Spin Polarization Efficiency Research Articles

Boosting the Curie temperature of GaN monolayer through van der Waals heterostructures

The pursuit of van der Waals (vdW) heterostructures with high Curie temperature and strong perpendicular magnetic anisotropy (PMA) is vital to the advancement of next generation spintronic devices. First-principles calculations are used to study the electronic structures and magnetic characteristics of GaN/VS2 vdW heterostructure under biaxial strain and electrostatic doping. Our findings show that a ferromagnetic ground state with a remarkable Curie temperature (477 K), much above room temperature, exists in GaN/VS2 vdW heterostructure and 100% spin polarization efficiency. Additionally, GaN/VS2 vdW heterostructure still maintains PMA under biaxial strain, which is indispensable for high-density information storage. We further explore the electron, magnetic, and transport properties of VS2/GaN/VS2 vdW sandwich heterostructure, where the magnetoresistivity can reach as high as 40%. Our research indicates that the heterostructure constructed by combining the ferromagnet VS2 and the non-magnetic semiconductor GaN is a promising material for vdW spin valve devices at room temperature.

Modulating the electronic transport properties of zigzag phosphorene nanoribbons through the coupling with the chromium triiodide molecules

Based on density functional theory (DFT) and non-equilibrium Green's function (NEGF) method, we have investigated spin-dependent electronic transport properties of zigzag phosphorene nanoribbons (ZPNRs) coupled with chromium triiodide (CrI3) molecules. Three different adsorption models have been considered: CrI3 molecule is placed on ZPNR surface (C-type), and CrI3 molecule is lateral adsorbed on the edge of ZPNR with its molecule panel vertical (V-type) or parallel (P-type). We discovered that ZPNR's electrical transport clearly displays spin polarization, owing to the coupling between ZPNR and CrI3 molecules. And spin-polarization is tightly dependent on the absorbing manners. Moreover, the negative differential resistance (NDR) characteristics can be observed from current-voltage curves and modulated by absorbing molecules. Furthermore, we also discovered that CrI3 is vertically adsorbed on the surface of the ZPNRs has a better spin-polarization efficiency than the other two cases.

A Theoretical Analysis of Spin-Dependent Tunneling in ZnO-Based Heterostructures

The electron tunneling in ZnO/ZnCdO semiconductor heterostructure is studied using the matrix method. The spin-polarization is examined due to Dresselhaus spin–orbit interaction and Rashba spin–orbit interaction. The total spin-polarization is mainly due to Dresselhaus spin–orbit interaction and the Rashba spin–orbit interaction is small. The high degree of spin polarization can be achieved at a high barrier width. With the increasing cadmium concentration in this heterostructure, the spin polarization efficiency is enhanced. The dwell time is reported and discussed.

Highly Durable Spin Filter Switching Based on Self-Assembled Chiral Molecular Motor.

Chiral molecules have recently received renewed interest as highly efficient sources of spin-selective charge emission known as chiral-induced spin selectivity (CISS), which potentially offers a fascinating utilization of organic chiral materials in novel solid-state spintronic devices. However, a practical use of CISS remains far from completion, and rather fundamental obstacles such as (i) external controllability of spin, (ii) function durability, and (iii) improvement of spin-polarization efficiency have not been surmounted to date. In this study, these issues are addressed by developing a self-assembled monolayer (SAM) of overcrowded alkene (OCA)-based molecular motor. With this system, it is successfully demonstrated that the direction of spin polarization can be externally and repeatedly manipulated in an extremely stable manner by switching the molecular chirality, which is achieved by a formation of the covalent bonds between the molecules and electrode. In addition, it is found that a higher stereo-ordering architecture of the SAM of OCAs tailored by mixing them with simple alkanethiols considerably enhances the efficiency of spin polarization per a single OCA molecule. All these findings provide the creditable feasibility study for strongly boosting development of CISS-based spintronic devices that can simultaneously fulfill the controllability, durability, and high spin-polarization efficiency.

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.

