Negative Tunnel Magnetoresistance Research Articles

Interfacial Resonance States-Induced Negative Tunneling Magneto-Resistance in Orthogonally Magnetized CoFeB/MgO/CoFeB

Interfacial Resonance States-Induced Negative Tunneling Magneto-Resistance in Orthogonally Magnetized CoFeB/MgO/CoFeB

Tunable long-range spin transport in a van der Waals Fe3GeTe2/WSe2/Fe3GeTe2 spin valve.

The seamless integration of two-dimensional (2D) ferromagnetic materials with similar or dissimilar materials can widen the scope of low-power spintronics. In this regard, a vertical van der Waals (vdW) heterostructure of 2D ferromagnets with semiconducting transition metal dichalcogenides (TMDCs) forms magnetic junctions with exceptional stability and electrical control. Interestingly, 2D metallic Fe3GeTe2 (FGT) reveals above room temperature Curie temperatures and has large magneto anisotropy due to spin-orbit coupling. In addition, it also possesses topological states and a large Berry curvature. Herein, we designed the FGT/WSe2/FGT vdW heterostructure with a uniform and sharp interface so that FGT could maintain its inherent electronic properties. Also, the uniform thickness of the barrier provides a smooth flow of spins through the junctions as tunneling exponentially decays with an increasing barrier thickness. However, strong energy-dependent spin polarization is crucial for achieving optimum spin valve properties, such as large tunneling magnetoresistance (TMR) along with the manipulation of the magnitude and sign reversal. We have observed a shifting of high-energy localized minority spin states toward low-energy regions, which causes spin polarization fluctuation between -42.5% and 41% over a wide range of bias voltage. This leads to a negative TMR% of ∼-100% at 0.1 V Å-1 and also a large positive TMR% at 0.2 V Å-1 and -0.4 V Å-1. Besides, the system exhibits a highly tunable large anomalous Hall conductivity (AHC) of 626 S cm-1. Interestingly, such unprecedented electronic behaviour with large and switchable spin polarization, anomalous Hall conductivity and TMR can be incorporated into MTJ devices, which provide electrical control and long-range spin transport. Additionally, the system emerges as a standout candidate in low-power spintronic devices (e.g., MRAM and magnetic sensors) owing to its distinctive energy-dependent electronic structure with a wide range of external bias.

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 our 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 having 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>

Spin effects on transport and zero-bias anomaly in a hybrid Majorana wire-quantum dot system

We examine the impact of spin effects on the nonequilibrium transport properties of a nanowire hosting Majorana zero-energy modes at its ends, coupled to a quantum dot junction with ferromagnetic leads. Using the real-time diagrammatic technique, we determine the current, differential conductance and current cross-correlations in the nonlinear response regime. We also explore transport in different magnetic configurations of the system, which can be quantified by the tunnel magnetoresistance. We show that the presence of Majorana quasiparticles gives rise to unique features in all spin-resolved transport characteristics, in particular, to zero-bias anomaly, negative differential conductance, negative tunnel magnetoresistance, and it is also reflected in the current cross-correlations. Moreover, we study the dependence of the zero-bias anomaly on various system parameters and demonstrate its dependence on the magnetic configuration of the system as well as on the degree of spin polarization in the leads. A highly nontrivial behavior is also found for the tunnel magnetoresistance, which exhibits regions of enhanced or negative values—new features resulting from the coupling to Majorana wire.

Interface structure and bias dependence of VN/AlN/VN tunnel junction: A semi-empirical calculation

Vanadium Nitride based magnetic heterostructures have opened up unprecedented opportunities in the field of spintronics owing to their half metallicity and a high value of spin polarization. In this paper, a tight-binding model of epitaxially grown VN/AlN/VN heterostructure is developed with different types of bonding at the left and right interface. The Slater–Koster tight-binding scheme is used to obtain the on-site and hopping energies of the model by fitting them to bulk DFT band structures. Electron transport across the VN/AlN/VN heterostructure is then studied using the non-equilibrium Green’s function method, which works well within the tight-binding framework. An asymmetry in the interface bonding of the structure was found to produce a resonance peak in the transmission spectra at around 0.5 V. Beyond 0.5 V, the tunnel junction registers a large antiparallel current resulting in a negative tunnel magnetoresistance. Moreover, the presence of a magnetic impurity in the insulating barrier enhances tunneling across the junction resulting in a large current that decreases with the value of intra-atomic Coulomb repulsion, U, at the impurity site.

