Tunnel Magnetoresistance Ratio Research Articles

Unconventional bias dependence of tunnel magnetoresistance induced by the Coulomb blockade effect

In conventional magnetic tunnel junctions (MTJs), the tunnel magnetoresistance (TMR) monotonically decreases with increasing bias voltage, which limits the bias voltage range for the operation of MTJs. In our study, using double-barrier MTJs composed of Fe/MgO/Fe/γ-Al2O3 grown on a Nb-doped SrTiO3 substrate, we demonstrate unconventional bias dependences of the TMR, in which the TMR ratio increases with increasing bias voltage. We reveal that this behavior originates from the sharp giant resistance peak near zero bias likely induced by the Coulomb blockade effect via Fe impurities in γ-Al2O3, which are diffused from the Fe layer. The observed TMR ratio is 23% at a bias voltage of −4 V at 3.5 K, which is a very high value in this large bias voltage range. Our results offer a novel way to improve the bias voltage dependence of TMR.

Thermally Robust Perpendicular SOT-MTJ Memory Cells With STT-Assisted Field-Free Switching

A back-end-of-line compatible 400 °C thermally robust perpendicular spin-orbit torque magnetic tunnel junction (p-SOT-MTJ) memory cell with a tunnel magnetoresistance ratio of 130% is demonstrated. It features an energy-efficient spin-transfer-torque-assisted field-free spin-orbit torque (SOT) switching and a novel interface-enhanced synthetic antiferromagnet (SAF). The optimal SAF with a Ru (9 Å) spacer sandwiched by Co/Pt multilayers has a high SAF coupling field of 2.8 kOe. The parallel magnetic coupling between the CoFeB-based reference layer and the bottom Co/Pt multilayer is enhanced by a magnet-coupling face-centered cubic textured Co/Pt (5 Å) multilayer buffer. The thermally induced Pt–Fe interdiffusion is effectively reduced by the W (3 Å) trilayers of texture-decoupling diffusion multibarrier. The Ta/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> -W and TaN/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> -W composite SOT channels are thick enough to be the etching stop and sustain 400 °C annealing without transforming to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula> -W. Using the harmonic Hall voltage measurement, the Ta/W and TaN/W channels exhibit the large effective spin Hall angle of approximately −0.21 and −0.27, respectively. Scaling magnetic tunnel junction (MTJ) down to 30 nm size can reduce the switching time due to single-domain switching based on the micromagnetic simulation. The damping constant of ~0.018 is obtained by the ferromagnetic resonance measurement. A bigger damping constant reduces the switching time as predicted by the calibrated simulation.

Lattice Softening in Metastable bcc CoxMn100−x (001) Ferromagnetic Layers for a Strain-Free Magnetic Tunnel Junction

In spintronics, one of the long-standing questions is why the $\mathrm{Mg}\mathrm{O}$-based magnetic tunnel junction (MTJ) is almost the only option for achieving a large tunneling magnetoresistance (TMR) ratio at room temperature, although this is not as large as the theoretical prediction. This study focuses on the development of an almost strain-free MTJ using metastable bcc ${\mathrm{Co}}_{x}{\mathrm{Mn}}_{100\text{\ensuremath{-}}x}$ ($\mathrm{Co}\text{\ensuremath{-}}\mathrm{Mn}$) ferromagnetic films. We investigate the degree of crystallization in MTJs consisting of $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Mn}/\mathrm{Mg}\mathrm{O}/\mathrm{Co}\text{\ensuremath{-}}\mathrm{Mn}$ in relation to their TMR ratios. Cross-section high resolution transmission electron microscopy reveals that almost consistent lattice constants of these layers for 66 \ensuremath{\le} x \ensuremath{\le} 83 with large TMR ratios of 229% at room temperature, confirming the soft nature of the $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Mn}$ layer with some dislocations at the $\mathrm{Mg}\mathrm{O}/{\mathrm{Co}}_{75}{\mathrm{Mn}}_{25}$ interfaces. Ab initio calculations confirm the crystalline deformation stability across a broad compositional range in $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Mn}$, proving the advantage of a strain-free interface for much larger TMR ratios.

