Spin-filter Devices Research Articles

All-spinel oxide Josephson junctions for high-efficiency spin filtering

Obtaining high efficiency spin filtering at room temperature using spinel ferromagnetic tunnel barriers has been hampered by the formation of antiphase boundaries due to their difference in lattice parameters between barrier and electrodes. In this work we demonstrate the use of LiTi2O4 thin films as electrodes in an all-spinel oxide CoFe2O4-based spin filter devices. These structures show nearly perfect epitaxy maintained throughout the structure and so minimise the potential for APBs formation. The LiTi2O4 in these devices is superconducting and so measurements at low temperature have been used to explore details of the tunnelling and Josephson junction behaviour.

High efficiency spin-valve and spin-filter in a doped rhombic graphene quantum dot device

Spin-polarized transport through a rhombic graphene quantum dot (rGQD) attached to armchair graphene nanoribbon (AGNR) electrodes is investigated by means of the Green’s function technique combined with single-band tight-binding (TB) approach including a Hubbard-like term. The Hubbard repulsion was included within the mean-field approximation. Compared to anti-ferromagnetic (AFM), we show that the ferromagnetic (FM) ordering of the rGQD corresponds to a smaller bandgap, thus resulting in an efficient spin injector. As a consequence, the electron transport spectrum reveals a spin valve effect, which is controlled by doping with B/N atoms creating a p-n-type junction. The calculations point out that such systems can be used as spin-filter devices with efficiency close to a 100%.

Direct optical observation of spin accumulation at nonmagnetic metal/oxide interface

We report the direct observation of uniform in-plane spin accumulation at room temperature by magneto optical Kerr effect, at the interface formed between nonmagnetic metal (Cu, Ag) and oxide (Bi2O3). Recent reports show spin to charge conversion at these interfaces suggesting the presence of Rashba like spin orbit coupling (SOC). The formation of spin accumulation is the result of current induced spin polarization at our interfaces (direct Rashba–Edelstein effect), without external magnetic field or proximity to ferromagnetic materials. We observe opposite orientation of spin accumulation at Cu/Bi2O3 and Ag/Bi2O3 interfaces reflecting their opposite sign of Rashba SOC (Rashba parameter). Moreover, estimation of spin accumulation from values of Rashba parameters obtained by independent spin pumping measurements, agrees well with the difference in amplitude of our normalized Kerr signals for Cu/Bi2O3 and Ag/Bi2O3 interfaces. Uniform in-plane spin accumulation due to Rashba-Edelstein effect can be applied for spin filter devices and efficient driving force for magnetization switching.

Spin-filter silicene devices induced by asymmetric electrodes

Using nonequilibrium Green's function in combination with the density functional theory, we investigate the spin-dependent transport properties of three silicene-based heterojunctions. The calculated results show that the heterojunctions are promising multifunctional devices in spintronics due to their nearly perfect bipolar spin-filter effect and high rectification ratio. Also shown are the obvious negative differential resistance behaviors for both spin channels. By analyzing the spin-resolved transmission spectra and the band structure of ZSiNR electrodes, as well as the spatial resolved local density of states, the mechanisms for these intriguing properties are demonstrated in detail.

Prediction of electronic structure of van der Waals interfaces: Benzene adsorbed monolayer MoS2, WS2 and WTe2

The electronic structure of benzene adsorbed monolayer MoS2, WS2 and WTe2 has been investigated by first-principles calculations with van der Waals forces. The benzene adsorbed monolayer MoS2, WS2 and WTe2 with spin-orbital coupling are found to be direct-band-gap semiconductors. All the benzene adsorbed model show the semiconducting characteristic. The band gap and spin splitting of the benzene adsorbed monolayer MoS2, WS2 and WTe2 are slightly regulated. The calculated results show the potential applications in the optoelectronic devices, spin-filter devices, etc.

Perfect spin-filter devices based on zigzag zinc oxide nanoribbons

Spin-polarized electron transport through a zigzag zinc oxide nanoribbon (ZnONR) has been studied using first-principles transport simulations. Ribbons without edges passivated show 100% spin polarization at small bias voltage independently of width. The ribbons with edge zinc atoms passivated maintain absolute spin-filtering effect in much larger bias region. The results demonstrate that zigzag ZnONRs act as perfect spin-filters in the absence of magnetic electrodes and external fields.