Magnetodielectric and TMR effects in P(VDF-HFP)/SrFe12O19 composite films for magnetic field – Tuned photodetector application

Magnetoelectric (ME) multiferroic materials possess extensive research interest as a result of their use in a variety of sectors. In this work, the optical properties of plain P(VDF-HFP) film and P(VDF-HFP)/SrFe12O19 composite films are investigated through UV-Vis absorption spectroscopy. In addition, the ME coupling of SrFe12O19 nanofibers reinforced P(VDF-HFP) composite films is investigated though magnetodielectric (MD) measurements. The parameters such as dielectric constant, capacitance, dielectric loss and impedance of the composite films are found to change with the applied magnetic field which is attributed to the direct ME coupling of the composite films. The prepared ME films show a negative tunneling magnetoresistance (TMR) effect with maximum TMR% of 14 % for the film (PSNF20) with 20 wt% of SrFe12O19 nanofiber loading. Photodetectors are fabricated using the films as active elements and the performance of the photodetectors is studied under various intensities of visible light radiation. All the devices exhibit good photosensitivity, responsivity and specific detectivity at an illumination intensity of 20 mW/cm2. The photodetector (PD20) fabricated using PSNF20 as an active element has the maximum rise/decay time, photosensitivity, responsivity and specific detectivity values as 468 /553 ms, 2.7 × 103, 22 mA/W and 6.77 × 1013 Jones, respectively at 20 mW/cm2. In addition, the performance of the devices is analyzed by subjecting the active element to external magnetic field. The photosensitivity, responsivity and specific detectivity all improve, showing that the magnetic field has an effect on the photodetector. The above parameters are maximum for PD20 device and the values are 4.3 × 103, 26 mA/W and, 8.01 × 1013 Jones, respectively at 4000 Oe and 20 mW/cm2. Thus, the prepared flexible magnetoelectric films of P(VDF-HFP)/SrFe12O19 nanofiber composites have been shown to be good options for the magnetic field tuned photodetector applications such as environmental sensing monitor, safety and security, optical communication systems, magnetic sensors, piezoelectric sensors and spintronics devices.

Temperature, Bias, Angle, and Thickness‐Dependent Magnetoresistance in a Vertical Spin Valve Structure CoFe/TiO2/CoFe

Metal‐oxide spin valve junctions are the building blocks for spintronic devices and are to be utilized for miniaturized magnetic sensors. Here, the fabrication and characterization of the vertical spin valve (VSV) based on the CoFe/TiO2/CoFe structure are described. A spacer layer (TiO2) of different thicknesses in the spin valve is utilized and the effect on MR is studied. This VSV showed significant positive magnetoresistance (MR) at different temperatures from low to room temperature. The maximum value of tunneling MR is investigated to be 3.4% at 30 K and 1.03% at room temperature (300 K), and the spin polarization obtained at 30 K is 12.8%. The MR of the spin valve is investigated by changing the orientation of the device at different angles with respect to an applied magnetic field, the switching points shifted toward the higher magnetic fields and the signal became wider. Interestingly, a negative tunneling MR is observed when the thickness of the spacer layer (TiO2 = 5.5 nm) is enhanced which may be due to the spin filtering effect. The demonstrated devices identify TiO2 as favorable spacer material in spin valves and open a way to integrate high‐performance memory storage devices.

Electrically controlled positive and negative magnetoresistance in ferromagnetic WSe2 junction

Electrical manipulation of transition metal dichalcogenides is very important to develop spintronics and valleytronics. We theoretically study the electron transport through a ferromagnetic/ferromagnetic insulating/ferromagnetic WSe2 tunnel junction. It is found that the tunneling magnetoresistance (TMR) could change from positive to negative values with a measurable conductance. The prominent positive and negative TMR with wide parameter regimes can be achieved and manipulated by the potential and exchange field. The negative TMR behavior could be regarded as a signature of spin polarization, since the conductance is always spin-polarized when the negative TMR occurs. The positive and negative TMR still survives under finite temperature and disorder. Furthermore, a spin-valley filter can also be realized in the proposed junction. The results suggest an electrical control of the ferromagnetic WSe2 junction.