DC and AC tunneling magnetoelectric responses of cobalt lateral nanogranular films

We demonstrate a new structural optimization of cobalt–(barium fluoride) lateral nanogranular films, achieving independently variable direct current (DC) and alternating current (AC) tunneling effects, in the film plane and out of the plane, respectively. Lateral nanogranular films were successfully fabricated via programmable tandem sputtering. Flat-shaped magnetic Co nanogranules were periodically inserted between dielectric barium fluoride layers of different thicknesses. All the lateral nanogranular films were insulating, with electrical resistivities ρ exceeding 1013 μΩ·cm, even with a Co content as high as 30 at.%. A lateral tunneling magnetoresistance (TMR) effect and vertical tunneling magnetodielectric (TMD) effect were found in the same lateral nanogranular film with 30 at.% Co. For a nanogranular film with lateral Co nanoparticles, the sensitivity of both the TMR and TMD ratio were improved at small magnetic fields compared with conventional films with spherical Co nanoparticles, reflecting its small saturation magnetic field. This layer-ordered metal–insulator nanogranular structure has potential applications in AC/DC hybrid elements as a high-sensitivity magnetic sensor.

High-Sensitivity Tunnel Magnetoresistance Sensors Based on Double Indirect and Direct Exchange Coupling Effect* Supported by the Framework Project of SGCC (Grant No. 5700-202058381A-0-0-00), and the National Key Research and Development Program of China (Grant No. 2017YFA0206200).

Detection of ultralow magnetic field requires magnetic sensors with high sensitivity and low noise level, especially for low operating frequency applications. We investigated the transport properties of tunnel magnetoresistance (TMR) sensors based on the double indirect exchange coupling effect. The TMR ratio of about 150% was obtained in the magnetic tunnel junctions and linear response to an in-plane magnetic field was successfully achieved. A high sensitivity of 1.85%/Oe was achieved due to a designed soft pinned sensing layer of CoFeB/NiFe/Ru/IrMn. Furthermore, the voltage output sensitivity and the noise level of 10.7 mV/V/Oe, 10 nT/Hz1/2 at 1 Hz and 3.3 nT/Hz1/2 at 10 Hz were achieved in Full Wheatstone Bridge configuration. This kind of magnetic sensors can be used in the field of smart grid for current detection and sensing.

Crucial role of interfacial s−d exchange interaction in the temperature dependence of tunnel magnetoresistance

The tunnel magnetoresistance (TMR) is one of the most important spintronic phenomena but its reduction at finite temperature is a severe drawback for applications. Here, we reveal a crucial determinant of the drawback, that is, the $s\text{\ensuremath{-}}d$ exchange interaction between conduction $s$ and localized $d$ electrons at interfacial ferromagnetic layers. By calculating the temperature dependence of the TMR ratio in Fe/MgO/Fe(001), we show that the obtained TMR ratio significantly decreases with increasing temperature owing to the spin-flip scattering in the ${\mathrm{\ensuremath{\Delta}}}_{1}$ state induced by the $s\text{\ensuremath{-}}d$ exchange interaction. The material dependence of the coupling constant ${J}_{sd}$ is also discussed on the basis of a nonempirical method.

Magnetic memory driven by topological insulators

Giant spin-orbit torque (SOT) from topological insulators (TIs) provides an energy efficient writing method for magnetic memory, which, however, is still premature for practical applications due to the challenge of the integration with magnetic tunnel junctions (MTJs). Here, we demonstrate a functional TI-MTJ device that could become the core element of the future energy-efficient spintronic devices, such as SOT-based magnetic random-access memory (SOT-MRAM). The state-of-the-art tunneling magnetoresistance (TMR) ratio of 102% and the ultralow switching current density of 1.2 × 105 A cm−2 have been simultaneously achieved in the TI-MTJ device at room temperature, laying down the foundation for TI-driven SOT-MRAM. The charge-spin conversion efficiency θSH in TIs is quantified by both the SOT-induced shift of the magnetic switching field (θSH = 1.59) and the SOT-induced ferromagnetic resonance (ST-FMR) (θSH = 1.02), which is one order of magnitude larger than that in conventional heavy metals. These results inspire a revolution of SOT-MRAM from classical to quantum materials, with great potential to further reduce the energy consumption.