Structural and magnetic properties of Ni78Fe22 thin films sandwiched between low-softening-point glasses and application in spin devices

We investigate the structural and magnetic properties of Ni78Fe22 thin films sandwiched between low-softening-point (LSP) glasses, which can be used in spin quantum cross (SQC) devices utilizing stray magnetic fields generated from magnetic thin-film edges. We also calculate the stray magnetic field generated between the two edges of Ni78Fe22 thin-film electrodes in SQC devices and discuss the applicability to spin-filter devices. Using the established fabrication technique, we successfully demonstrate the formation of LSP-glass/Ni78Fe22/LSP-glass structures with smooth and clear interfaces. The coercivity of the Ni78Fe22 thin films is enhanced from 0.9 to 103Oe by increasing the applied pressure from 0 to 1.0MPa in the thermal pressing process. According to the random anisotropy model, the enhancement of the coercivity is attributed to the increase in the crystal grain size. The stray magnetic field is also uniformly generated from the Ni78Fe22 thin-film edge in the direction perpendicular to the cross section of the LSP-glass/Ni78Fe22/LSP-glass structures. Theoretical calculation reveals that a high stray field of approximately 5kOe is generated when the distance between two edges of the Ni78Fe22 thin-film electrodes is less than 5nm and the thickness of Ni78Fe22 is greater than 20nm. These experimental and calculation results indicate that Ni78Fe22 thin films sandwiched between LSP glasses are useful as electrodes for SQC devices, serving as spin-filter devices.

The electronic transport and Spin Seebeck Effect of MnAs(001)/ InP(001) heterogeneous junctions

We report the electronic transport properties of the MnAs(001)/InP(001) magnetic tunnel junctions by first principles calculations. The half-metallic MnAs was predicted as a result of 100% spin polarization and then minority spin current is absolutely inhibited when the bias voltage is applied to terminals of both MnAs and InP. Majority spin current is inhibited when the bias voltage is less than 0.8 V and “relaxed” when the bias voltage increases. Spin-up and spin-down currents flowing in opposite directions are proved to be induced by temperature difference between left and right electrodes called Seebeck Effect rather than external electrical bias. Thermal current was demonstrated perfect spin filtering effects to indicating possible use for fabrication of highly-efficient spin filter devices in this study.

Co thickness dependence of structural and magnetic properties in spin quantum cross devices utilizing stray magnetic fields

We investigate the Co thickness dependence of the structural and magnetic properties of Co thin-film electrodes sandwiched between borate glasses in spin quantum cross (SQC) devices that utilize stray magnetic fields. We also calculate the Co thickness dependence of the stray field between the two edges of Co thin-film electrodes in SQC devices using micromagnetic simulation. The surface roughness of Co thin films with a thickness of less than 20 nm on borate glasses is shown to be as small as 0.18 nm, at the same scanning scale as the Co film thickness, and the squareness of the hysteresis loop is shown to be as large as 0.96–1.0. As a result of the establishment of polishing techniques for Co thin-film electrodes sandwiched between borate glasses, we successfully demonstrate the formation of smooth Co edges and the generation of stray magnetic fields from Co edges. Theoretical calculation reveals that a strong stray field beyond 6 kOe is generated when the Co thickness is greater than 10 nm at a junction gap distance of 5 nm. From these experimental and calculation results, it can be concluded that SQC devices with a Co thickness of 10–20 nm can be expected to function as spin-filter devices.

Spin-Filtering Transport in Double Parallel Quantum Wires on a Graphene Sheet* *Supported by the National Natural Science Foundation of China under Grant Nos. 11174214, 11204192, the NSAF Joint Fund Jointly set up by the National Natural Science Foundation of China and the Chinese Academy of Engineering Physics under Grant Nos. U1230201 and U1430117

We theoretically investigate the spin filtering transport of double parallel quantum wires (QWs) side-coupled to a graphene sheet and sandwiched between two ferromagnetic (FM) leads. The dependences of the wire-graphene coupling strength, wire-wire coupling strength, as well as the spin polarization of the ferromagnetic leads are studied. It is found that the wire-graphene coupling strength tends to reduce the current and the wire-wire coupling strength can first reinforce and then decrease the current. The spin polarization strength has an enhanced (identical) effect on the current under the parallel (anti-parallel) alignment of the FM leads, which gives rise to an obvious spin-filter and tunnel magnetoresistance (TMR) effect. Our results suggest that such a theoretical model can stimulate some experimental investigations about the spin-filter devices.

Half-metallicity modulation of hybrid BN-C nanotubes by external electric fields: a first-principles study.