Spin- and valley-polarized transport and magnetoresistance in asymmetric ferromagnetic WSe2 tunnel junctions

Transition metal dichalcogenides (TMDs) offer a new platform for theoretical study of two-dimensional materials and their applications, beyond graphene. Previous studies indicate that monolayers of TMDs become ferromagnetic when put in proximity to conventional ferromagnetic, leading to remarkable spin and valley transport properties when combined with the intrinsic spin-orbit coupling (SOC) inherent in these materials. In this work, we show that a magnetic tunnel junction setup consisting of an asymmetric ferromagnetic/ferromagnetic/normal ${\mathrm{WSe}}_{2}$ junction gives rise to a fully spin- and valley-polarized current for both parallel and antiparallel magnetization configurations in the presence of an off-resonance light. Due to the strong SOC of ${\mathrm{WSe}}_{2}$ together with off-resonance light, both the spin and valley polarizations are tunable and switchable. We also find that the tunneling magnetoresistance (TMR) could be tuned to 1 by off-resonance light. The relatively much stronger SOC in the conduction and valence bands of ${\mathrm{WSe}}_{2}$ compared to other compounds of the dichalcogenides family in collaboration with an off-resonance light leads to significant negative TMR at weak enough exchange fields. In appropriate parameter regimes, the TMR oscillations from negative to positive values can be tuned by the off-resonance light. Our results suggest that magnetic tunnel junctions involving ${\mathrm{WSe}}_{2}$ have very promising potential for applications in magnetic memory and spin and valleytronics devices.

Negative tunnel magnetoresistance in a quantum dot induced by interplay of a Majorana fermion and thermal-driven ferromagnetic leads* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11834005 and 11674139).

We investigate the spin-related currents and tunnel magnetoresistance through a quantum dot, which is side-coupled with a Majorana fermion zero mode and two thermal-driven ferromagnetic electrodes. It is found that the interplay of Majorana fermion and electrodes’ spin polarization can induce a nonlinear thermal-bias spin current. This interplay also decreases the total magnitude of spin or charge current, in either parallel or antiparallel configuration. In addition, a thermal-driven negative tunnel magnetoresistance is found, which is an unique feature to characterize Majorana fermion. With large temperature difference, a step phenomenon is observed in gate tuned spin-up current. When the coupling between quantum dot and topological superconductor is strong enough, this step will evolve into a linear relation, revealing Majorana fermion’s robustness.

Electronic Transport Through Double Quantum Dot Coupled to Majorana Bound States and Ferromagnetic Leads

We have studied theoretically the properties of electrical current and tunnel magnetoresistance (TMR) through a serially connected double quantum dot (DQD) sandwiched between two ferromagnetic leads by using the nonequilibrium Green’s function technique. We consider that each of the DQD couples to one mode of the Majorana bound states (MBSs) formed at the ends of a topological superconductor nanowire with spin-dependent coupling strength. By adjusting the sign of the spin polarization of dot–MBS coupling strength and the arrangement of magnetic moments of the two leads, the currents’ magnitude can be effectively enhanced or suppressed. Under some conditions, a negative TMR emerges which is useful in detection of the MBSs, a research subject currently under extensive investigations. Moreover, the amplitude of the TMR can be adjusted in a large regime by variation of several system parameters, such as direct hybridization strength between the MBSs or the dots and the positions of the dots’ energy levels. Such tunable currents and TMR may also find use in high-efficiency spintronic devices or information processes.

Investigating the effect of geometrical asymmetry on conductance and TMR ratio in the ZnO rock salt-based MTJ: a DFT study

Effects of geometrical asymmetry on spintronic properties of Fe/ZnO/Fe magnetic tunnel junction based on zinc oxide barrier tunnel with rock-salt crystalline structure is studied. Simulations are performed using density functional theory, and substituted layers of C, Mg, Al, Mo, and Ta are used to make geometrically asymmetric structures. The results indicate that this asymmetry has a substantial influence on the properties of the spin-dependent electronic transport, conductance, and the tunneling magneto-resistance (TMR) ratio of the pristine symmetric structure. Additionally, it is shown that geometrical asymmetry results in a sharp decrease in the TMR ratio in one of these junctions and causes a negative TMR ratio in the other four asymmetric structures. Due to the large conductance of the three pristine, C and Al substituted structures in the PA configuration, these structures can be used to generate the current with pure spin for experimental purposes.

