Inverse Tunneling Magnetoresistance Research Articles

Structural design of magnetostrictive sensing glove and its application for gesture recognition

PurposeGesture recognition plays an important role in many fields such as human–computer interaction, medical rehabilitation, virtual and augmented reality. Gesture recognition using wearable devices is a common and effective recognition method. This study aims to combine the inverse magnetostrictive effect and tunneling magnetoresistance effect and proposes a novel wearable sensing glove applied in the field of gesture recognition.Design/methodology/approachA magnetostrictive sensing glove with function of gesture recognition is proposed based on Fe-Ni alloy, tunneling magnetoresistive elements, Agilus30 base and square permanent magnets. The sensing glove consists of five sensing units to measure the bending angle of each finger joint. The optimal structure of the sensing units is determined through experimentation and simulation. The output voltage model of the sensing units is established, and the output characteristics of the sensing units are tested by the experimental platform. Fifteen gestures are selected for recognition, and the corresponding output voltages are collected to construct the data set and the data is processed using Back Propagation Neural Network.FindingsThe sensing units can detect the change in the bending angle of finger joints from 0 to 105 degrees and a maximum error of 4.69% between the experimental and theoretical values. The average recognition accuracy of Back Propagation Neural Network is 97.53% for 15 gestures.Research limitations/implicationsThe sensing glove can only recognize static gestures at present, and further research is still needed to recognize dynamic gestures.Practical implicationsA new approach to gesture recognition using wearable devices.Social implicationsThis study has a broad application prospect in the field of human–computer interaction.Originality/valueThe sensing glove can collect voltage signals under different gestures to realize the recognition of different gestures with good repeatability, which has a broad application prospect in the field of human–computer interaction.

Inverse tunnel magnetoresistance of magnetic tunnel junctions with a NiCo2O4 electrode

Inverse spinel oxide NiCo2O4 (NCO) is known to exhibit ferrimagnetic characteristics and electrical conductivity. First-principles calculations predict NCO to be a half-metal with a negative polarization of −100%. In this study, we fabricated epitaxial NCO/MgO/Fe magnetic tunnel junctions by reactive molecular beam epitaxy and observed an inverse tunnel magnetoresistance (TMR) effect of −19.1% at 14 K, indicating that NCO has negative spin polarization. The TMR ratio monotonically decreased with increasing temperature, which was attributed to the temperature dependence of the NCO surface magnetization due to the thermal excitation of spin waves. In addition, the TMR ratio displayed strong bias voltage dependence, decreasing to less than half of the maximum value at +20 and −30 mV. These findings support the use of NCO in spintronic devices and should lead to further developments in oxide spintronics.

Ab Initio Study of Magnetic Tunnel Junctions Based on Half-Metallic and Spin-Gapless Semiconducting Heusler Compounds: Reconfigurable Diode and Inverse Tunnel-Magnetoresistance Effect

Magnetic tunnel junctions (MTJs) have attracted strong research interest within the last decades due to their potential use as nonvolatile memory such as magnetoresistive random access memory as well as for magnetic logic applications. Half-metallic magnets (HMMs) have been suggested as ideal electrode materials for MTJs to achieve an extremely large tunnel-magnetoresistance (TMR) effect. Despite their high TMR ratios, MTJs based on HMMs do not exhibit current rectification, i.e., a diode effect, which was achieved in a magnetic tunnel junction concept [E. \ifmmode \mbox{\c{S}}\else \c{S}\fi{}a\ifmmode \mbox{\c{s}}\else \c{s}\fi{}\ifmmode \imath \else \i \fi{}o\ifmmode \breve{g}\else \u{g}\fi{}lu et al., ACS Appl. Electron. Mater. 1, 1552--9 (2019)] based on HMMs and type-II spin-gapless semiconductors (SGSs). The proposed concept has recently been experimentally demonstrated using Heusler compounds. In the present work, we investigate from first-principles MTJs based on type-II SGS and HMM quaternary Heusler compounds $\mathrm{Fe}\mathrm{V}\mathrm{Ta}\mathrm{Al}$, $\mathrm{Fe}\mathrm{V}\mathrm{Ti}\mathrm{Si}$, $\mathrm{Mn}\mathrm{V}\mathrm{Ti}\mathrm{Al}$, and $\mathrm{Co}\mathrm{V}\mathrm{Ti}\mathrm{Sb}$. Our ab initio quantum transport calculations based on a nonequilibrium Green's function method have demonstrated that the MTJs under consideration exhibit current rectification with relatively high on:off ratios. We show that, in contrast to conventional semiconductor diodes, the rectification bias voltage window (or breakdown voltage) of the MTJs is limited by the spin gap of the HMM and SGS Heusler compounds. A unique feature of the present MTJs is that the diode effect can be configured dynamically, i.e., depending on the relative orientation of the magnetization of the electrodes, the MTJ allows the electrical current to pass either in one or the other direction, which leads to an inverse TMR effect. The combination of nonvolatility, reconfigurable diode functionality, tunable rectification voltage window, and high Curie temperature of the electrode materials makes the proposed MTJs very promising for room-temperature spintronic applications and opens ways to magnetic memory and logic concepts as well as logic-in-memory computing.

