Spin Tunneling Research Articles

Electrical switching of spin-polarized current in multiferroic tunneling junctions

The generation and manipulation of spin-polarized current are critical for spintronic devices. In this work, we propose a mechanism to generate and switch spin-polarized current by an electric field in multiferroic tunnel junctions (MFTJs), with symmetric interface terminations in an antiparallel magnetic state. In such devices, different spin tunneling barriers are realized by the magnetoelectric coupling effect, resulting in a spin-polarized current. By reversing the electric polarization of the ferroelectric layer, the spin polarization of current is efficiently switched for the exchange of spin tunneling barriers. By first-principles quantum transport calculations, we show that a highly spin-polarized current is obtained and manipulated by the electric field in hafnia-based MFTJs based on the proposed mechanism. We also demonstrate that four resistance states are realized in Co/HfO2/Co junctions with asymmetric interface terminations. This work provides a promising approach for realizing the electrical control of spin current for spintronic applications.

Change in the Magnetoresistance of a Spin Tunnel Element upon Inhomogeneous Magnetization Reversal with the Formation of Domains

Change in the Magnetoresistance of a Spin Tunnel Element upon Inhomogeneous Magnetization Reversal with the Formation of Domains

High-Efficient Spin Injection in GaN at Room Temperature Through A Van der Waals Tunnelling Barrier

Achieving high-efficient spin injection in semiconductors is critical for developing spintronic devices. Although a tunnel spin injector is typically used, the construction of a high-quality tunnel barrier remains a significant challenge due to the large lattice mismatch between oxides and semiconductors. In this work, van der Waals h-BN films with the atomically flat interface were engaged as the tunnel barrier to achieve high spin polarization in GaN, and the spin injection and transport in GaN were investigated systematically. Based on the Hanle precession and magnetic resistance measurements, CoFeB was determined as an optimal spin polarizer, bilayer h-BN tunnelling barrier was proven to yield a much higher spin polarization than the case of monolayer, and appropriate carrier concentration as well as higher crystal equality of n-GaN could effectively reduce the defect-induced spin scattering to improve the spin transport. The systematic understanding and the high efficiency of spin injection in this work may pave the way to the development of physical connotations and the applications of semiconductor spintronics.

Enhancement of spin diffusion length in tunneling junctions by benzene ring insertion into the saturated alkyl sulfonic acid

We fabricated Fe3O4/alkyl-sulfonic acid and Fe3O4/alkyl-benzene sulfonic molecules hybrid nanoparticles using self-assembled monolayers to investigate the impact of benzene ring insertion on the spin tunneling through molecules. The results showed that benzene ring insertion decreased the tunnel barrier height, slightly tuned the magnetoresistance values, and enhanced the intramolecular spin diffusion length by ∼1.5 times. We proposed an extended model to explore the interfacial magnetic properties. The fitting results indicated that benzene ring insertion changed the magnetic properties of the shell layer. This work first demonstrated that benzene ring insertion enhanced the intramolecular spin diffusion length during the tunneling process.

Quantum Phase Transition in the Spin Transport Properties of Ferromagnetic Metal-Insulator-Metal Hybrid Materials.

Perpendicular magnetic tunnel junctions provide a technologically important design platform for studying metal-insulator-metal heterostructure materials. Accurate characterization of the sensitivity of their electronic structure to proximity coupling effects based on first-principles calculations is key in the fundamental understanding of their emergent collective properties at macroscopic scales. Here, we use an effective field theory that combines ab initio calculations of the electronic structure within density functional theory with the plane waves calculation of the spin polarised conductance to gain insights into the proximity effect induced magnetoelectric couplings that arise in the transport of spin angular momentum when a monolayer tunnel barrier material is integrated into the magnetic tunnel junction. We find that the spin density of states exhibits a discontinuous change from half-metallic to the metallic character in the presence of monolayer hexagonal boron nitride when the applied electric field reaches a critical amplitude, and this signals a first order transition in the transport phase. This unravels an electric-field induced quantum phase transition in the presence of a monolayer hexagonal boron nitride tunnel barrier quite unlike molybdenum disulphide. The role of the applied electric field in the observed phase transition is understood in terms of the induced spin-flip transition and the charge transfer at the constituent interfaces. The results of this study show that the choice of the tunnel barrier layer material plays a nontrivial role in determining the magnetoelectric couplings during spin tunnelling under external field bias.

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.