Multi-heterostructured spin-valve junction of vertical FLG/MoSe2/FLG

Two-dimensional (2D) layered materials and their heterostructures have opened a new avenue for next-generation spintronic applications, benefited by their unique electronic properties and high crystallinity with an atomically flat surface. Here, we report magnetoresistance of vertical magnetic spin-valve devices with multi-layer (ML) MoSe2 and its heterostructures with few-layer graphene (FLG). We employed a micro-fabrication procedure to form ultraclean ferromagnetic–non-magnetic–ferromagnetic interfaces to elucidate the intrinsic spin-transferring mechanism through both an individual material and combinations of 2D layered materials. However, it is revealed that the polarity of tunneling magnetoresistance (TMR) is independent of non-magnetic spacers whether the spin valve is composed of a single material or a hybrid structure, but it strongly depends on the interfaces between ferromagnetics (FMs) and 2D materials. We observed positive spin polarizations in ML-MoSe2 and FLG/ML-MoSe2/FLG tunnel junctions, whereas spin-valve devices comprised of FLG/ML-MoSe2 showed a reversed spin polarization and demonstrated a negative TMR. Importantly, in Co/FLG/ML-MoSe2/FLG/NiFe devices, the polarization of spin carriers in the FM/FLG interface remained conserved during tunneling through MoSe2 flakes in spin-transferring events, which is understandable by Julliere’s model. In addition, large TMR values are investigated at low temperatures, whereas at high temperatures, the TMR ratios are deteriorated. Furthermore, the large values of driving ac-current also quenched the amplitude of TMR signals. Therefore, our observations suggest that the microscopic spin-transferring mechanism between ferromagnetic metals and 2D materials played a momentous role in spin-transferring phenomena in vertical magnetic spin-valve junctions.

Detection of Majorana Bound States by Sign Change of the Tunnel Magnetoresistance in a Quantum Dot Coupled to Ferromagnetic Electrodes

We study quantum transport and tunneling magnetoresistance (TMR) through an interacting quantum dot (QD) attached to ferromagnetic electrodes in the Coulomb blockade regime. The QD is also side-coupled to a superconductor nanowire hosting Majarana bound states (MBSs). It is found that when the electrodes’ magnetic moments are arranged in antiparallel, the current’s intensity will be enhanced to be larger than that of the parallel configuration by the hybridization between the QD and the MBSs. This change in the current induces anomalous negative TMR unique to the existence of MBSs, providing an efficient detection way of the MBSs. The negative TMR is weakened by the overlap between the two bound states but obviously enhanced by the left-right asymmetry between the QD and the electrodes. We also find that the TMR value changes non-monotonously with the spin polarization of the electrodes. Our results may find real use in energy saving spintronic devices and quantum information processing.

Magnetoelectric response and tunneling magnetoresistance behavior of flexible P(VDF-H FP)/Cobalt ferrite nanofiber composite films

Fabrication of magnetoelectric (ME) polymer composite films by embedding ferromagnetic cobalt ferrite (CoFe2O4) nanofibers into electroactive poly(vinylidene fluoride–hexafluoropropylene) (P(VDF-HFP)) matrix is reported. Single-phase CoFe2O4 nanofibers made of cubic spinel nano crystallites are synthesized by using electro-spinning method, whereas the solution-casting technique is adapted to prepare flexible polymer composite films. The influence of CoFe2O4 nanofiber on structural, functional, magnetic, ferroelectric, and magnetoresistance properties of the composite films is investigated. The cross-coupling between ferroelectric and ferromagnetic orderings is ensured, by the variations of ferroelectric response at different magnetic fields. These magnetoelectric films are found to exhibit a negative tunneling magnetoresistance (TMR) effect with maximum TMR% of 28 for the film with 20 wt% of CoFe2O4 loading. The dielectric constant and electrical energy storage density of the films are increased with the addition of CoFe2O4 nanofiber. The ME films exhibiting TMR and high energy density can be the potential candidates for multifunctional device applications such as memory and spintronics devices, magnetic sensor, and bio-sensor.