On the basis of density functional theory, we systematically investigate the electronic and magnetic properties of hybrid BN-C nanotubes, Cx(BN)y where x + y = 12, with and without an external electric field. The BN-C nanotubes are totally distinct from pristine boron-nitride and carbon nanotubes. The electronic properties of Cx(BN)y change significantly with composition: from the nonmagnetic semiconductors to the half-metals. The half-metallicity is attributed to the competition among the band gap, which is related to the width of C domain, the width of BN domain, and the intrinsic polarization field. Application of the external fields can enhance or counterbalance the polarization fields and change the band gaps. The half-metallicity can be modulated. In BN-rich tubes, such as C2(BN)10, the energy gap can be engineered from 0.50 eV to 0.95 eV and in C3(BN)9, the ground state is converted from the nonmagnetic state into the anti-ferro-magnetic one. In other tubes, the half-metallicity can be enhanced or destroyed by different external fields. The modulation indicates that hybrid BN-C nanotubes can work as the components of the spin-filter devices.

Spin regulation in composite spin-filter barrier devices

Magnetic insulators are known to provide large effective Zeeman fields that are confined at an interface, making them especially powerful in modifying adjacent one- or two-dimensional electronic structures. Utilizing this phenomenon and the other important property of magnetic insulators--spin filtering--here we report the generation and subsequent detection of a large interface field, as large as tens of tesla in EuS/Al/EuS heterostructures with metallic coulomb islands confined within a magnetic insulator barrier. The unique energy profile across this sandwich structure produces spin-assisted charge transfer across the device, generating a spontaneous spin current and voltage. These unique properties can be practical for controlling spin flows in electronic devices and for energy harvesting.

Nitrogen and Boron substitutional doped zigzag silicene nanoribbons: Ab initio investigation

We performed a spin polarized density-function theory study of the stabilities, electronic and magnetic properties of zigzag silicene nanoribbons (ZSiNRs) substitutionally doped with a single N or B atom located at various sites ranging from edge to center of the ribbon. From minimization of the formation energy, it is found that the substitutional doping is favorable at edge of the ribbon. A single N or B atom substitution one edge Si atom of ZSiNRs can greatly suppress the spin-polarizations of the impurity atom site and its vicinity region, and leads to a transition from antiferromagnetic (AFM) state to ferromagnetic (FM) state, which is attributed to the splitting of the original spin degenerate edge bands. A single N atom doped ZSiNRs still keep semiconductor property but a single B atom doped ZSiNRs exhibit a half-metallic character. Our results reveal that substitution doped ZSiNRs have potential applications in Si-based nanoelectronics, such as field effect transisitors (FETs), negative differential resistance (NDR) and spin filter (SF) devices.

Optical and electronic properties of graphene nanoribbons upon adsorption of ligand-protected aluminum clusters

We have carried out first-principles calculations to investigate how the electronic and optical features of graphene nanoribbons are affected by the presence of atomic clusters. Aluminum clusters of different sizes and stabilized by organic ligands were deposited on graphene nanoribbons from which the energetic features of the adsorption plus electronic structure were treated within density-functional theory. Our results point out that, depending on their size and structure shape, the clusters perturb distinctively the electronic properties of the ribbons. We suggest that such selective response can be measured through optical means revealing that graphene nanoribbons can work as an efficient characterization medium of atomic clusters. In addition, we demonstrate that atomic clusters can fine-tune the electronic and spin-polarized states of graphene ribbons from which novel spin-filter devices could be designed.

Rashba spin−orbit coupling effect on tunneling time in semiconductor spintronic junctions

Based on the transfer-matrix method and the group velocity concept, we investigate theoretically the spin tunneling time through single and double barriers of diluted magnetic semiconductor structures in the presence of the Rashba spin−orbit coupling (RSOC) effect. The calculation of transmission probability is based on an effective mass quantum-mechanical approach in the presence of an external magnetic field applied along the growth direction of the junction. The results show that the RSOC has great different influence on spin-dependent tunneling time of electrons with spin up and spin down in these structures. We also study the effect of zero-field conduction band offset on spin-dependent transmission properties. It is found that in the presence of RSOC and a positive zero-field conduction band offset as high as 10 meV, the results show a high degree of spin polarization and spin separation in the tunneling time in the considered system and this aspect may be utilized in designing new spin filter devices.

Angular dependence of shot noise in the presence of Rashba spin–orbit coupling in semiconductor spintronics junctions

We investigate spin-dependent current and shot noise, taking into account the Rashba spin–orbit coupling (RSOC) effect in double diluted magnetic semiconductor (DMS) barrier resonant tunneling diodes. The calculation is based on an effective mass approach. The magnetization of DMS is calculated by the mean-field approximation in low magnetic field. The spin-splitting of DMS depends on the sp–d exchange interaction. We also examine the dependence of transport properties of CdTe/CdMnTe heterostructures on applied voltage and relative angle between the magnetization of two DMS layers. It is found that the RSOC has great different influence on the transport properties of tunneling electrons with spin-up and spin-down, which have different contributions to the current and the shot noise. Also, we can see that the RSOC enhances the spin polarization of the system, which makes the nanostructure a good candidate for new spin filter devices. Thus, these numerical results may shed light on the next applications of quantum multilayer systems and make them a good choice for future spintronics devices.