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

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

Breaking the symmetry of spin-sublattices in antiferromagnet by interfacial tailoring in the L1-MnPt/NaCl/Fe junction

The detection of magnetization states of the antiferromagnets is a precondition to read the binary bits (0 and 1) for the magnetic recording. The magnetoresistance effect has been utilized to detect the magnetization states in commercially ferromagnetic spintronic devices. However, reading the magnetization states of antiferromagnets is extremely hard because of the absence of net magnetic moment in the antiferromagnetic materials. Here, to generate the tunneling magnetoresistance effect in an antiferromagnetic device with the compensated surface, a method of interlayer selection and interfacial tailoring was proposed to break the symmetry of spin-sublattices in antiferromagnets. The stacking structure of the device is L10-MnPt/NaCl/Fe. The C-type antiferromagnet L10-MnPt was used as the antiferromagnetic electrode, and Fe was the other electrode. NaCl was selected as the barrier layer to break the symmetry of spin-sublattices in L10-MnPt by considering the lattice constants of different materials. It is confirmed that the device with the Mn terminal shows the C4v symmetry in which the spin-sublattices symmetry of L10-MnPt was broken. As a result, the tunneling magnetoresistance ratio of the present device can be up to 200%, which is much larger than the reported ratios of both anisotropic magnetoresistance and the spin Hall magnetoresistance effect in the antiferromagnetic devices, showing a great potential application to read the magnetization states of the antiferromagnetic devices. The spin-dependent transport mechanism of the present device was analyzed in detail by the kǁ-resolved transmission and the local density of states. This study would be beneficial to promote the development of antiferromagnetic spintronics.

Enhanced Magnetoresistance under Bias Voltage in Fe / MgO / MgAl2O4 / MgO / Fe Trilayer Tunneling Barrier Junction

Experiments on magnetic tunnel junctions (MTJs) show that the use of a (001)-oriented spinel ${\mathrm{Mg}\mathrm{Al}}_{2}{\mathrm{O}}_{4}$ barrier improves the robustness of the tunneling magnetoresistance (TMR) ratio against bias voltage [Sukegawa et al., Appl. Phys. Lett. 96, 212505 (2010)]; however, the maximum TMR ratio is very small compared with that of the $\mathrm{Mg}\mathrm{O}$-based MTJ. To overcome this problem, we propose a MTJ with a trilayered tunnel-barrier junction, $\mathrm{Fe}$/$\mathrm{Mg}\mathrm{O}$/${\mathrm{Mg}\mathrm{Al}}_{2}{\mathrm{O}}_{4}$/$\mathrm{Mg}\mathrm{O}$/$\mathrm{Fe}$, from first-principles calculations. The presence of the $\mathrm{Mg}\mathrm{O}$ interlayer between $\mathrm{Fe}$ and ${\mathrm{Mg}\mathrm{Al}}_{2}{\mathrm{O}}_{4}$ has the effect of enhancing the TMR ratio to more than $1000\mathrm{%}$ at zero bias. The large TMR is maintained under a bias voltage. The results indicate the potential of a hybrid-type tunnel barrier that combines the advantages of MTJs containing a single $\mathrm{Mg}\mathrm{O}$ barrier (high TMR) and a single ${\mathrm{Mg}\mathrm{Al}}_{2}{\mathrm{O}}_{4}$ barrier (robustness to bias voltages). The $\mathrm{Mg}\mathrm{O}$ interlayer is found to play a key role in suppressing the transmittance of the minority-spin channel, and thus, the tunneling conductance of antiparallel magnetization is significantly reduced.

