Spin Filter Tunnel Research Articles

The shot noise in quasi-one-dimensional single spin filter magnetic heterojunctions

Based on the free electronic model and Büttike’s theory on shot noise, we investigated the shot noise of spin electrons in ferromagnetic/non-ferromagnetic insulator/spin filter layer/normal metal (FM/I/SF/NM) quasi-one-dimensional single spin filter tunneling junctions. The calculation results show that the barrier heights and thicknesses of the insulator and spin filter layers strongly affect the values and phases of the Fano factors of spin electrons. Compared with the traditional sandwich structures of ferromagnetic/spin filter layer/normal metal (FM/SF/NM) and ferromagnetic/non-ferromagnetic insulator/normal metal (FM/I/NM), when the thicknesses of the insulator and spin filter layers are very small, in single spin filter magnetic heterojunctions (FM/I/SF/NM), the insulator layer can suppress the Fano factors, while the spin filter layer is easier to separate the Fano factors of electrons whose spin directions are different. Further analysis shows that whether the Rashba coupling in the spin filter layer has an obvious influence on the Fano factors of spin electrons depends on the thickness of the spin filter layer. On the other hand, the Fano factors are strongly dependent on the sizes and directions of the molecular fields of the ferromagnet and spin filter layers. In addition, the electrons are incident with low energy or high energy, and the barrier heights and thicknesses of the spin filter layer and the thicknesses of the insulation layer have a significant effect on the spin-direction-dependent Fano factors of electrons.

Tuning magnetoresistance of chromium chloride tunnel junction through the interface and multi-field effect

<sec>Magnetic tunnel junctions (MTJs) serve as essential platforms for investigating spin transport properties, magnetic phase transitions, and anisotropy in magnetic materials. Recently two-dimensional van der Waals antiferromagnetic insulators like chromium chloride (CrCl<sub>3</sub>) or chromium iodide (CrI<sub>3</sub>) have been used to develop spin-filtering magnetic tunnel junctions (sf-MTJs), improving the device performance for material property exploration and spintronic applications. However, it is crucial to recognize that the spin-filtering effect is not the sole determining factor of tunneling magnetoresistance (TMR) in these junctions; the interface magnetic exchange interactions and adjustable electrode density of states (DOS) fluctuations, response to applied electric or magnetic fields, can also influence the tunneling current.</sec><sec>In this study, we fabricate MTJ devices by using mechanically-exfoliated few-layer CrCl<sub>3</sub> as the tunnel barrier and few-layer graphene (FLG) as electrodes through dry transfer technique. Conducting low-temperature quantum transport measurements, we observe unconventional TMR behaviors, including bias-voltage-dependent TMR, oscillatory tunneling current under high magnetic fields, and tunable tunneling current via gate voltage.</sec><sec>A qualitative model of elastic tunneling current is employed to analyze the spin and band characteristics of the MTJ device. The observed bias-voltage-dependent TMR is attributed to the changes in the tunneling mechanism due to magnetic proximity effect, which induces magnetization in the FLG electrode near the FLG/CrCl<sub>3</sub> interface. The antiparallel alignment of polarized spin to CrCl<sub>3</sub>’s magnetization results in injected charge carriers facing a higher tunnel barrier, leading to negative TMR at lower bias voltages. As the bias voltage increases, the magnetic proximity effect lessens, and the device reverts to its conventional spin-filtering functionality. The oscillatory tunneling current is explained by the graphene electrode’s quantum oscillatory density of states behavior under vertical magnetic fields, which can be controlled by the applied gate voltage.</sec><sec>This study contributes to the understanding of previously unexplored TMR phenomena in two-dimensional MTJs, deepening our insights into carrier transport properties in these heterostructures and broadening avenues for investigating the physical properties of two-dimensional magnetic materials and their spintronic applications.</sec>

Magnetic-ferroelectric synergic control of multilevel conducting states in van der Waals multiferroic tunnel junctions towards in-memory computing.

van der Waals (vdW) multiferroic tunnel junctions (MFTJs) based on two-dimensional materials have gained significant interest due to their potential applications in next-generation data storage and in-memory computing devices. In this study, we construct vdW MFTJs by employing monolayer Mn2Se3 as the spin-filter tunnel barrier, TiTe2 as the electrodes and In2S3 as the tunnel barrier to investigate the spin transport properties based on first-principles quantum transport calculations. It is highlighted that apparent tunneling magnetoresistance (TMR) and tunneling electroresistance (TER) effects with a maximum TMR ratio of 6237% and TER ratio of 1771% can be realized by using bilayer In2S3 as the tunnel barrier under finite bias. Furthermore, the physical origin of the distinguished TMR and TER effects is unraveled from the k||-resolved transmission spectra and spin-dependent projected local density of states analysis. Interestingly, four distinguishable conductance states reveal the implementation of four-state nonvolatile data storage using one MFTJ unit. More importantly, in-memory logic computing and multilevel data storage can be achieved at the same time by magnetic switching and electrical control, respectively. These results shed light on vdW MFTJs in the applications of in-memory computing as well as multilevel data storage devices.