Coherent control of spin tunneling in a driven spin–orbit coupled bosonic triple well

We investigate the coherent control of spin tunneling for a spin–orbit (SO) coupled boson trapped in a driven triple well. In the high-frequency limit, the quasienergies of the system are obtained analytically and the fine energy band structures are shown. By regulating the driving parameters, we reveal that the directed spin-flipping or spin-conserving tunneling of an SO-coupled boson occurs along different pathways and in different directions. The analytical results are demonstrated by numerical simulations and good agreements are found. Further, an interesting scheme of quantum spin tunneling switch with or without spin-flipping is presented. The results may have potential applications in the design of spintronic devices.

Improving signal-to-noise ratio of magnetic tunnel junction based radio frequency detector via spin-torque ferromagnetic resonance.

This article focuses on the spin-torque ferromagnetic resonance (STFMR) technique, which was developed and optimized to investigate spin-transfer effects in magnetic tunnel junctions (MTJ) and spin Hall effect phenomena in ferromagnet/non-magnetic heavy metal bilayer systems. The devices for STFMR are typically fabricated with co-planar waveguides with contact pads for applying radio frequency or direct current, Irf(Idc). The device under investigation was a CoFeB/MgO/CoFeB based MTJ with a resistance-area product of 1.5 Ω (μm)2 having a circular cross section with a diameter of 180nm and tunneling magneto-resistance in the range of 60%-80%. The development of the STFMR setup and its optimization for achieving higher signal-to-noise ratio (SNR) is discussed using two modulation schemes, namely, radio-frequency modulation and field modulation (FM). The FM-STFMR method reduces frequency-dependent noise and offers a higher SNR of 30 dB compared to other modulation schemes in the literature. In addition, a vector network analyzer based STFMR technique is developed, which provides a simple and fast means for characterizing MTJ devices. Furthermore, to calculate the exact power reaching the MTJ, impedance mismatch is calculated using the de-embedding method. The magnitude of in-plane torkance and out-of-plane torkance with dc bias is measured, and the results are found to be consistent with the results of STFMR techniques. The results show that the magnitude of out-of-plane torkance is substantially smaller than that of in-plane torkance in MTJ.

Enhancement of voltage controlled magnetic anisotropy (VCMA) through electron depletion

Recent advancement in the switching of perpendicular magnetic tunnel junctions with an electric field has been a milestone for realizing ultra-low energy memory and computing devices. To integrate with current spin-transfer torque-magnetic tunnel junction and spin–orbit torque-magnetic tunnel junction devices, the typical linear fJ/V m range voltage controlled magnetic anisotropy (VCMA) needs to be significantly enhanced with approaches that include new materials or stack engineering. A possible bidirectional and 1.1 pJ/V m VCMA effect has been predicted by using heavily electron-depleted Fe/MgO interfaces. To improve upon existing VCMA technology, we have proposed inserting high work function materials underneath the magnetic layer. This will deplete electrons from the magnetic layer biasing the gating window into the electron-depleted regime, where the pJ/V m and bidirectional VCMA effect was predicted. We have demonstrated tunable control of the Ta/Pd(x)/Ta underlayer's work function. By varying the Pd thickness (x) from 0 to 10 nm, we have observed a tunable change in the Ta layer's work function from 4.32 to 4.90 eV. To investigate the extent of the electron depletion as a function of the Pd thickness in the underlayer, we have performed DFT calculations on supercells of Ta/Pd(x)/Ta/CoFe/MgO, which demonstrate that electron depletion will not be fully screened at the CoFe/MgO interface. Gated pillar devices with Hall cross geometries were fabricated and tested to extract the anisotropy change as a function of applied gate voltage for samples with various Pd thicknesses. The electron-depleted Pd samples show three to six times VCMA improvement compared to the electron accumulated Ta control sample.

Important Heat Contribution by Tunneling Spin Scattering in Magnetic Tunnel Junction

We investigated the heat contribution by tunneling spin scattering of magnetic tunnel junction in parallel (P) and antiparallel (AP) states using finite element simulation. It showed a maximum temperature increase of 23.2% from P to AP, which is even more significant than the heating asymmetry caused by different current directions. A remarkable enhancement of the temperature gradient was confirmed for contributing to the extra thermal spin-transfer torque to the free layer under a specific current direction. Both the contributions of enhanced temperature and temperature gradient should be the possible reasons for the asymmetry of the critical switching current density from P to AP and AP to P. We also extended the research to a double-barrier structure, clarifying its high performance from the aspect of heat generation. Our demonstration may offer efficient tunneling magnon excitation by using the tunneling spin scattering heat.

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.

