Spin-polarized Tunneling Research Articles

Unneling Barrier Thickness Dependence of Spin Polarization of Ferromagnet in Magnetic Tunnel Junctions

Abstract For designing low-impedance magnetic tunnel junctions (MTJs), it has been found that tunneling magnetoresistance (TMR) strongly correlates with the insulating barrier thickness, imposing a fundamental question on the relationship between spin polarization of ferromagnet and the insulating barrier thickness in MTJs. Here, we investigate the influence of alumina barrier thickness on tunneling spin polarization (TSP) through a combination of theoretical calculations and experimental verification. Our simulating results reveal a significant impact of barrier thickness on TSP, exhibiting an oscillating decay of TSP with the barrier layer thinning. Experimental verification is realized on FeNi/AlO$_x$/Al superconducting tunnel junctions to directly probe the spin polarization of FeNi ferromagnet using Zeeman-split tunneling spectroscopy technique. These findings provide valuable insights for the design of high-performance spintronic devices, particularly in applications such as MRAM, where precise control over the insulating barrier layer is crucial.

GaAs quantum transport: Scrutinizing the effects of pressure, temperature and Rashba-Dresselhaus spin-orbit interactions for next-generation spintronic devices

This research investigates the spin-dependent transmission attributes and associated Giant Magnetoresistance (GMR) effect emerging from spin-polarized electron tunneling in a GaAs quantum well superlattice (QWS). The influence of Rashba and Dresselhaus spin-orbit interactions (SOIs) on transport properties is analyzed under the framework of the scattering matrix method. Our quantitative findings revealed that these spin-orbit couplings (SOCs) introduce inherent spin-filtering mechanisms, significantly influencing transmission probabilities. Furthermore, numerical estimations proved that external perturbations including pressure, temperature and structural modifications considerably impact the quantum transport characteristics of the scrutinized system. Additionally, major outcomes suggest that precise manipulation of spin-dependent transport in GaAs QWS holds promise for advancing spintronics technology. It is found that with well-tuned parameter values, the SOCs behave as a spin-dependent barrier modifying the band shape and its anisotropy. This study offers novel insights into the fundamental physics governing spin-dependent transport within quantum superlattices with promising implications for the development of advanced spintronic devices.

Half-Metallic Transport and Spin-Polarized Tunneling through the van der Waals Ferromagnet Fe4GeTe2.

We examine the coherent spin-dependent transport properties of the van der Waals (vdW) ferromagnet Fe4GeTe2 using density functional theory combined with the nonequilibrium Green's function method. Our findings reveal that the conductance perpendicular to the layers is half-metallic, meaning that it is almost entirely spin-polarized. This property persists from the bulk to a single layer, even under significant bias voltages and with spin-orbit coupling. Additionally, using dynamical mean field theory for quantum transport, we demonstrate that electron correlations are important for magnetic properties but minimally impact the conductance, preserving almost perfect spin-polarization. Motivated by these results, we then study the tunnel magnetoresistance (TMR) in a magnetic tunnel junction consisting of two Fe4GeTe2 layers with the vdW gap acting as an insulating barrier. We predict a TMR ratio of ∼500%, which can be further enhanced by increasing the number of Fe4GeTe2 layers in the junction.

Spin-textured Volkov–Pankratov states and their tunnel magnetoresistance response

Volkov–Pankratov (VP) states are a family of sub-gap states that appear at the smooth interface/domain wall between topologically distinct gaped states. We carry out quantum transport simulations on one- and two-dimensional lattice models to demonstrate the emergence of such states in the edge spectrum of a quantum spin Hall system subjected to a smoothly varying exchange field that switches its sign at a given spatial point. We show the VP states possess non-trivial spin textures that can be characterized by a winding number in real space. It is further demonstrated that the application of an electric field along the edge provides control of this spin texture without altering the winding number. Finally, we illuminate how these spin textures can be read off via the local tunnel magnetoresistance (TMR) response of spin-polarized tunnel probes attached to the edge and the TMR can be controlled by purely electrical means akin to a Datta–Das type spin transistor.