Tunnel-magneto resistance of double-barrier tunnel junctions in the presence of spin-orbit coupling within spin-filter barriers

The tunnel magneto-resistance ratio is investigated for double-barrier tunnel junctions with spin-filter (magnetic insulators) barriers. The transmission probability is calculated in the tight-binding model by the non-equilibrium Green's function formalism considering spin-orbit coupling within barriers. The formation of resonant states inside a non-magnetic spacer gives rise to an enhancement in tunnel magneto-resistance ratio. Double spin-filter tunnel junctions have also shown a higher tunnel magneto-resistance ratio compared to that in double-barrier magnetic tunnel junctions. These findings will be valuable to improve the performance of spintronics devices.

Spin-Filter Magnetic Tunnel Junctions With Tunable Enhanced Tunnel Magneto-Resistance

Spin-Filter Magnetic Tunnel Junctions With Tunable Enhanced Tunnel Magneto-Resistance

Effect of spin-orbit coupling within the spin-filter layer on the tunnel magneto-resistance in spin-filter magnetic tunnel junctions

The tunnel magneto-resistance ratio is investigated for spin-filter magnetic tunnel junctions in the presence of spin–orbit coupling within a spin-filter layer. The non-equilibrium Green’s function formalism is utilized to calculate the transmission function in the linear-response limit. The results show that a larger tunnel magneto-resistance is achieved for spin-filter magnetic tunnel junctions compared to that for conventional magnetic tunnel junctions due to the existence of a spin-filter layer. Therefore, the current findings can be introduced new routes to improve the field of spintronics.

First-Principles Study of Induced Magnetism in Tungsten Vanadium Selenide Alloys for Spintronic Applications.

The possibility of inducing magnetization in tungsten selenide monolayer by alloying with vanadium selenide was investigated through first-principles calculations. Electronic, optical, and magnetic properties of different W1–xVxSe2 alloy compositions were studied extensively. As the proportion of vanadium atoms in the alloys increased, a phase transition from semiconducting to metallic to semiconducting was discovered. All alloy compositions demonstrated induced magnetism with a long-range ferromagnetic order. Interestingly, in the case of the W0.25V0.75Se2 alloy, spin-up states in the band diagram showed a finite band gap, while a nonzero band gap was found for spin-down states. The W0.25V0.75Se2 alloy can be used as a spin filter tunneling barrier exploiting this fascinating property. High spin polarization of the tunnel current was found for the alloy. Furthermore, under the Curie temperature, electrical conductivity for the spin-up channel was found to be zero, while conductivity for the spin-down channel was around 1019 (Ω cm s)−1 when the chemical potential was 0.2 eV greater than the Fermi energy. Changes in optical properties were also investigated through time-dependent density functional theory calculations. The findings of this study will be beneficial for proposing new magnetic monolayer alloys for application in nanoscale spintronic devices.

Spin-Filter Magnetic Tunnel Junctions Based on A-Type Antiferromagnetic CrSBr with Giant Tunnel Magnetoresistance

CrSBr is a stable two-dimensional (2D) van der Waals (vdW) magnet with intralayer ferromagnetic and interlayer antiferromagnetic couplings. Here, we propose to use CrSBr as the barrier in spin-filter (sf) MTJ and establish the devices based on graphene/CrSBr/graphene structures. Employing density functional theory (DFT) combined with the nonequilibrium Green’s function approach, we investigated the transmission details, and the results show TMR values above 330%, 2 × 107% and 105% with two-, four- and six-layer CrSBr at zero bias, respectively. Subsequently, we systematically analyze the transmission spectra, transmission eigenstates, electrostatic potentials, band structures and local density of states to elaborate the underlying mechanism of the TMR effect in the sf-MTJs. Our results indicate the great prospect of CrSBr-based sf-MTJs in applications, and provide guidance for futural experiments.