Room-temperature polarization of individual nuclear spins in diamond via anisotropic hyperfine coupling and coherent population trapping

We employ the electronic spin of a single nitrogen-vacancy (NV) defect in diamond to detect and control the quantum state of remote nuclear spins coupled by hyperfine interaction. More precisely, our work focuses on individual ^{13}text {C} nuclei featuring a moderate hyperfine coupling strength (sim 1 MHz) with the NV’s electron spin. Two different methods providing an efficient room-temperature polarization of these peculiar ^{13}text {C} nuclear spins are described. The first one is based on a polarization transfer from the NV electron spin to the ^{13}text {C} nucleus, which is mediated by the anisotropic component of the hyperfine interaction. The second one relies on coherent population trapping (CPT) within a Lambda -type energy-level configuration in the microwave domain, which enables to initialize the ^{13}text {C} nuclear spin in any quantum state superposition on the Bloch sphere. This CPT protocol is performed in an unusual regime for which relaxation from the excited level of the Lambda -scheme is externally triggered by optical pumping and separated in time from coherent microwave excitations. For these two polarization techniques, we investigate the impact of optical illumination on the nuclear spin polarization efficiency. This work adds new methods to the quantum toolbox used for coherent control of individual nuclear spins in diamond, which might find applications in quantum metrology.Graphical

Vacancy tuned thermoelectric properties and high spin filtering performance in graphene/silicene heterostructures

The main contribution of this paper is to study the spin caloritronic effects in defected graphene/silicene nanoribbon (GSNR) junctions. Each step-like GSNR is subjected to the ferromagnetic exchange and local external electric fields, and their responses are determined using the nonequilibrium Green’s function (NEGF) approach. To further study the thermoelectric (TE) properties of the GSNRs, three defect arrangements of divacancies (DVs) are also considered for a larger system, and their responses are re-evaluated. The results demonstrate that the defected GSNRs with the DVs can provide an almost perfect thermal spin filtering effect (SFE), and spin switching. A negative differential thermoelectric resistance (NDTR) effect and high spin polarization efficiency (SPE) larger than 99.99% are obtained. The system with the DV defects can show a large spin-dependent Seebeck coefficient, equal to Ss ⁓ 1.2 mV/K, which is relatively large and acceptable. Appropriate thermal and electronic properties of the GSNRs can also be obtained by tuning up the DV orientation in the device region. Accordingly, the step-like GSNRs can be employed to produce high efficiency spin caloritronic devices with various features in practical applications.

Spin polarization and magneto-dielectric coupling in Al-modified thin iron oxide films -microwave mediated sol-gel approach

Production of single-phase materials with multifunctional properties is still a challenge faced by material scientists. In addition, obtaining high spin polarization efficiency in the materials that exhibit multifunctional properties is a big issue. A novel approach is suggested in this work for obtaining multifunctionality and spin polarization in the same material. This approach has combined the effect of microwave radiations and aluminum (Al) doping in iron oxide thin films during synthesis. Combined effect of microwave radiations and Al doping results in controlling / tuning the structural transitions in iron oxide thin films. Pristine and 2–10 wt% Al doped iron oxide thin films are prepared and studied in detail. Raman analysis shows that 2 and 4 wt% Al concentration results in γ-Fe2O3 + Fe3O4 phase with 71.3% and 64.5% of γ-Fe2O3 content, respectively. XRD and Raman analyses confirm the transition from γ-Fe2O3 to Fe3O4 thin films at Al concentrations of 6–10 wt%. Structural transformation shows that microwave radiations catalyzes that Al3+ions to occupy the vacancies on B sites of iron oxide thus, lead to the formation of Fe3O4. Observation of Verwey transition ~ 126 K also supports the transition in phases of iron oxide with increase in saturation magnetization from 251.3emu/cm3 (pristine films) to 405.6emu/cm3 (8 wt% Al concentration). High dielectric constant of ~ 135.5 (log f = 5.0) is observed for 8 wt% Al concentration. Conductivity and detailed impedance & modulus analyses depict Mott’s hopping phenomenon along with presence of different relaxation times. Coupling between magnetic and dielectric properties is observed at room temperature. Magnetoresistance curves indicate spin polarization efficiency of ~24%.

Role of transport polarization in electrocatalysis: A case study of the Ni-cluster/Graphene interface

As cathodes, iron-series (Fe, Co, Ni) clusters supported by carbon materials exhibit outstanding electrocatalytic reduction activities in many electrocatalytic applications. To date, this general characteristic of iron-series clusters that should be related to the inherent attributes of these electrodes has not been fully understood from the perspectives of thermodynamics and electronic structure alone. Electron transport is a necessary process in electrocatalysis, and therefore, the study of the change of the electronic state in electron transport is beneficial for understanding this general characteristic of iron-series cluster catalysts. In this work, the electron transport properties, including the conductivity and transport spin-polarization at the Ni-cluster/graphene interface are carefully investigated as an example of carbon-supported iron-series electrodes. Using first-principles calculations within the framework of the nonequilibrium Green’s function density functional theory (NEGF-DFT), we reveal that the electronic transport states of the coupled Ni-cluster/graphene are strongly changed compared to those of their isolated Ni-cluster and graphene component. It is found that graphene dominates the overall conductivity of the interface, while the morphology of Ni-clusters controls the spin polarization efficiency. High spin polarization can lead to the self-excitation effect of the electrons that raises the energy of the electronic system, improves the thermodynamics of the reduction reaction and promotes catalytic activity. Our work hints that iron-series elements (Fe, Co, Ni) based electrodes may generally show transport polarization that is likely to give rise to a high electrocatalytic reduction activity and such transport polarizability can be used as a new factor in the further exploration and design for electrocatalytic materials.