Magneto-optical tunability of impedance through electronic structure modification in ZnO–rGO/LSMO/ITO spintronic devices

In this paper, we have investigated red light (∼660 nm) and magnetic field dependence of impedance across (100−x)% ZnO(zinc oxide)–x% rGO(reduced graphene oxide)/La0.7Sr0.3MnO3(LSMO)/ITO (x=0,0.6,0.8,100) heterostructure devices. Field-induced scattering due to the spin filter effect and spin polarized tunneling (SPT) have been extracted from the zinc oxide–reduced graphene oxide nanocomposite/LSMO space charge region (ZnO–rGO/LSMO SCR) and the LSMO active region of the devices, respectively. Higher SPT leads to higher LSMO SCR scattering across the devices. Devices with higher rGO contents could not be fitted with two RC circuits as resistance values because the two phenomena are incomparable with each other. Light-induced scattering has been observed at the ZnO–rGO nanocomposite active region and ZnO–rGO/LSMO SCR of the devices. For composite devices with x=0.8 and 0.6, higher photocarrier generation at ZnO–rGO nanocomposite active layer leads to enhanced scattering at LSMO SCR with light illumination. Light-dependent scattering at both regions, however, follows almost same decreasing trend for bare devices with x=0, 100. The decreasing trend of light-dependent scattering for ZnO/LSMO/ITO and rGO/LSMO/ITO bare devices suddenly gets reversed and, eventually, follows an increasing trend at magnetic field ambiance of 0.5 and 1 kOe, respectively. The LSMO SCRs of the bare devices got enhanced with the field, leading to a light-dependent response similar to composite devices at the higher field.

The spin-polarized tunnel current and the spin-transfer torque in triple-barrier junctions with external electrodes and quantum wells made of ferromagnetic materials

The electron transport in triple-barrier junctions consisting of external ferromagnetic layers and three non-magnetic barriers separated by two inner ferromagnetic layers is analysed. It is assumed that the magnetic moments in the external electrodes are oriented in the same direction, whereas the magnetic moments of the inner layers can form any angle with this direction. The possibility of the gigantic enhancement of the normal torque component has been found in the junctions with a non-colinear orientation of magnetic moments in the inner layers. This enhancement may be much larger than in the double junctions. The current and the in-plane torque are most amplified when the quantum-well states in both inner layers have similar energies and can form the complex resonant states located slightly below the Fermi level. When the tunnelling by these complex resonant states occurs, the torque is greatest in the magnetic configuration other than that where the current is maximally enhanced. Additionally, the angular dependence of the torque differs significantly from that observed in single tunnel junctions.

Rashba effect on finite temperature magnetotransport in a dissipative quantum dot transistor with electronic and polaronic interactions

The Rashba spin–orbit coupling induced quantum transport through a quantum dot embedded in a two-arm quantum loop of a quantum dot transistor is studied at finite temperature in the presence of electron–phonon and Hubbard interactions, an external magnetic field and quantum dissipation. The Anderson-Holstein-Caldeira-Leggett-Rashba model is used to describe the system and several unitary transformations are employed to decouple some of the interactions and the transport properties are calculated using the Keldysh technique. It is shown that the Rashba coupling alone separates the spin-up and spin-down currents causing zero-field spin-polarization. The gap between the up and down-spin currents and conductances can be changed by tuning the Rashba strength. In the absence of a field, the spin-up and spin-down currents show an opposite behaviour with respect to spin–orbit interaction phase. The spin-polarization increases with increasing electron–phonon interaction at zero magnetic field. In the presence of a magnetic field, the tunneling conductance and spin-polarization change differently with the polaronic interaction, spin–orbit interaction and dissipation in different temperature regimes. This study predicts that for a given Rashba strength and magnetic field, the maximum spin-polarization in a quantum dot based device occurs at zero temperature.

Optimum contact resistance for two-terminal magnetoresistance in a lateral spin valve

The two-terminal magnetoresistance (2T-MR) due to spin accumulation in a lateral spin valve is determined for devices with a Si channel and Fe/MgO tunnel contacts of varying MgO thickness. Established theory predicts that the 2T-MR exhibits a pronounced maximum for contact resistances comparable to the spin resistance rs of the channel. At large contact resistance (≫rs), the 2T-MR is, indeed, very small, despite the large tunnel spin polarization (TSP) of the contacts (90%). When the contact resistance is reduced toward rs, the 2T-MR increases, but much less than expected because for thinner MgO the TSP decays. For devices with the thinnest MgO and contact resistances near the predicted optimum, the 2T-MR is actually lower, owing to the smaller TSP (14%). The optimum and scaling of the 2T-MR are, thus, profoundly affected by the variation of the TSP with contact resistance. This is relevant for the design of practical two-terminal devices, including those with channel materials other than Si.