Giant tunneling magnetoresistance induced by bias voltage in spin-filter van der Waals magnetic tunnel junctions with an interlayer antiferromagnetic semiconductor barrier

Spin-filter van der Waals Magnetic tunnel junctions (sf-vdW MTJs) formed by an interlayer antiferromagnetic (AFM) vdW semiconductor as a barrier have exhibited promising prospects in achieving a high tunneling magnetoresistance (TMR) ratio in MTJs. Here, using first-principles calculations, we investigate the spin-dependent transport and the TMR effect of sf-vdW MTJs formed by sandwiching bilayer and trilayer $\mathrm{Ni}{\mathrm{Br}}_{2}$ barriers between two graphite electrodes ($\mathrm{Gr}/\mathrm{Ni}{\mathrm{Br}}_{2}/\mathrm{Gr}$ sf-vdW MTJs). Similar to the experimental results of sf-vdW MTJs formed by a few-layer $\mathrm{Cr}{\mathrm{I}}_{3}$ barrier, the TMR ratios of the $\mathrm{Gr}/\mathrm{Ni}{\mathrm{Br}}_{2}/\mathrm{Gr}$ sf-vdW MTJs increase first with the increase of bias voltage and decrease with the further increase of bias voltage after reaching the highest points because the conduction bands of the interlayer ferromagnetic (FM) $\mathrm{Ni}{\mathrm{Br}}_{2}$ barrier at the $K$ points go into the bias window earlier than those of the interlayer AFM $\mathrm{Ni}{\mathrm{Br}}_{2}$ barrier with the increase of bias voltage. Compared to the TMR ratios of about 170% and 206% at zero bias voltage, the TMR ratios of the $\mathrm{Gr}/\mathrm{Ni}{\mathrm{Br}}_{2}/\mathrm{Gr}$ sf-vdW MTJs with bilayer and trilayer $\mathrm{Ni}{\mathrm{Br}}_{2}$ barriers are largely increased about 34 and 67 times by the optimized bias voltage, respectively. Correspondingly, a giant TMR ratio of about 6000% and 14 000% can be achieved in the $\mathrm{Gr}/\mathrm{Ni}{\mathrm{Br}}_{2}/\mathrm{Gr}$ sf-vdW MTJs with bilayer and trilayer $\mathrm{Ni}{\mathrm{Br}}_{2}$ barriers at 0.14 and 0.125 V bias voltage, respectively. Our results elucidate the mechanism of bias voltage induced giant TMR ratio in the $\mathrm{Gr}/\mathrm{Ni}{\mathrm{Br}}_{2}/\mathrm{Gr}$ sf-vdW MTJs and provide promising routes for developing MTJs with a high TMR ratio.

Recent developments on 2D magnetic materials: challenges and opportunities

The emergence of two-dimensional (2D) magnetic materials exhibiting strong magnetization at ultrathin limits above room temperature is promising for miniaturization of devices beyond Moore’s law for future energy efficient nanoelectronic devices. Here, the current status, different mechanisms for the existence of magnetism, spin current injection, and other magnetic properties of monolayer to few layers of various 2D magnetic materials are reviewed. Some of the promising applications of these materials are spintronics devices such as spin valves, spin tunnel field-effect transistors, and spin-filtering magnetic tunnel junctions. Due to the tunable electronic properties of these 2D materials, it is quite interesting to inject the spin current with suitable ferromagnetic contacts. For instance, black phosphorus is a layered material with a small Schottky barrier height capable of injecting spin current. This review includes many recently explored 2D magnetic materials ranging from exfoliated 2D crystals to CVD grown materials from single to several layers, demonstrating tunable layer-dependent magnetic properties. We also explore some of the promising theoretical study based on 2D magnetic compounds such as 2D alkali-based chromium chalcogenides, which shows ferromagnetic as well as semiconducting behavior. The layer-dependent magnetic ordering has been observed in layered compounds like 1T-CrTe2, VSe2, CrI3, and Fe3GeTe2, which have great potential for the future applications in magnetic-based electronic devices. Finally, we emphasize the challenges, opportunities, and future directions of the 2D magnetic materials, where new discoveries might have outstanding impact in transformational scientific breakthroughs towards memory, spintronics, optoelectronics, and other multifunctional device applications.