The dependence of TMR on the barrier thickness, bias voltage and asymmetry in Fe/ZnO/Fe MTJs: A DFT study

Density functional theory (DFT) and non-equilibrium Green's function (NEGF) methods are used to simulate spin-polarized transport properties in Fe(001)/ZnO(001)/Fe(001) magnetic tunnel junctions (MTJs) with an even number of ZnO monolayers (ML). The barrier thickness conductance, tunneling magneto-resistance (TMR) ratio, and tunneling current are investigated for parallel and antiparallel alignment of the magnetization of electrodes. The results indicate that in zero and non-zero bias voltages the slower decay of conductance in the parallel alignment configuration, compared to antiparallel alignment, leads to increased TMR ratio for thicker barriers. Additionally, it is shown that, across all thicknesses of the ZnO barrier, the TMR ratio is highly sensitive to the applied bias voltage and decreases with increasing voltage. Moreover, analysis of the 4-ML MTJ reveals that, for the same density of states (DOS) in the symmetric structure, increasing the DOS at the Fermi level of antiparallel alignment configuration in the asymmetric case results in a negative TMR ratio. The results suggest that a ZnO rock-salt type potential barrier is as suitable as the traditional MgO barrier for spintronic purposes in MTJs.

Andreev transport through single-molecule magnets

The Andreev transport through a large-spin magnetic molecule, such as a single molecular magnet, attached to superconducting and ferromagnetic leads is studied theoretically by means of the real-time diagrammatic technique. It is shown that due to the proximity effect, molecular Andreev bound states form in the system, with energies depending on the intrinsic parameters of the molecule. We study the spin-resolved Andreev current, conductance, and tunnel magnetoresistance in both the linear and nonlinear response regimes and find regions of negative differential conductance, as well as either enhanced or negative tunnel magnetoresistance. The mechanisms leading to those effects are thoroughly discussed. It is also shown that the tunnel magnetoresistance can provide information about particular spin multiplets responsible for the Andreev reflection processes.

Large Magnetoresistance in Fe3O4/4,4′-Bipyridine/Fe3O4 Organic Magnetic Tunnel Junctions

Organic magnetic tunnel junctions (OMTJs) are promising systems thanks to their chemically tunable electronic property, long spin lifetime and easy functionalizations. Here, the spin-dependent electronic transport properties in Fe3O4/4,4’-bipyridine/Fe3O4 OMTJs are investigated by first-principles quantum transport calculations. Since the transport properties of junctions are sensitive to the device details, two types of terminations of Fe3O4 electrodes are considered. The device with tetrahedral Fe termination shows an anomalous negative tunnel magnetoresistance (TMR), i.e., which has a higher and lower junction resistance in the parallel and anti-parallel magnetization configurations, respectively. When the contact termination is octahedral Fe, a large positive TMR of 180% appears. The difference in TMR sign of two OMTJs originates from the electrons transmission mediated by frontier molecular levels coupled differently to Fe d states. Furthermore, TMR can be effectively controlled by applied electrical...

Quantum oscillation of conductance and negative tunneling magnetoresistance in velocity-modulated graphene spin-valve device

Mismatch effect of renormalized Fermi velocity of massive Dirac fermions on the spin transport properties and tunneling magnetoresistance in a gapped graphene-based ferromagnetic/velocity barrier/ferromagnetic (FG/VB/FG’) junction is investigated. The electrostatic potential created by the applied voltage on the VB region can generate spin-dependent collimation of Dirac fermions. The quantum beating pattern in the spin conductance oscillation are shown as a function of the Fermi energy at low velocity ratio (the Fermi velocity inside the barrier to that outside the barrier). The Fermi-velocity mismatch effect between graphene junction give rises to the oscillating behavior of negative tunneling magnetoresistance at the velocity ratio less than one.

Negative tunneling magnetoresistance of Fe/MgO/NiO/Fe magnetic tunnel junction: Role of spin mixing and interface state

Motivated by a recent tunneling magnetoresistance (TMR) measurement in which the negative TMR is observed in MgO/NiO-based magnetic tunnel junctions (MTJs), we have performed systematic calculations of transmission, current, and TMR of Fe/MgO/NiO/Fe MTJ with different thicknesses of NiO and MgO layers based on noncollinear density functional theory and non-equilibrium Green's function theory. The calculations show that, as the thickness of NiO and MgO layers is small, the negative TMR can be obtained which is attributed to the spin mixing effect and interface state. However, in the thick MTJ, the spin-flipping scattering becomes weaker, and thus, the MTJs recover positive TMR. Based on our theoretical results, we believe that the interface state at Fe/NiO interface and the spin mixing effect induced by noncollinear interfacial magnetization will play important role in determining transmission and current of Fe/MgO/NiO/Fe MTJ. The results reported here will be important in understanding the electron tunneling in MTJ with the barrier made by transition metal oxide.