Pure thermal spin current and perfect spin-filtering with negative differential thermoelectric resistance induced by proximity effect in graphene/silicene junctions

The spin-dependent Seebeck effect (SDSE) and thermal spin-filtering effect (SFE) are now considered as the essential aspects of the spin caloritronics, which can efficiently explore the relationships between the spin and heat transport in the materials. However, there is still a challenge to get a thermally-induced spin current with no thermal electron current. This paper aims to numerically investigate the spin-dependent transport properties in hybrid graphene/silicene nanoribbons (GSNRs), using the nonequilibrium Green’s function method. The effects of temperature gradient between the left and right leads, the ferromagnetic exchange field, and the local external electric fields are also included. The results showed that the spin-up and spin-down currents are produced and flow in opposite directions with almost equal magnitudes. This evidently shows that the carrier transport is dominated by the thermal spin current, whereas the thermal electron current is almost disappeared. A pure thermal spin current with the finite threshold temperatures can be obtained by modulating the temperature, and a negative differential thermoelectric resistance is obtained for the thermal electron current. A nearly zero charge thermopower is also obtained, which further demonstrates the emergence of the SDSE. The response of the hybrid system is then varied by changing the magnitudes of the ferromagnetic exchange field and local external electric fields. Thus, a nearly perfect SFE can be observed at room temperature, whereas the spin polarization efficiency is reached up to 99%. It is believed that the results obtained from this study can be useful to well understand the inspiring thermospin phenomena, and to enhance the spin caloritronics material with lower energy consumption.

Dynamic N14 nuclear spin polarization in nitrogen-vacancy centers in diamond

We studied the dynamic nuclear spin polarization of nitrogen in negatively charged nitrogen-vacancy (NV) centers in diamond both experimentally and theoretically over a wide range of magnetic fields from 0 to 1100 G covering both the excited-state level anti-crossing and the ground-state level anti-crossing magnetic field regions. Special attention was paid to the less studied ground-state level anti-crossing region. The nuclear spin polarization was inferred from measurements of the optically detected magnetic resonance signal. These measurements show that a very large (up to $96 \pm 2\%$) nuclear spin polarization of nitrogen can be achieved over a very broad range of magnetic field starting from around 400 G up to magnetic field values substantially exceeding the ground-state level anti-crossing at 1024 G. We measured the influence of angular deviations of the magnetic field from the NV axis on the nuclear spin polarization efficiency and found that, in the vicinity of the ground-state level anti-crossing, the nuclear spin polarization is more sensitive to this angle than in the vicinity of the excited-state level anti-crossing. Indeed, an angle as small as a tenth of a degree of arc can destroy almost completely the spin polarization of a nitrogen nucleus. In addition, we investigated theoretically the influence of strain and optical excitation power on the nuclear spin polarization.

Impact of Process Variability on Write Error Rate and Read Disturbance in STT-MRAM Devices

Spin-transfer torque magnetic random-access memory (STT-MRAM) is the most promising next-generation memory technology that combines the advantages of mainstream memory technologies, such as SRAM, DRAM, and flash. In the STT-MRAM, a magnetic tunnel junction (MTJ) is used as a bit-cell to store the data, and its magnetic properties have a critical role in thermal noise-aware STT-switching operations. This work analyzes the impact of MTJ material and geometric parameter variations, such as saturation magnetization (MS), magnetic anisotropy (HK), damping factor (α), spin polarization efficiency factor (η), oxide thickness (tOX), free layer thickness (tF), tunnel magnetoresistance (TMR), and cross-sectional area of free layer (AF) variations on write error rate (WER) and read disturbance rate (RDR) for reliable write and read operations, respectively. To evaluate the scalability of MRAM devices, we investigate both WER and RDR with a wide range of MTJ diameters between 90 and 30 nm, which corresponds to mainstream technology nodes from 40 up to 14 nm advance node. In our work, the Fokker-Planck (FP) numerical approach is mainly utilized for an efficient analysis, which allows for parametric variation and evaluates its impact on switching. Although the impact of material and geometric parameter variations on WER is decreased as MTJ scales down from 90 to 30 nm, the variation effect can be still critical with small MTJ diameter, and the most significant influential variation is η, MS, HK, and α in that order. By contrast, the impact of material and geometric parameter variation on RDR increases in MTJ scaling, and we show that negative variations of HK and MS could be a critical bottleneck in 30 and 40 nm MTJ diameters. Our work finally emphasizes the necessity of the WER and RDR analysis by considering the parameter variation in MTJ scaling for practical STT-MRAM development.