Giant Spin-Orbit Interaction Due to Rotating Magnetic Fields in Graphene Nanoribbons

We theoretically study graphene nanoribbons in the presence of spatially varying magnetic fields produced e.g. by nanomagnets. We show both analytically and numerically that an exceptionally large Rashba spin orbit interaction (SOI) of the order of 10 meV can be produced by the non-uniform magnetic field. As a consequence, helical modes exist in armchair nanoribbons that exhibit nearly perfect spin polarization and are robust against boundary defects. This paves the way to realizing spin filter devices in graphene nanoribbons in the temperature regime of a few Kelvins. If a nanoribbon in the helical regime is in proximity contact to an s-wave superconductor, the nanoribbon can be tuned into a topological phase sustaining Majorana fermions.

SAM-like arrangement of thiolated graphene nanoribbons: decoupling the edge state from the metal substrate

Density functional theory calculations have been used to analyze the electronic and magnetic properties of ultrathin zigzag graphene nanoribbons (ZGNRs) with different edge saturations. We have compared a symmetric hydrogen saturation of both edges with an asymmetric saturation in which one of the edges is saturated with sulphur atoms or thiol groups, while the other one is kept hydrogen saturated. The adsorption of such partially thiolated ZGNRs on Au(111) has also been explored. We have considered vertical and tilted adsorption configurations of the ribbons, reminiscent of those found for thiolated organic molecules in self-assembled monolayers (SAM) on gold substrates. We have found that saturation with sulphur atoms or thiol groups removes the corresponding edge state from the Fermi energy and kills the accompanying spin polarization. However, this effect is so local that the electronic and magnetic properties of the mono-hydrogenated edge (H-edge) remain unaffected. Thus, the system develops a spin moment mainly localized at the H-edge. This property is not modified when the partially thiolated ribbon is attached to the gold substrate, and is quite independent of the width of the ribbon. Therefore, the upright adsorption of partially thiolated ZGNRs can be an effective way to decouple the spin-polarized channel provided by the H-edge from an underlying metal substrate. These observations might open a novel route to build spin-filter devices using ZGNRs on gold substrates.

Beating Analysis of Shubnikov de Haas Oscillation in In<sub>0.53</sub>Ga<sub>0.47</sub>As Double Quantum Well toward Spin Filter Applications

We recently determined the values of intrinsic spin-orbit (SO) parameters for In 0.52 Al 0.48 As/In 0.53 Ga 0.47 As(10 nm)/In 0.52 Al 0.48 As (InGaAs/InAlAs) quantum wells (QW), lattice-matched to (001) InP, from the weak localization/antilocalization analysis of the low-temperature magneto-conductivity measurements [1]. We have then studied the subband energy spectra for the InGaAs/InAlAs double QW system from beatings in the Shubnikov de Haas (SdH) oscillations. The basic properties obtained here for the double QW system provides useful information for realizing nonmagnetic spin-filter devices based on the spin-orbit interaction [2]. Copyright © 2012 The Institute of Electronics, Information and Communication Engineers.

Fabrication of Multiferroic Co-Substituted BiFeO₃ Epitaxial Films on SrTiO₃ (100) Substrates by Radio Frequency Magnetron Sputtering.

The 10 at.% Co-substituted BiFeO3 films (of thickness 50 nm) were successfully prepared by radio frequency (r.f.) magnetron sputtering on SrTiO3 (100) substrates with epitaxial relationships of [001](001)Co-BiFeO3//[001](001)SrTiO3. In this study, a single phase Co-substituted BiFeO3 epitaxial film was fabricated by r.f. magnetron sputtering. Sputtering conditions such as Ar, O2 gas pressure, annealing temperature, annealing atmosphere, and sputtering power were systematically changed. It was observed that a low Ar gas pressure and low sputtering power is necessary to suppress the formation of the secondary phases of BiOx. The Co-substituted BiFeO3 films were crystalized with post-annealing at 600 °C in air. The process window for single phase films is narrower than that for pure BiFeO3 epitaxial films. By substituting Fe with Co in BiFeO3, the magnetization at room temperature increased to 20 emu/cm3. This result suggests that Co-substituted BiFeO3 films can be used in spin-filter devices.