomeric separation, asymmetric catalysis, chiral optoelectronic materials, chiral spin filters, and biomedical uses, are highlighted accordingly. Finally, the scientific challenges and future perspectives for assembling materials with SMCI are also discussed.