Magnetostrictive Tactile Sensor Array Based on L-Shaped Galfenol Wire and Application for Tilt Detection

A magnetostrictive tactile sensor unit with a large force measurement range and high sensitivity is designed using L-shaped Galfenol wires, tunnel magnetoresistive elements, and cylindrical permanent magnets. The output voltage model of the sensor unit is established based on the inverse magnetostrictive effect, tunnel magnetoresistance, and elastodynamic theory. Structures of the sensor unit and array are optimized using COMSOL. The static, dynamic, and temperature characteristics are tested. The interaction between the output voltages of each unit in a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3\times $ </tex-math></inline-formula> 3 tactile sensor array is described. The experimental results show that the sensitivity of the sensor unit is 85.5 mV/N at 0-10 N and 28.4 mV/N at 10-16 N under a biased magnetic field of 1.671 kA/m. The dynamic force sensitivity of the sensor unit does not vary by more than 1.4%. The magnetic field lateral coupling and longitudinal coupling of the sensor array do not exceed a maximum of 2.3%. The array can accurately distinguish the force distribution when multiple contacts are subjected to force at the same time. The sensitivity does not vary by more than 3.9% over the ambient temperature range of 23- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$59~^{\circ }\text{C}$ </tex-math></inline-formula> . The tactile sensor array is mounted on the mechanical fingertips to grasp bar-shaped objects with different tilt angles. The output voltage of the sensor array obtained from the robotic grasping experiments is analyzed using Probabilistic Neural Network, which can determine the tilt angle of the grasped objects, and the accuracy rate of object tilt recognition is 96.74%.

Design and Characterization of High-Sensitivity Magnetostrictive Tactile Sensor Array

A new high-sensitivity magnetostrictive tactile sensor array is designed based on the inverse magnetostrictive effect and tunnel magnetoresistance to enable intelligent robots to have a sensitive sense of touch and perform delicate operations. The output voltage model of the tactile sensor is developed. The optimal bias field and spacing of the sensor unit are determined using COMSOL Multiphysics. Test platforms for static and dynamic characteristics are built. The output characteristics of the tactile sensor under static and dynamic forces are tested. The sensitivity is compared with that of other types of tactile sensors. The tactile sensor units are set into a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times3$ </tex-math></inline-formula> sensor array and the coupling effect of the output voltage of each unit in the array is tested. The experimental results show that the experimental value of the output voltage matches the calculated value with a maximum error of 3.47%. The sensitivity of the tactile sensor is 323 mV/N at 0–4 N static force, which is higher compared to other types of tactile sensors. The sensitivity of the sensor unit changes less than 1.15% at 1–4 Hz dynamic force. The interaction of output voltage between units in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times3$ </tex-math></inline-formula> tactile sensor array does not exceed 1.5% and is negligible. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times3$ </tex-math></inline-formula> tactile sensor array is attached to a three-finger manipulator for grasping objects with a different hardness. The results of the study have guiding significance to the design of high-sensitivity tactile sensors for intelligent robots.

Spintronic transport through a double quantum dot-based spin valve with noncollinear magnetizations

We study the magnetoresistive properties of a spin valve based on a double quantum dot attached to ferromagnetic leads with noncollinear alignment of magnetic moments. It is assumed that each dot is strongly coupled to its own ferromagnetic electrode, while the hopping between the dots is relatively weak. The calculations are performed by using the perturbation theory in the coupling between the dots, while the local density of states of a quantum dot attached to a given external lead is determined with the aid of the numerical renormalization group method. We demonstrate that the examined device can exhibit considerable positive or inverse tunnel magnetoresistance. It can be also a source of highly spin-polarized current. Importantly, the spin-resolved transport properties can be controlled by gate and bias voltages and depend on the angle between the magnetizations of the ferromagnets.