Two Terminal Perpendicular Spin Transfer Torque Magnetic Random Access Memory Based Stochastic Nature Spin Neuron Device for Spiking Neural Network

In recent years, research on neuromorphic chips of spin-based devices capable of low-power, ultra-high-speed switching operation has been actively conducted. There were reports of spin-based neuromorphic devices using spin-transfer-torque (STT), which are assisted by an external magnetic field [1] and spin-orbit-torque (SOT) spin neurons via heavy metal that have a high spin Hall angle. [2-4] Since these spin neurons operate with pulse bias, they are suitable for calculating spiking neural network (SNN), which is a low-power artificial neural network (ANN) that operates similar to the human brain. However, the previously announced devices required an external magnetic field or implemented in a three-terminal method (16F2), making fabrication impossible for high density and low-power devices. In this study, a 300 nm-scale 2-terminal (4F2) perpendicular STT-based (p-STT) spin neuron device capable of switching operation without an external magnetic field was fabricated to demonstrate stochastic neuron nature. Furthermore, an SNN was constructed using fitting data and an MNIST pattern recognition test was conducted.As shown in Fig. 1-(a), a 300 nm pattern process was processed via E-beam lithography of a double MgO p-STT magnetic tunnel junction (MTJ) with a tunnel-magnetoresistance (TMR) ratio of 158.13%. Figure 1-(b) is a transmission electron microscopy (TEM) image of a 300 nm scale p-STT spin based neuron. Figure 1-(c) is a DC sweep R-V graph of a spin neuron. It was confirmed that the resistance was 985 Ω in parallel state (LRS) and 2137 Ω in anti-parallel state (HRS) relatively, and the difference of resistance was 1114 Ω. To measure the stochastic characteristics depending on the set voltage, the set voltage pulse amplitude was varied from -0.61 mV to -0.695 mV with pulse width of 250 μs. It was confirmed that switching is impossible below -0.61 V, and switching occurs with one pulse above -0.695 V. In addition, as a result of the measurement the switching probability depending on the set voltage pulse was fitted with a sigmoid function. Furthermore, we performed a pattern recognition simulation assuming that p-STT neurons were applied in a single-layer SNN consisting of 784 input layers and 100 output layers. In this simulation, the images used for pattern recognition were MNIST handwritten digit images with a size of 28X28 pixels, 60000 images were used for training, and 10000 images not used for training were used for testing. In our presentation, we will review in detail the stochastic nature of the p-STT neuron depending on the spike amplitude and SNN pattern recognition.This research was supported by National R&D Program through the National Research Foundation of Korea(NRF) funded by Ministry of Science and ICT(2021M3F3A2A01037733) Reference [1] Kondo, Kei, et al. "A two-terminal perpendicular spin-transfer torque based artificial neuron." Journal of Physics D: Applied Physics 51.50 (2018): 504002.[2] Kurenkov, Aleksandr, et al. "Artificial neuron and synapse realized in an antiferromagnet/ferromagnet heterostructure using dynamics of spin–orbit torque switching." Advanced Materials 31.23 (2019): 1900636.[3] Ostwal, Vaibhav, et al. "Spin-torque devices with hard axis initialization as Stochastic Binary Neurons." Scientific reports 8.1 (2018): 1-8.[4] Reidy, Brendan, and Ramtin Zand. "SOT-MRAM based Sigmoidal Neuron for Neuromorphic Architectures." arXiv preprint arXiv:2006.01238 (2020). Figure 1

Effect of a ferromagnetic STM cobalt tip on a single Co-phthalocyanine molecule adsorbed on a ferromagnetic substrate

The effect of a ferromagnetic scanning tunneling microscope (STM) cobalt tip on the electronic, magnetic and electronic transport properties of a Co-phthalocyanine (CoPc)/Co(111) junction has been investigated in the framework of density functional theory in conjunction with Landauer transport and the non-equilibrium Green function formalism. It is shown that the spin magnetic moment of the CoPc molecule can be flipped by varying the distance between the STM tip and the CoPc molecule when passing from the tunneling regime to the contact regime. Our calculations show that such spin flip of the CoPc molecule leads to a change of the sign of the Tunneling Magneto-Resistance ratio (TMR). The change from the tunneling to contact regime also leads to large changes in the total and spin-polarized I–V characteristics.