Generation of spin-polarized electronic currents using perpendicularly magnetized cobalt ferrite spin-filtering barriers grown on spinel-type-conductive layers

We demonstrated the generation of perpendicularly spin-polarized electronic currents using a tunnel spin-filtering effect through insulative Fe-rich cobalt ferrite CoxFe3−xO4+δ (I-CFO) barriers with perpendicular magnetic anisotropy (PMA). The I-CFO films grown on conductive Fe-rich cobalt ferrite CoyFe3−yO4 (C-CFO) films, which were deposited on additional I-CFO buffer layers on MgO(001) substrates, exhibited PMA induced by an epitaxial strain. Magnetic tunnel junctions (MTJs), which comprise C-CFO electrode layers, I-CFO barrier layers, and perpendicularly magnetized Co/{Tb/Co}15/Co spin detection layers, showed a tunnel magnetoresistance (TMR) effect. This indicated that spin-polarized tunnel currents were injected into the spin detection layers. A spin injection efficiency of −28% was observed for the MTJs with an I-CFO barrier of 3.0 nm in thickness at 100 K. The voltage dependence of the TMR effect indicates that the spin-injection efficiency is affected by voltage-dependent changes in the effective spin-dependent barrier width. The combination of spinel-type C-CFO and I-CFO films with well-controlled compositions and lattice strains is, therefore, applicable as a spin-injection source for spintronics devices when perpendicularly spin-polarized electronic currents are required.

Magnetic field dependent metal insulator transition by monovalent doping (Na+) in PrMnO3: Investigation through structural, magnetic and transport properties

A detailed study on the structural, magnetic, and transport properties of a monovalent (Na+) doped Pr0.75Na0·25MnO3 (PNMO) compound has been carried out. Temperature (T) and magnetic field (H) dependent magnetization (M), and resistivity (ρ) measurements have been carried out which reveal a second-order magnetic phase transition around TC = 60 K accompanied by a metal-insulator transition around TMI = 138 K. Magnetization measurements also confirm the coexistence of antiferromagnetic (AFM) and ferromagnetic (FM) components in the low-temperature regime. M − H has been used to calculate the magnetocaloric behavior and the system is found to show a maximum magnetic entropy change i.e. –ΔSmax as 2.54 J kg −1.K −1 for the field change of 5 T. Temperature dependence resistivity has been measured under the applied magnetic field and the system exhibits a colossal magnetoresistance (≅ 81%). Spin-polarized tunneling (SPT) model is applied to explain magnetoresistance (MR) phenomenon under applied magnetic field. Analysis of zero-field resistivity data signifies that a transition from Mott-VRH to Efros-Shklovskii-VRH (ES-VRH) conduction occurs in the low-temperature region in this present PNMO system.

Scaling with MgO thickness of the proximity exchange field in a Fe/MgO/Si contact

The scaling of the proximity exchange field, which ferromagnetic Fe exerts on the spin accumulation at the MgO/Si interface of Fe/MgO/Si tunnel contacts, was investigated using the inverted Hanle effect. Data as a function of the applied bias voltage shows that the exchange field varies with bias in the same way as the square of the tunnel spin polarization (TSP). The decay of the exchange field with increasing MgO thickness ${t}_{\mathrm{MgO}}$ is rather weak and nonexponential, which we argue to be related to the improved epitaxial quality of the MgO, and the, consequently, improved spin-filtering properties at larger MgO thickness. Using the TSP data to correct for this yields an exponential decay of the exchange field with ${t}_{\mathrm{MgO}}$ with a decay length of $\ensuremath{\lambda}=\ensuremath{-}0.42$ nm. Comparing this to the decay length for the tunnel conductance ($\ensuremath{\lambda}=\ensuremath{-}0.21$ nm) suggests that the exchange interaction is dominated by two-step tunneling processes.

Enhanced magnetocaloric effect by modifying the intergranular phase via V-addition in La2/3Ca1/3MnO3 composites

Vanadium oxides (V2O5) with relatively low melting point (963 K) was found to be an effective sintering aid for forming multiphase nanocomposite of La2/3Ca1/3MnO3. The polycrystalline perovskites with a nominal composition of La2/3Ca1/3Mn1−xVxO3 (0 ≤ x ≤ 0.12) were prepared by the liquid-phase sintering method. The structure, magnetic properties and magnetocaloric effect (MCE) were investigated. The study on microstructure revealed that the composites consist of a major crystalline phase of La2/3 Ca1/3MnO3 and intergranular phases of Ca10V6O25 and La2MnO4.15 at the grain boundary. All the samples undergo the first-order phase transition at the Curie temperature. V-addition leads to a reduction of the Curie temperature, but significantly improves the MCE. The maximal magnetic entropy change of 3.8 J/kg K was achieved for x = 0.08 under an applied field of 0.5 T, almost twice as high as the un-doped sample. The result implies that the control of intergranular phases is a potential way to improve the MCE in perovskite manganites. The enhancement of MCE could be attributed to spin-polarized tunneling or spin-dependent scattering of polarized electrons at the grain boundaries.