Topological superconductivity in semiconductor–superconductor–magnetic-insulator heterostructures

Hybrid superconductor-semiconductor heterostructures are promising platforms for realizing topological superconductors and exploring Majorana bound states physics. Motivated by recent experimental progress, we theoretically study how magnetic insulators offer an alternative to the use of external magnetic fields for reaching the topological regime. We consider different setups, where: (1) the magnetic insulator induces an exchange field in the superconductor, which leads to a splitting in the semiconductor by proximity effect, and (2) the magnetic insulator acts as a spin-filter tunnel barrier between the superconductor and the semiconductor. We show that the spin splitting in the superconductor alone cannot induce a topological transition in the semiconductor. To overcome this limitation, we propose to use a spin-filter barrier that enhances the magnetic exchange and provides a mechanism for a topological phase transition. Moreover, the spin-dependent tunneling introduces a strong dependence on the band alignment, which can be crucial in quantum-confined systems. This mechanism opens up a route towards networks of topological wires with fewer constraints on device geometry compared to previous devices that require external magnetic fields.

Spin-Filtering Ferroelectric Tunnel Junctions as Multiferroic Synapses for Neuromorphic Computing.

As nanoelectronic synapses, memristive ferroelectric tunnel junctions (FTJs) have triggered great interest due to the potential applications in neuromorphic computing for emulating biological brains. Here, we demonstrate multiferroic FTJ synapses based on the ferroelectric modulation of spin-filtering BaTiO3/CoFe2O4 composite barriers. Continuous conductance change with an ON/OFF current ratio of ∼54 400% and long-term memory with the spike-timing-dependent plasticity (STDP) of synaptic weight for Hebbian learning are achieved by controlling the polarization switching of BaTiO3. Supervised learning simulations adopting the STDP results as database for weight training are performed on a crossbar neural network and exhibit a high accuracy rate above 97% for recognition. The polarization switching also alters the band alignment of CoFe2O4 barrier relative to the electrodes, giving rise to the change of tunneling magnetoresistance ratio by about 10 times and even the reversal of its sign depending upon the resistance states. These results, especially the electrically switchable spin polarization, provide a new approach toward multiferroic neuromorphic devices with energy-efficient electrical manipulations through potential barrier design. In addition, the availability of spinel ferrite barriers epitaxially grown with ferroelectric oxides also expends the playground of FTJ devices for a broad scope of applications.

Spin-Polarized Electron Transfer in Multilayers with Different Types of Rough Interfaces

In this study, the influence of interfacial roughness of multilayers structures on spin filter tunneling is investigated at low biases as a function of interfacial roughness type. The results show that the roughness causes the reduction of resonant tunneling, maximum achievable tunneling magneto resistance (TMR), and spin-filtering efficiency of tunneling structures. Based on the numerical results, decreasing the electronic devices’ efficiency is straight related to roughening of interfacial. Additionally, the effect of different temperatures is investigated on the spin polarization (SP). The values of SP decreased sensibly by increasing the temperature.

Recent Advances in Two-Dimensional Magnets: Physics and Devices towards Spintronic Applications.

The emergence of low-dimensional nanomaterials has brought revolutionized development of magnetism, as the size effect can significantly influence the spin arrangement. Since the first demonstration of truly two-dimensional magnetic materials (2DMMs) in 2017, a wide variety of magnetic phases and associated properties have been exhibited in these 2DMMs, which offer a new opportunity to manipulate the spin-based devices efficiently in the future. Herein, we focus on the recent progress of 2DMMs and heterostructures in the aspects of their structural characteristics, physical properties, and spintronic applications. Firstly, the microscopy characterization of the spatial arrangement of spins in 2D lattices is reviewed. Afterwards, the optical probes in the light-matter-spin interactions at the 2D scale are discussed. Then, particularly, we systematically summarize the recent work on the electronic and spintronic devices of 2DMMs. In the section of electronic properties, we raise several exciting phenomena in 2DMMs, i.e., long-distance magnon transport, field-effect transistors, varying magnetoresistance behavior, and (quantum) anomalous Hall effect. In the section of spintronic applications, we highlight spintronic devices based on 2DMMs, e.g., spin valves, spin-orbit torque, spin field-effect transistors, spin tunneling field-effect transistors, and spin-filter magnetic tunnel junctions. At last, we also provide our perspectives on the current challenges and future expectations in this field, which may be a helpful guide for theorists and experimentalists who are exploring the optical, electronic, and spintronic properties of 2DMMs.

Voltage Control of a van der Waals Spin-Filter Magnetic Tunnel Junction.