Surface acoustic wave induced modulation of tunneling magnetoresistance in magnetic tunnel junctions

We show that a surface acoustic wave (SAW) applied across the terminals of a magnetic tunnel junction (MTJ) decreases both the (time-averaged) parallel and antiparallel resistances of the MTJ, with the latter decreasing much more than the former. This results in a decrease in the tunneling magnetoresistance ratio. The coercivities of the free and fixed layers of the MTJ, however, are not affected significantly, suggesting that the SAW does not cause large-angle magnetization rotation in the magnetic layers through the inverse magnetostriction (Villari) effect at the power levels used. This study sheds light on the dynamical behavior of an MTJ under periodic compressive and tensile strain.

Laser induced crystallization of Co\u2013Fe\u2013B films

Local crystallization of ferromagnetic layers is crucial in the successful realization of miniaturized tunneling magnetoresistance (TMR) devices. In the case of Co–Fe–B TMR devices, used most successfully so far in applications and devices, Co–Fe–B layers are initially deposited in an amorphous state and annealed post-deposition to induce crystallization in Co–Fe, thereby increasing the device performance. In this work, first direct proof of locally triggered crystallization of 10 nm thick Co–Fe–B films by laser irradiation is provided by means of X-ray diffraction (XRD) using synchrotron radiation. A comparison with furnace annealing is performed for benchmarking purposes, covering different annealing parameters, including temperature and duration in the case of furnace annealing, as well as laser intensity and scanning speed for the laser annealing. Films of Co–Fe–B with different stoichiometry sandwiched between a Ru and a Ta or MgO layer were systematically assessed by XRD and SQUID magnetometry in order to elucidate the crystallization mechanisms. The transformation of Co–Fe–B films from amorphous to crystalline is revealed by the presence of pronounced CoFe(110) and/or CoFe(200) reflexes in the XRD θ-2θ scans, depending on the capping layer. For a certain window of parameters, comparable crystallization yields are obtained with furnace and laser annealing. Samples with an MgO capping layer required a slightly lower laser intensity to achieve equivalent Co–Fe crystallization yields, highlighting the potential of laser annealing to locally enhance the TMR ratio.

Electronic structures of various (001) and (111) surfaces, interfaces and spin transport properties of half-metallic fully compensated ferrimagnet Cr2Se

Half-metallic fully compensated ferrimagnets have advantages of extremely low net magnetic moments and large spin polarization. By employing the first-principles calculations within density functional theory combined with the nonequilibrium Green’s function, we investigated the electronic structures of Cr2Se (001), (111) surfaces and Cr2Se/CdS interface, as well as the transport properties of Cr2Se/CdS/Cr2Se magnetic tunnel junction (MTJ). Our calculations show that bulk half-Heusler alloy Cr2Se is a half-metallic fully compensated ferrimagnet with a direct spin-down bandgap. Although the half-metallic character of bulk is destroyed in all surface terminations owing to the emergence of surface states, CrBSe-(001) termination holds the lowest surface free energy and possesses the highest surface spin polarization (SP) of 83.6%. Calculations on electronic structures of Cr2Se/CdS heterostructures show that CrBSe-S-top type interface structure holds the lowest interface free energy and high interface SP of 71.6%. Moreover, the spin transport properties of Cr2Se/CdS/Cr2Se MTJ device are studied and the calculated tunnel magnetoresistance (TMR) ratio reaches up to a high value of 4.98×103, which shows that the device has great application potential in spintronic devices.

Full-Electrical Writing and Reading of Magnetization States in a Magnetic Junction with Symmetrical Structure and Antiparallel Magnetic Configuration.

Full-electrical writing and reading of magnetization states are vital for the development of next-generation spintronic devices with high density and ultralow-power consumption. Here, we proposed a method to realize the full-electrical writing and reading of magnetization states via a structural design, which only requires a symmetrical device structure and an antiparallel magnetic configuration. CrBr3, h-BN, and 1T-MnSe2 were selected to construct the device of CrBr3/h-BN/1T-MnSe2/h-BN/CrBr3, where the magnetization of two CrBr3 layers was fixed to the antiparallel state. By changing the direction and magnitude of the applied electric field, it is proved that the magnetization of 1T-MnSe2 could be reversed. Moreover, the device energies before and after the magnetization reversal are the same when the applied electric field is removed due to the structural symmetry. Meanwhile, the magnetic anisotropy energy of 1T-MnSe2 could induce an energy barrier, to guarantee the nonvolatile magnetization reversal in the present device. In addition, the tunnel magnetoresistance ratio was found up to 421%, showing a promising application to full-electrically write and read magnetization in spintronics. The present study likely promotes the development of full-electrical and ultralow-power spintronics devices.