Tunnel Spin-Polarization of Ferromagnetic Metals and Ferrimagnetic Oxides and Its Effect on Tunnel Magnetoresistance

This work presents an examination and unification of fragmented data on spin polarization in half-metallic, ferrimagnetic oxides. It also includes well understood ferromagnetic metals for comparison. The temperature and disorder dependencies of the spin polarization are evaluated. Both the temperature dependence of the tunnel magnetoresistance and, for the very first time, its temperature coefficient are calculated based on the simplified Julliére model. The tunnel magnetoresistance in the magnetic tunnel junctions deteriorates due to the temperature dependence of the spin polarization the lower the Curie temperature is. As a result, magnetic tunnel junctions—consisting of ferromagnetic oxides with a Curie temperature not far above room temperature—are not promising for room temperature applications. Additionally, ferrimagnetic oxides possessing a Curie temperature below 650 K are not suitable for room temperature applications because of an unacceptable temperature coefficient exceeding −2%.

Changes in the coercivity fields of magnetoresistance hysteresis loops under the influence of a spin-polarized current flowing through the half-metal CrO2 nanocomposite system

Using the example of a pressed sample consisting of chromium dioxide nanoparticles coated with insulating shells, we study the relationship between the electron transport system and magnetic subsystem in granular spin-polarized metals. It is shown that the spin-polarized tunneling transport current can affect the coercivity fields of the percolation cluster formed in the sample with decreasing temperature.

Giant Influence of Clustering and Anti-Clustering of Disordered Surface Roughness on Electronic Tunneling

This work reveals the giant influence of spatial distribution of disordered surface roughness on electron tunneling, which is of immediate relevance to the magneto tunnel device and imaging technologies. We calculate the spin-dependent tunneling in Fe/vacuum/Fe junction with disordered surface roughness with the first-principles non-equilibrium dynamical cluster theory. It is found that, at high concentration of surface roughness, different spatial distributions, including the clustered, anti-clustered and completely random roughness characterized by Warren–Cowley parameters, present large deviations from each other in all spin channels. By changing from clustered to anti-clustered roughness, it is surprising that spin polarization of tunneling in parallel configuration (PC) can be drastically reversed from –0.52 to 0.93, while complete randomness almost eliminates the polarization. It is found that the anti-clustered roughness can dramatically quench the tunneling of minority spin in both PC and anti-PC by orders of magnitude, but significantly enhance the transmission of majority spin in PC (by as large as 40%) compared to the results of clustered roughness, presenting distinct influences of differently correlated surface roughness. The spatial correlation of disordered surface roughness can significantly modify the surface resonance of Fe minority spin.

Large magnetocapacitance beyond 420% in epitaxial magnetic tunnel junctions with an MgAl2O4 barrier

Magnetocapacitance (MC) effect has been observed in systems where both symmetries of time-reversal and space-inversion are broken, for examples, in multiferroic materials and spintronic devices. The effect has received increasing attention due to its interesting physics and the prospect of applications. Recently, a large tunnel magnetocapacitance (TMC) of 332% at room temperature was reported using MgO-based (001)-textured magnetic tunnel junctions (MTJs). Here, we report further enhancement in TMC beyond 420% at room temperature using epitaxial MTJs with an MgAl2O4(001) barrier with a cation-disordered spinel structure. This large TMC is partially caused by the high effective tunneling spin polarization, resulted from the excellent lattice matching between the Fe electrodes and the MgAl2O4 barrier. The epitaxial nature of this MTJ system sports an enhanced spin-dependent coherent tunneling effect. Among other factors leading to the large TMC are the appearance of the spin capacitance, the large barrier height, and the suppression of spin flipping through the MgAl2O4 barrier. We explain the observed TMC by the Debye-Fröhlich modelled calculation incorporating Zhang-sigmoid formula, parabolic barrier approximation, and spin-dependent drift diffusion model. Furthermore, we predict a 1000% TMC in MTJs with a spin polarization of 0.8. These experimental and theoretical findings provide a deeper understanding on the intrinsic mechanism of the TMC effect. New applications based on large TMC may become possible in spintronics, such as multi-value memories, spin logic devices, magnetic sensors, and neuromorphic computing.