Atomically thin chromium triiodide (CrI3) has recently been identified as a layered antiferromagnetic insulator, in which adjacent ferromagnetic monolayers are antiferromagnetically coupled. This unusual magnetic structure naturally comprises a series of antialigned spin filters, which can be utilized to make spin-filter magnetic tunnel junctions with very large tunneling magnetoresistance (TMR). Here we report voltage control of TMR formed by four-layer CrI3 sandwiched by monolayer graphene contacts in a dual-gated structure. By varying the gate voltages at fixed magnetic field, the device can be switched reversibly between bistable magnetic states with the same net magnetization but drastically different resistance (by a factor of 10 or more). In addition, without switching the state, the TMR can be continuously modulated between 17,000% and 57,000%, due to the combination of spin-dependent tunnel barrier with changing carrier distributions in the graphene contacts. Our work demonstrates new kinds of magnetically moderated transistor action and opens up possibilities for voltage-controlled van der Waals spintronic devices.

Spin splitting in EuO(111)/Si(111) spin-filter tunnel junctions with atomically sharp interface

We demonstrate the tunneling in spin-split barriers made of ferromagnetic EuO grown on Si(111) substrates by molecular beam epitaxy. For 6 nm thick EuO films with high crystal quality and atomically sharp interfaces, we find a barrier height lowering driven by the spin splitting below the Curie temperature of 35 K. We determined the splitting energy to be 0.56 ± 0.03 eV at 20 K which results in a spin polarization above 90%.

Structural and magnetic properties of La0.7Sr0.3MnO3/LaCoO3 heterostructures

Perovskite manganites are well known for their complex magnetic and electronic properties. Their heterostructures are even more perplexing, specifically when interfaced with magnetic or nonmagnetic insulators as they offer particular promise for applications such as spin filter tunnel junctions. This study explores the structural and magnetic properties of one such heterostructure, namely, La0.7Sr0.3MnO3/LaCoO3 (LSMO/LCO) bilayers as the thickness of the LCO layer is varied while keeping that of the LSMO layer fixed. Exceeding a threshold value while gradually increasing the LCO layer thickness, prominent side lobes are observed in the reciprocal space map signifying the formation of periodic structural striped domains in LCO. The magnetic properties of the bilayers, specifically the interfacial couplings, are markedly influenced by the variation of the LCO thickness with noteworthy consequences that include a tunable negative exchange bias.

Negative differential resistance, perfect spin-filtering effect and tunnel magnetoresistance in vanadium-doped zigzag blue phosphorus nanoribbons.

We investigate the electronic and transport properties of vanadium-doped zigzag blue phosphorus nanoribbons by first-principles quantum transport calculations. We study the spin-dependent transport properties and obtain current-voltage curves showing obvious spin polarization and negative differential behaviors. These interesting transport behaviors can be explained by the band structure of the vanadium-doped zigzag blue phosphorus nanoribbons. The tunnel magnetoresistance and spin-filtering effects under different magnetic configurations originate predominately from the symmetry matching between the band structures of the electrodes. According to our results, vanadium-doped zigzag blue phosphorus nanoribbons can be used as a perfect spin filter with a large tunnel magnetoresistance. This also indicates that blue phosphorus nanoribbons are a promising candidate for their future application in spintronics.

Analytic expression for the giant fieldlike spin torque in spin-filter magnetic tunnel junctions

We propose analytic expressions for fieldlike, ${T}_{\ensuremath{\perp}}$, and spin-transfer, ${T}_{\ensuremath{\parallel}}$, spin torque components in the spin-filter-based magnetic tunnel junction (SFMTJ), by using the single-band tight-binding model with the nonequilibrium Keldysh formalism. In consideration of multireflection processes between noncollinear magnetization of the spin-filter (SF) barrier and the ferromagnetic (FM) electrode, the central spin-selective SF barrier plays an active role in the striking discovery ${T}_{\ensuremath{\perp}}\ensuremath{\gg}{T}_{\ensuremath{\parallel}}$, which can be further identified by the unusual barrier thickness dependence of giant ${T}_{\ensuremath{\perp}}$. Our general expressions reveal the sinusoidal angular dependence of both spin torque components, even in the presence of the SF barrier.

θ0 thermal Josephson junction

We predict the thermal counterpart of the anomalous Josephson effect in superconductor/ferromagnet/superconductor junctions with non-coplanar magnetic texture. The heat current through the junction is shown to have the phase-sensitive interference component proportional to $\cos(\theta - \theta_0)$, where $\theta$ is the Josephson phase difference and $\theta_0$ is the texture-dependent phase shift. In the generic tri-layer magnetic structure with the spin-filtering tunnel barrier $\theta_0$ is determined by the spin chirality of magnetic configuration and can be considered as the direct manifestation of the energy transport with participation of spin-triplet Cooper pairs. In case of the ideal spin filter the phase shift is shown to be robust against spin relaxation caused by the spin-orbital scattering. Possible applications of the coupling between heat flow and magnetic precession are discussed.