Large Voltage-Tunable Spin Valve Based on a Double Quantum Dot

We study the spin-dependent transport properties of a spin valve based on a double quantum dot. Each quantum dot is assumed to be strongly coupled to its own ferromagnetic lead, while the coupling between the dots is relatively weak. The current flowing through the system is determined within perturbation theory in the hopping between the dots, whereas the spectrum of a quantum-dot--ferromagnetic-lead subsystem is determined by means of the numerical renormalization group method. The spin-dependent charge fluctuations between ferromagnets and quantum dots generate an effective exchange field, which splits the double-dot levels. Such a field can be controlled, separately for each quantum dot, by the gate voltages or by changing the magnetic configuration of the external leads. We demonstrate that the considered double-quantum-dot spin-valve setup exhibits enhanced magnetoresistive properties, including both normal and inverse tunnel magnetoresistance. We also show that this system allows for the generation of highly spin-polarized currents, which can be controlled by purely electrical means. The considered double quantum dot with ferromagnetic contacts can thus serve as an efficient voltage-tunable spin valve characterized by high output parameters.

Large Inverse Tunnel Magnetoresistance in Magnetic Tunnel Junctions with an Fe3O4 Electrode

Spinel-type magnetite ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ is predicted to be a half-metal material with negative spin polarization. However, magnetic tunnel junctions (MTJs) using an ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ electrode exhibit a small tunnel magnetoresistance (TMR) effect, the sign of which has not been established experimentally. The development of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ as an excellent TMR material requires a better understanding of the characteristics of the interface and the phase transition of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ called the Verwey transition. We fabricate MTJs using epitaxial ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$-based stacks on $\mathrm{Mg}\mathrm{O}(001)$ substrates and find a large inverse TMR ratio of \ensuremath{-}55.8% at 80 K, which corresponds to 126% by the optimistic definition of the TMR ratio. The temperature dependence of the TMR ratio is significantly affected by the Verwey transition. Moreover, we investigate the dependence of TMR on oxygen partial pressure during ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ deposition. It is found that the magneto-transport properties of the MTJs show different behaviors depending on the oxygen partial pressure because the Verwey transition is sensitive to the oxygen concentration. Furthermore, the electronic and magnetic properties at the interfacial regions are investigated by x-ray magnetic spectroscopy and first-principles calculation. These findings greatly support the use of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ in spintronic devices and should lead to further developments in oxide spintronics.

Observation of tunneling magnetoresistance effect in L10-MnAl/MgO/Co2MnSi/MnAl perpendicular magnetic tunnel junctions

We report on the tunneling magnetoresistance (TMR) effect in fully perpendicular magnetic tunnel junctions (p-MTJs) with the core structure of L10-MnAl/MgO/Co2MnSi/MnAl. The multilayer is epitaxially grown on GaAs (0 0 1) substrate by molecular-beam epitaxy (MBE), and both the top and bottom MnAl layers show well perpendicular magnetic anisotropy (PMA). Meanwhile, an inverse TMR effect with the MR ratio of 10% is observed at 5 K, which is attributed to the intrinsic negative spin polarization of MnAl in contrast to the positive one in Co2MnSi. This work proposes a MnAl-based fully p-MTJ for future STT-MRAM applications.

Magnetoresistive properties of a double magnetic molecule spin valve in different geometrical arrangements

Abstract Spin-dependent transport through a spin valve consisting of two large-spin molecules, such as single molecular magnets, weakly coupled to external ferromagnetic electrodes is theoretically studied. The main focus is on the dependence of the tunneling current, its spin polarization, differential conductance and tunnel magnetoresistance on the arrangement of molecules embedded in the tunnel junction. These quantities are calculated by using the real-time diagrammatic technique in the lowest-order perturbation theory with respect to the coupling strength. When the molecules’ geometry is parallel, both an enhanced as well as inverse tunnel magnetoresistance can develop depending on the molecule’s occupation. On the other hand, if the two molecules form a serial geometry, a strong current-suppression occurs due to the Pauli spin blockade effect, resulting in negative differential conductance. In this transport regime we also find large tunnel magnetoresistance, which now exhibits strong asymmetry with respect to the bias voltage reversal. In addition, we show that an enhanced magnetoresistance and negative differential conductance can develop when a T-shaped molecular geometry is taken under consideration. These effects are explained by invoking particular occupation of molecular states and nonequilibrium spin accumulation that builds up in the case of the antiparallel magnetic configuration of the device.