Abnormal specific heat at low temperatures and transport properties in Mg-Cu-Ag-Gd bulk metallic glass

We report the pronounced low-temperature specific-heat Cp anomalies in the Mg59.5Cu22.9Ag6.6Gd11 bulk metallic glass (BMG). The extrapolated electron's temperature coefficient γ0K is up to 681.8 mJ/(mol-Gd⋅Κ2) at 0 K, which is a heavy-fermion-like behavior. The low temperature specific heat indicates an enhancement of the conduction-electron effective mass m∗ below 7.5 K, suggesting that the Mg59.5Cu22.9Ag6.6Gd11 BMG is not free-electron-like solid. The excess specific heat in the Mg-based BMG is interpreted with tunneling states and spin glass state (magnetism) which are determined by subtracting electrons' and phonons' contribution to the specific heat below 12 K. The Boson peak (BP) temperature is located at 27 K, which is much higher than the reported values of other BMGs. And, a BP height of 0.047 mJ/(mol·K4) is obtained due to reduced free volume during copper mold casting with a slow cooling rate. The electrical resistivity was also investigated between 2 and 300 K, which has a negative temperature coefficient of resistivity (TCR) below 35 K (Kondo temperature, TK) and a positive value of 3.9 × 10−4/K above 35 K. There is a minimum at about 35 K for the electrical resistivity, which can be explained by the Kondo effect. For the resistivity above 35 K, it can be explained by the Faber-Ziman model due to the T-dependence change of structure factor.

Tunneling spin current in systems with spin degeneracy

We study theoretically spin current generation from a band insulator with PT symmetry, which is associated with Zener tunneling in strong dc electric fields. Each band in this system is doubly degenerate with opposite spins, but spin rotational symmetry is not preserved in general. We consider the condition for spin current generation in connection with the nature of the wave function, which ultimately depends on a geometric quantity known as the shift vector. From an analysis of a two-band model, we find that the shift vector is necessary for spin current generation in a PT symmetric system. We also present zigzag chain models that have shift vectors, and confirm from numerical calculations that a nonzero tunneling spin current occurs in spin-degenerate systems.

Giant magnetoresistance and tunneling electroresistance in multiferroic tunnel junctions with 2D ferroelectrics.

Multiferroic tunneling junctions (MFTJs), composed of two magnetic electrodes separated by an ultrathin ferroelectric (FE) thin film as a barrier, have received great attention in multi-functional devices. Recent theoretical and experimental works have revealed that ferroelectric polarization exists at room temperature in two-dimensional ferroelectric (2D FE) materials within the ultrathin thickness. Here we propose a novel MFTJ Ni/bilayer In2Se3/BN/Ni, in which the resistance of the tunneling spin polarization electrons can be modulated by different magnetization alignments of the electrode and electric polarization direction of the 2D FE In2Se3 layer, leading to multiple tunneling resistance states. The tunneling magnetoresistance (TMR) and electroresistance (TER) of MFTJs are enhanced by the inserted h-BN layer, achieving an ON/OFF TER ratio of 4188% as well as a TMR ratio of 581% with a much lower resistance area. The giant tunneling resistance ratio, multiple resistance states, and ultra-low energy consumption in 2D FE-based MFTJs suggest their great potential in non-destructive non-volatile memories.

Transport spin polarization of noncollinear antiferromagnetic antiperovskites

Spin-polarized currents play a key role in spintronics. Recently, it has been found that antiferromagnets with a non-spin-degenerate band structure can efficiently spin-polarize electric currents, even though their net magnetization is zero. Among the antiferromagnetic metals with magnetic space group symmetry supporting this functionality, the noncollinear antiferromagnetic antiperovskites ANMn$_3$ (A = Ga, Ni, Sn, and Pt) are especially promising. This is due to their high N\'eel temperatures and a good lattice match to perovskite oxide substrates, offering possibilities of high structural quality heterostructures based on these materials. Here, we investigate the spin polarization of antiferromagnetic ANMn$_3$ metals using first-principles density functional theory calculations. We find that the spin polarization of the longitudinal currents in these materials is comparable to that in widely used ferromagnetic metals, and thus can be exploited in magnetic tunnel junctions and spin transfer torque devices. Moreover, for certain film growth directions, the out-of-plane transverse spin currents with a giant charge-to-spin conversion efficiency can be achieved, implying that the ANMn$_3$ antiperovskites can be used as efficient spin sources. These properties make ANMn$_3$ compounds promising for application in spintronics.