Magnetoresistance effect in a vertical spin valve fabricated with a dry-transferred CVD graphene and a resist-free process

One of the most prominent and effective applications of graphene in the field of spintronics is its use as a spacer layer between ferromagnetic metals in vertical spin valve devices, which are widely used as magnetic sensors. The magnetoresistance in such devices can be enhanced by a selection of suitable spacer materials and proper fabrication procedures. Here, we report the use of dry-transferred single- and double-layer graphene, grown by chemical vapor deposition (CVD), as the spacer layer and the fabrication procedure in which no photo-resist or electron-beam resists is used. The measured maximum magnetoresistance of NiFe/CVD-Graphene/Co junction is 0.9% for the single- and 1.2% for the double-layer graphene at 30 K. The spin polarization efficiency of the ferromagnetic electrodes is about 6.7% and 8% for the single- and the double-layer graphene, respectively, at the same temperature. The bias-independent magnetoresistance rules out any contamination and oxidation of the interfaces between the ferromagnet and the graphene. The magnetoresistance measured as a function of tilted magnetic field at different angles showed no changes in the maximum value, which implies that the magnetoresistance signal is absent from anisotropic effects.

Spin filtering in GaAs/Al0.3Ga0.7As multiple quantum wells

In this study, spin polarization efficiency and spin-dependent transmission of electrons passing through a multiple quantum well structure with constant total effective length (60 nm) made of GaAs/Al0.3Ga0.7As, which was grown along [001] direction, were investigated. The calculations were done with the quantum transmitting boundary method. We presented the number of wells in a constant total length as a tuning tool for spin filtering purposes. In some energy intervals, 100% spin polarization and spin-dependent transmission were observed.

High Spin Polarization and Thermoelectric Efficiency of Half-Metallic Ferromagnetic CrYSn (Y=Ca, Sr) of Half-Heusler Compounds

In the present work we have performed self-consistent ab-initio calculation using the full-potential linearized augmented plane-wave method (FP-LAPW), based on the density functional theory (DFT) as implemented in the Wien2k code to study the structural, electronic, magnetic, thermodynamic and thermoelectric properties of the half-heusler compound CrYSn ([Formula: see text], Sr) using generalized gradient approximation (GGA) described by Perdew–Burke–Ernzerhof (PBE), GGA+U and the modified Beck–Johnson correction (mBJ), the obtained results show that the compound is stable in the ferromagnetic state (FM) in [Formula: see text] phase on one hand and has a half-metallic character (metallic nature in spin up channel and semiconductor one in spin down channel with an indirect gap) on the other hand thus, the compound is a good candidate for spintronic applications, moreover it shows a very interesting thermoelectric predisposition in the minority spin or spin down channel at room temperature consisting of a very high Seebeck coefficient, high electrical conductivity and figure of merit near unity for the two compounds. The thermodynamic properties of CrCaSn and CrSrSn compounds using Gibbs code are studied for the first time. This study showed that these compounds can be used in extreme thermodynamic conditions. Since no experimental data were reported until now concerning this compound, our theoretical predictions of electronic, thermodynamic and thermoelectric properties are likely to be experimentally verified.

Lead position and lead-ring coupling effects on the spin-dependent transport properties in a two-dimensional network of quantum nanorings in the presence of Rashba spin–orbit interaction

The effects of lead positions and lead-ring coupling regimes are investigated for spin-related transport through a two-dimensional network of quantum nanorings (2DNQRs) considering Rashba spin–orbit interaction (RSOI) and magnetic flux. A matrix representation of the transmission and reflection coefficients through a single ring connected to the arbitrary number of leads has been introduced. As a specific example of 2DNQRs, the conductance, spin polarization and system efficiency are obtained via a triangular network of quantum rings (TNQRs). TNQRs are completely opaque or transparent versus RSOI strength and wave vector (k) of the incident electron. The periodicity of these functions along the k axis depends on the lead positions and is independent of lead-ring coupling. Also, the symmetric geometry and strong lead-ring coupling regime significantly improve the performance of the system as a multipurpose spintronic device (i.e., a perfect spin filter, spin splitter, spin switching, Stern–Gerlach device and an electronic switching device).