Spin-dependent electronic transport characteristics in Fe4N/BiFeO3/Fe4N perpendicular magnetic tunnel junctions

Perpendicular magnetic tunnel junctions (MTJs) have attracted increasing attention owing to the low energy consumption and wide application prospects. Herewith, against Julliere's formula, an inverse tunnel magnetoresistance (TMR) appears in tetragonal Fe4N/BiFeO3/Fe4N perpendicular MTJs, which is attributed to the binding between the interface resonant tunneling state and central (bordered) hot spots. Especially, antiferromagnetic BiFeO3 shows an extra spin-polarized resonant state in the barrier, which provides a magnetic-barrier factor to affect the tunneling transport in MTJs. Meanwhile, due to the spin-polarized transport in Fe4N/BiFeO3/Fe4N MTJs, the sign of TMR can be tuned by the applied bias. The tunable TMR and resonant magnetic barrier effect pave the way for clarifying the tunneling transport in other junctions and spintronic devices.

Antiferroelectric Tunnel Junctions

Antiferroelectric tunnel junctions of La0.7Sr0.3MnO3/PbZrO3/Co fabricated by pulsed laser deposition and sputtering are reported for the first time. The current–voltage curves highlight the monostability type (threshold) resistive switching corresponding to the antiferroelectric behavior for the PbZrO3 barrier down to ≈4 nm thickness. Tunneling electroresistance values up to 109%, which is much higher than the usual values in ferroelectric tunnel junctions, are observed at room temperature. The resistive switching is attributed to a combination of the nonpolar to polar switching induced by an external field, leading to significant changes in the barrier heights, and the crossover from direct tunneling to Fowler–Nordheim tunneling. At low temperatures, the devices exhibit an inverse tunneling magnetoresistance, in which the resistance is higher in the parallel magnetization state, most probably caused by interfacial Zr and Co ions' hybridization influence on the spin polarization.

Resonant TMR inversion in LiF/EuS based spin-filter tunnel junctions

Resonant tunneling can lead to inverse tunnel magnetoresistance when impurity levels rather than direct tunneling dominate the transport process. We fabricated hybrid magnetic tunnel junctions of CoFe/LiF/EuS/Ti, with an epitaxial LiF energy barrier joined with a polycrystalline EuS spin-filter barrier. Due to the water solubility of LiF, the devices were fully packaged in situ. The devices showed sizeable positive TMR up to 16% at low bias voltages but clearly inverted TMR at higher bias voltages. The TMR inversion depends sensitively on the thickness of LiF, and the tendency of inversion disappears when LiF gets thick enough and recovers its intrinsic properties.

Inverse tunnel magnetoresistance in epitaxial FeCo/MgO/Fe tunnel junctions patterned by in situ shadow-masks

Fully epitaxial FeCo/MgO/Fe magnetic tunnel junctions on silicon substrates were fabricated using in situ shadow-masks in an electron-beam deposition system. An inverse tunneling magnetoresistance (TMR) of −39% was observed at 77 K after annealing, which was not obtained in MTJs grown in better vacuum with the same device structure. This inverse TMR is attributed to the oxidation of the FeCo/MgO interface, which provides a negative spin polarization. Our work highlights the importance of interfacial properties on tunneling magnetoresistance and points to a simple processing route to achieve inverse TMR by carefully controlling the oxidation condition of the bottom layer.

Unidirectional Spin-Dependent Molecule-Ferromagnet Hybridized States Anisotropy in Cobalt Phthalocyanine Based Magnetic Tunnel Junctions.

Organic or molecular spintronics is a rising field of research at the frontier between condensed matter physics and chemistry. It aims to mix spin physics and the richness of chemistry towards designing new properties for spin electronics devices through engineering at the molecular scale. Beyond the expectation of a long spin lifetime, molecules can be also used to tailor the spin polarization of the injected current through the spin-dependent hybridization between molecules and ferromagnetic electrodes. In this Letter, we provide direct evidence of a hybrid interface spin polarization reversal due to the differing hybridization between phthalocyanine molecules and each cobalt electrode in Co/CoPc/Co magnetic tunnel junctions. Tunnel magnetoresistance and anisotropic tunnel magnetoresistance experiments show that interfacial hybridized electronic states have a unidirectional anisotropy that can be controlled by an electric field and that spin hybridization at the bottom and top interfaces differ, leading to an inverse tunnel magnetoresistance.

Large positive spin polarization and giant inverse tunneling magnetoresistance in Fe/PbTiO3/Fe multiferroic tunnel junction

We perform first-principles electronic structure and spin-dependent transport calculations of a multiferroic tunnel junction (MFTJ) with an epitaxial Fe/PbTiO3/Fe heterostructure. We predict a large positive spin-polarization (SP) and an intriguing giant inverse tunneling magnetoresistance (TMR) ratio in this tunnel junction. We demonstrate that the tunneling properties are determined by ferroelectric (FE) polarization screening and electronic reconstruction at the interface with lower electrostatic potential. The intricate complex band structure of PbTiO3, in particular the lowest decay rates concerning Pb 6pz and Ti 3dz2 states near the Γ¯ point, gives rise to the large positive SP of the tunneling current in the parallel magnetic configuration. However, the giant inverse TMR ratio is attributed to the minority-spin electrons of the interfacial Ti 3dxz+3dyz orbitals which have considerably weight in the extended area around the Γ¯ point at the Fermi energy and causes remarkable contributions to the conductance in the antiparallel magnetic configuration.