Gigantic tunneling magnetoresistance in magnetic Weyl semimetal tunnel junctions.

We investigate the tunneling magnetoresistance in magnetic tunnel junctions (MTJs) comprised of Weyl semimetal contacts. We show that chirality-magnetization locking leads to a gigantic tunneling magnetoresistance ratio, an effect that does not rely on spin filtering by the tunnel barrier. Our results indicate that the conductance in the anti-parallel configuration is more sensitive to magnetization fluctations than in MTJs with normal ferromagnets, and predicts a TMR as large as 104 % when realistic magnetization fluctuations are accounted for. In addition, we show that the Fermi arc states give rise to a non-monotonic dependence of conductance on the misalignment angle between the magnetizations of the two contacts.

Electric-field-assisted non-volatile magnetic switching in a magnetoelectronic hybrid structure

SummaryElectric-field (E-field) control of magnetic switching provides an energy-efficient means to toggle the magnetic states in spintronic devices. The angular tunneling magnetoresistance (TMR) of an magnetic tunnel junction (MTJ)/PMN-PT magnetoelectronic hybrid indicates that the angle-dependent switching fields of the free layer can decrease significantly subject to the application of an E-field. In particular, the switching field along the major axis is reduced by 59% from 28.0 to 11.5 Oe as the E-field increases from 0 to 6 kV/cm, while the TMR ratio remains intact. The switching boundary angle decreases (increases) for the parallel (antiparallel) to antiparallel (parallel) state switch, resulting in a shrunk switching window size. The non-volatile and reversible 180° magnetization switching is demonstrated by using E-fields with a smaller magnetic field bias as low as 11.5 Oe. The angular magnetic switching originates from competition among the E-field-induced magnetoelastic anisotropy, magnetic shape anisotropy, and Zeeman energy, which is confirmed by micromagnetic simulations.

Barrier-dependent electronic transport properties in two-dimensional MnBi2Te4-based van der Waals magnetic tunnel junctions

Tunneling magnetoresistance (TMR) and spin filtering effects in the magnetic tunnel junctions (MTJs) have drawn much attention for potential spintronic applications based on magnetic manipulation of electric transport. However, the traditional MTJs cannot meet the demand for rapid miniaturization of electronic components. Thus, van der Waals (vdW) MTJs with a few atomic layers stacked vertically are ideal candidates for atomic scale devices. In this work, by employing the non-equilibrium Green's function combined with density-functional theory, we systemically study the spin-dependent electronic transport properties across MnBi2Te4 (MBT)-based vdW MTJs with three typical barrier layers, i.e., monolayer hexagonal boron nitride (h-BN), monolayer graphene, and vacuum. By using graphite as the electrode of these junctions, we find that a high TMR ratio up to 4000% and almost 100% spin filtering ratio are realized in MBT|h-BN|MBT MTJ at low bias voltages. Moreover, a remarkable negative differential resistance effect is observed in MBT|h-BN|MBT and MBT|Graphene|MBT junctions. The observed barrier-dependent quantum transport phenomenon is explained by the transmission coefficient. Our unique design of these vdW structures reasonably overcomes the bottleneck of current leakage and avoids the interface contact issues and paves the way for the exploration of spintronics devices with better performance.

Jump of tunneling magnetoresistance in magnetic nanocontacts with mismatched cross section

We have studied the influence of the transverse size of a magnetic tunnel nanojunction on the magnitude of the magnetoresistance. During modeling, the size of the right contact was fixed, while the size of the left one gradually changed until they coincided. We found a sharp drop in the tunneling magnetoresistance (TMR) in nanocontacts with mismatched cross section. This can be explained by the peculiarities of the spatial distribution of the electron density, which is different for majority and minority-spin states. Thus, our study indicates that the quantum size effect can also be a factor affecting the TMR ratio. Taking into account the observed effect can be important when designing the TMR-based nanodevices.