Improved lifetime of a high spin polarization superlattice photocathode

Negative Electron Affinity (NEA) activated surfaces are required to extract highly spin-polarized electron beams from GaAs-based photocathodes, but they suffer extreme sensitivity to poor vacuum conditions that results in rapid degradation of quantum efficiency. We report on a series of unconventional NEA activations on surfaces of bulk GaAs with Cs, Sb, and O2 using different methods of oxygen exposure for optimizing photocathode performance. One order of magnitude improvement in lifetime with respect to the standard Cs–O2 activation is achieved without significant loss of electron spin polarization and quantum efficiency by codepositing Cs, Sb, and O2. A strained GaAs/GaAsP superlattice sample activated with the codeposition method demonstrated similar enhancement in lifetime near the photoemission threshold while maintaining 90% spin polarization.

Electronic properties and spintronic applications of carbon phosphide nanoribbons

Carbon phosphide (CP) monolayer, a hybrid of graphene and phosphorene that simultaneously preserves their high electrical mobility, finite band gap, and air stability, has been successfully synthesized in a recent experiment. In this work, we study the electronic and transport properties of CP nanoribbons in $\ensuremath{\alpha}$-phase with different edge configurations using first-principle density functional theory simulation. We find that the electronic properties of the CP nanoribbon exhibit contrasting behaviors depending on the edge termination configurations. In the presence of hydrogen edge passivation, the nanoribbon mimics black phosphorus nanoribbons with field-effect-tunable band gap. Conversely, for a bare edge terminated by a phosphorus atom, the nanoribbon becomes graphene-like, exhibiting large spin splitting in the ferromagnetic state and can be tuned into a half metal under an external transverse electric field in the antiferromagnetic state. Intriguingly, when the external transverse electric field exceeds a critical value, the charge transport channel becomes strongly localized to only one edge, and the edge localization can be tuned by an external electric field. Leveraging on the unusual spin-resolved transport properties of the CP nanoribbon, we demonstrate the operation of a bipolar spin filter with exceptional spin polarization efficiency approaching 100%. Our findings reveal the potential of the CP nanoribbon as a spintronic material that fuses the strengths of both graphene and phosphorene.

Semiconductor-to-metallic spin-filtering and positive and negative magnetoresistance effects in C3N with nickel electrodes

Abstract Carbon-based spintronic devices have attracted extensive attention recently because they have long spin relaxation times and lengths. As a new member of the carbon material family, C3N may have great application prospects in the field of spintronics. Though a previous work proposed the model of spintronic devices based on bare C3N nanoribbons, these devices are difficult to implement experimentally. This is due to the fact that edge reconstruction will occur in the bare C3N nanoribbons. In this work, we systematically investigate the spin-polarized properties of 2D C3N sheet and its corresponding hydrogen-passivated nanoribbons with ferromagnetic Ni electrodes. Semiconductor-to-metallic spin-filtering and positive magnetoresistance effect are observed in 2D C3N sheet stacked on top of the Ni electrodes. Especially impressive is the enhancement of spin-polarization efficiency and the emergence of negative magnetoresistance effect after the 2D C3N sheet is converted into hydrogen-passivated nanoribbons. These results indicate that C3N has strong potential for nanoscale spintronics applications.

Magneto-electronic properties, carrier mobility and strain effects of InSe nanoribbon

The monolayer InSe has been successfully fabricated recently and studied intensely. Here, we investigate the geometrical stability and various physical properties such as electronic and magnetic feature, carrier mobility and strain effects for InSe nanoribbons. Our calculations show that armchair nanoribbons, regardless of the bare-edged or H-saturated ones, are semiconductors with an indirect bandgaps, but the bandgap size is increased greatly by H-saturation. Their electron mobility is predicted to be moderately large (from ~102 to ~103 cm2 V−1 s−1) with the holes being less mobile for wider ribbons, and the carrier polarity phenomenon becomes more prominently for H-saturation. The zigzag InSe nanoribbons are found to be magnetic metals with a bigger magnetic moment and the ferromagnetic ground state at the single edge. The magnetism stems from unpaired electrons at the In-rich edge. More interestingly, it is found that the externally applied mechanical strain can effectively tune the spin polarization efficiency at the Fermi level to two stepwise stages, suggesting that the strain can act as a tool for developing a mechanical switch to control spin-polarized transport under lower bias. The detailed analysis suggests that this strain-tuning mechanism can be attributed to the ionic and covalent bond-configuration competition due to the strain-induced bond-length alterations, which leads to the unpaired electron redistribution in magnetic atoms or vanishing.