Breakdown by magnetic field in a La0.7Sr0.3MnO3/MgO/Fe spin valve

A La0.7Sr0.3MnO3/MgO/Fe spin valve with inverse tunneling magnetoresistance (TMR) was fabricated on a (100) SrTiO3 substrate by radio frequency magnetron sputtering. Giant TMR ratios up to 540% were obtained. The breakdown of the spin valve was observed at high magnetic field, which was attributed to the joint action of the invalidation of MgO barrier and the shift of Fermi energy in La0.7Sr0.3MnO3 at high magnetic field.

New materials research for high spin polarized current

The author reports here a thorough investigation of structural and magnetic properties of Co2FeAl0.5Si0.5 Heusler alloy films, and the tunnel magnetoresistance effect for junctions with Co2FeAl0.5Si0.5 electrodes, spin injection into GaAs semiconductor from Co2FeAl0.5Si0.5, and spin filtering phenomena for junctions with CoFe2O4 ferrite barrier. It was observed that tunnel magnetoresistance ratio up to 832%(386%) at 9K (room temperature), which corresponds to the tunnel spin polarization of 0.90 (0.81) for the junctions using Co2FeAl0.5Si0.5 Heusler electrodes by optimizing the fabrication condition. It was also found that the tunnel magnetoresistance ratio are almost the same between the junctions with Co2FeAl0.5Si0.5 Heusler electrodes on Cr buffered (100) and (110) MgO substrates, which indicates that tunnel spin polarization of Co2FeAl0.5Si0.5 for these two direction are almost the same. The next part of this paper is a spin filtering effect using a Co ferrite. The spin filtering effect was observed through a thin Co-ferrite barrier. The inverse type tunnel magnetoresistance ratio of −124% measured at 10K was obtained. The inverse type magnetoresistance suggests the negative spin polarization of Co-ferrite barrier. The magnetoresistance ratio of −124% corresponds to the spin polarization of −0.77 by the Co-ferrite barrier. The last part is devoted to the spin injection from Co2FeAl0.5Si0.5 into GaAs. The spin injection signal was clearly obtained by three terminal Hanle measurement. The spin relaxation time was estimated to be 380ps measured at 5K.

Inverse TMR in a nominally symmetric CoFe/AlOx/CoFe junction induced by interfacial Fe3O4 investigated by STEM-EELS

We have found inverse tunneling magnetoresistance (TMR) with a non-symmetric bias voltage dependence in a nominally symmetric Si (001)/Ag/CoFe/AlOx/CoFe/IrMn/Ag magnetic tunnel junction after field cooling. The O K edge fine structure extracted from electron energy loss spectroscopy spectrum images taken at the interfaces of junctions with inverse TMR shows a thin, discontinuous Fe3O4 layer at the CoFe/AlOx interfaces. The Fe L2,3 edge core level shifts are also consistent with those of Fe3O4. We find no Fe3O4 layer in junctions with normal TMR. We believe this Fe3O4 layer is responsible for the inverse TMR.

Temperature and Bias-Assisted Transport Properties of LSMO/AlO/CoFeB Magnetic Tunnel Junction

We have observed the transport properties in magnetic tunnel junction with the multilayered structure: (50 nm)/Al-O (1 nm)/CoFeB (5 nm)/Ta (10 nm)/Ru (10 nm). The maximum normal tunneling magnetoresistance (TMR) ratio was found to be 12.4% at a temperature range around 100 K, and the maximum inverse TMR ratio was observed to be 3% at a temperature of 5 K . The inverse TMR decreases with increasing temperature until it reaches a temperature range of around 50 K, whereby it switches to the normal TMR behavior. The appearance of inverse TMR at low temperatures is attributed to the presence of minority-spin density of states (DOS) of half-metal around the Fermi level present at the half-metal-insulator interface. The vanishing of inverse TMR and increase of normal TMR above certain temperature range is due to no contribution from the minority-spin DOS in the tunneling process. These results were found to be reproducible and the TMR ratio of more than 3% and 11% was obtained at room temperature before and after annealing at a temperature of 235 K for an annealing time of one hour, respectively.