Current Rectification in Junctions with Spin-Split Superconductors

Spin-split superconductors exhibit an electron-hole asymmetric spin-resolved density of states, but the symmetry is restored upon averaging over spin. On the other hand, asymmetry appears again in tunneling junctions of spin-split superconductors with a spin-polarized barrier. As demonstrated recently in both theory and experiment, this fact leads to a particularly strong thermoelectric effect in superconductor-ferromagnet structures. In this work we show another important effect stemming from the electron-hole asymmetry - current rectification. We calculate the charge current in spin-polarized tunnel junctions of normal metal and a spin-split superconductor with AC and DC voltage bias. In the DC case, the I-V curve is not fully antisymmetric and has a voltage-symmetric component due to spin polarization. This translates to the existence of a rectified current in the AC case, which is proportional to the spin polarization of the junction and strongly depends on the frequency of the applied bias. We discuss possible applications of the rectification effect, including a diode for superconducting electronics and radiation detectors. The analysis of the rectified charge current is supplemented by the discussion of heat current and relevant noise correlators, where electron-hole asymmetry also plays an important role, and which are useful for applications in detectors.

Tunnel magnetodielectric effect: Theory and experiment

The recently discovered tunnel magnetodielectric (TMD) effect—the magnetic field-induced increase in the dielectric permittivity (ε′) of nanogranular composites caused by the spin-dependent quantum mechanical charge tunneling—is of interest for both the scientific value that combines the fields of magnetoelectric and spintronics and multifunctional device applications. However, little is known about how large the maximum dielectric change Δε′/ε′ can achieve and why the Δε′/ε′ variations obey the dependence of square of normalized magnetization (m2), which are critically important for searching and designing materials with higher Δε′/ε′. Here, we perform approximate theoretical derivation and reveal that the maximum Δε′/ε′ can be estimated using intrinsic tunneling spin polarization (PT) and extrinsic normalized magnetization (m), that is, Δε′/ε′ = 2PT2m2. This formulation allows predicting over 200% of theoretical limit for m = 1 and accounts for the observed m2 dependence of Δε′/ε′ for a given PT. We experimentally demonstrate that x-dependence of Δε′/ε′ in (CoxFe100−x)–MgF2 films is phenomenologically consistent with this formulation. This work is pivotal to the design of ultra-highly tunable magnetoelectric applications of the TMD effect at room temperature.

Numerical simulations for ferromagnetic resonance of nano-size island structures probed by radio-frequency scanning tunneling microscopy

We numerically calculated ferromagnetic resonance (FMR) spectra taken on a single-domain nano-size ferromagnetic island structure in the configuration of radio-frequency (RF) scanning tunneling microscopy, where RF electromagnetic waves are introduced into the tunneling gap through the probe tip. In this scheme, near-field in-plane azimuthal RF magnetic field induces FMR of an out-of-plane magnetized island situated below the tip under the external out-of-plane magnetic field. The amount of the magnetization of the island is effectively reduced by the resonance and the reduction can be detected from the spin-polarized tunneling conductance. From the calculated spectra we found that the FMR signal becomes larger with a smaller tip-sample distance and a sharper tip. It is also revealed that the azimuthal RF magnetic field exerted on the island and therefore the FMR signal are enhanced when a tip is located near the edge of the island.

Spin mapping of intralayer antiferromagnetism and field-induced spin reorientation in monolayer CrTe2

Intrinsic antiferromagnetism in van der Waals (vdW) monolayer (ML) crystals enriches our understanding of two-dimensional (2D) magnetic orders and presents several advantages over ferromagnetism in spintronic applications. However, studies of 2D intrinsic antiferromagnetism are sparse, owing to the lack of net magnetisation. Here, by combining spin-polarised scanning tunnelling microscopy and first-principles calculations, we investigate the magnetism of vdW ML CrTe2, which has been successfully grown through molecular-beam epitaxy. We observe a stable antiferromagnetic (AFM) order at the atomic scale in the ML crystal, whose bulk is ferromagnetic, and correlate its imaged zigzag spin texture with the atomic lattice structure. The AFM order exhibits an intriguing noncollinear spin reorientation under magnetic fields, consistent with its calculated moderate magnetic anisotropy. The findings of this study demonstrate the intricacy of 2D vdW magnetic materials and pave the way for their in-depth analysis.