Temperature dependent transition of conduction mechanism from carrier injection to multistep tunneling in Fe3O4 (111)/Alq3/Co organic spin valve

We have presented experimental results addressing the origin of spin valve (SV) magneto-resistance (MR), in both injection and tunnel conduction regimes, in our fabricated Fe 3 O 4 (111)/Alq 3 /Co SV device. Experimental evidences have shown that any alternative MR process, such as ‘tunneling anisotropic MR’ is not at the origin of this SV MR. Spin resolved density of states of electrodes indicate that both the conduction mechanisms induce different spin dependent scattering which inturn modify the MR signal of the device. This modification helped in maintaining a non-monotonous quenching of MR signal with increase in temperature. We have also proposed a phenomenological model for device operation where the concept of charge gap modification at Fermi level across Verwey transition is envisaged to offer this unique scenario of tuning the conduction mode and hence MR in this ferrite based organic SV. The model is also supported by an established theoretical study which considers high temperature phonon assisted tunneling through defect states at electrode–organic interface of the device. • Conduction mechanism transition from carrier injection to multistep tunneling. • Transition due to Verwey transition of Fe 3 O 4 . • Spin flip scattering at Alq 3 spacer above Verwey transition temperature. • Non-monotonous MR quenching with temperature. • Room temperature MR signal of hybrid spin valve.

Four-point probe stand for magnetoresistance measurement of unpatterned sample

An experimental stand for express diagnostics of multilayer spin tunnel structures has been developed. The current-in-plane tunnelling method (CIPT) requires no processing, is fast, and provides reliable data which are reflective of the deposition only. The stand is based on the four-probe method for measuring resistance at external alternating magnetic field. This technique can be applied after only a short processing route, thereby saving time and resources, and reducing the potential for damaging the junction.

Perfect spin filtering effect, tunnel magnetoresistance and thermoelectric effect in metals-adsorbed blue phosphorene nanoribbons

The thermoelectric transport properties of Fe- and Ti-adsorbed blue phosphorene nanoribbons are studied using the first principles approach. The research indicates that Fe- and Ti-adsorbed blue phosphorene nanoribbons exhibit spin-dependent Seebeck effect. The devices based on the Fe-adsorbed blue phosphorene nanoribbons show perfect spin filtering effect caused by 100% spin polarization and 106% tunnel magnetoresistance. The spin-up current and the spin-down current are reversed in Ti-adsorbed blue phosphorene nanoribbons, indicating that a large spin current and a small charge current can be generated. These systems also exhibit negative differential resistance induced by the current-voltage curve. The band structures and transport spectrums are utilized to explain the physical mechanism of spin current generation. Metals-adsorbed blue phosphorene nanoribbons can be considered novel thermoelectric materials and used to make multifunctional thermal spintronic devices.

Disentangling 4f -radical coupling and dissipative Landau-Zener quantum tunneling in a continuously measured single-ion magnet spin transistor

Both radical-lanthanide exchange coupling and crystal-field tunnel splitting are responsible for the ground to first-excited Kramers doublet energy gap $\mathrm{\ensuremath{\Delta}}{E}_{\text{KD}}$ in an Ising $4f$ single-ion magnet coupled to a noninnocent ligand. However, these two mechanisms cannot be easily disentangled via spectroscopy alone because their interplay results in a single absorption line. Here we present a microscopic theory for the continuous measurement of the dissipative Landau-Zener transitions recently discussed for a ${\mathrm{TbPc}}_{2}$ spin transistor [Phys. Rev. Lett. 118, 257701 (2017)] and show how this quantum transport experiment can be used to disentangle the magnetic coupling and the tunnel splitting contributions to $\mathrm{\ensuremath{\Delta}}{E}_{\text{KD}}$. In addition, our model elucidates the microscopic origin of the decoherent spin tunneling observed in the ${\mathrm{TbPc}}_{2}$ spin transistor, by explicitly exposing the role played by phonon-mediated spin relaxation and sequential electron transport through the noninnocent ligand, in a time-dependent magnetic field.

Spontaneous symmetry breaking induced thermospin effect in superconducting tunnel junctions

We discuss the charge and the spin tunneling currents between two Bardeen-Cooper-Schrieffer (BCS) superconductors, where one density of states is spin-split. In the presence of a large temperature bias across the junction, we predict the generation of a spin-polarized thermoelectric current. This thermo-spin effect is the result of a spontaneous particle-hole symmetry breaking in the absence of a polarizing tunnel barrier. The two spin components, which move in opposite directions, generate a spin current larger than the purely polarized case when the thermo-active component dominates over the dissipative one.