Large Tunnel Magnetoresistance Ratio Research Articles

Tunable High‐Temperature Tunneling Magnetoresistance in All‐van der Waals Antiferromagnet/Semiconductor/Ferromagnet Junctions

AbstractMagnetic tunnel junctions (MTJs) are widely applied in spintronic devices for efficient spin detection through the imbalance of spin polarization at the Fermi level. The van der Waals (vdW) property of 2D magnets with atomically flat surfaces and negligible surface roughness greatly facilitates the development of MTJs, primarily in ferromagnets. Here, A‐type antiferromagnetism in 2D vdW single‐crystal (Fe0.8Co0.2)3GaTe2 is reported with TN ≈ 203 K in bulk and ≈ 185 K in 9‐nm nanosheets. The metallic nature and out‐of‐plane magnetic anisotropy make it a suitable candidate for MTJ electrodes. By constructing heterostructures based on (Fe0.8Co0.2)3GaTe2/WSe2/Fe3GaTe2, a large tunneling magnetoresistance (TMR) ratio of 180% at low temperature is obtained, with the TMR signal persisting at near‐room temperature 280 K. Furthermore, the TMR is tunable by the electric field, and the MTJ device operates stably with a low applied bias down to 1 mV (≈0.6 nA), highlighting its potential for energy‐efficient spintronic devices. This work opens up new opportunities for 2D antiferromagnetic spintronics and quantum devices.

631% room temperature tunnel magnetoresistance with large oscillation effect in CoFe/MgO/CoFe(001) junctions

We demonstrate tunnel magnetoresistance (TMR) ratios of up to 631% at room temperature (RT) using CoFe/MgO/CoFe(001) epitaxial magnetic tunnel junctions (MTJs). The TMR ratio increased up to 1143% at 10 K. The large TMR ratios resulted from fine-tuning of atomic-scale structures of the MTJs, such as crystallographic orientations and MgO interface oxidation by interface insertion of ultrathin CoFe and Mg layers, which are expected to enhance the well-known Δ1 coherent tunneling transport. Interestingly, the TMR oscillation effect, which is not covered by the standard coherent tunneling theory, also became significant. A 0.32-nm period TMR oscillation with increasing MgO thickness dominates the transport in a wide range of MgO thicknesses; the peak-to-valley difference of the TMR oscillation exceeds 140% at RT, which is attributed to the appearance of large oscillatory components in the resistance area product.

Ab-initio predictions of mechanical, electronic, magnetic, and transport properties of bulk and heterostructure of a novel Fe-Cr based full Heusler chalcogenide

According to electronic structure calculations based on density functional theory, we predicted and studied the structural, mechanical, electronic, magnetic, and transport properties of a new full Heusler chalcogenide called Fe2CrTe in both its bulk and heterostructure forms. This system exhibits ferromagnetic and half-metallic (HM)-like behavior, with very high (about 95%) spin polarization at the Fermi level in its cubic phase. Interestingly, under tetragonal distortion, a clear minimum (with almost the same energy as the cubic phase) was found at a c/a value of ∼1.26, but it exhibits ferrimagnetic and fully metallic characteristics. The compound was found to be dynamically stable against lattice vibration in both phases. The elastic properties indicate that the compound is mechanically stable in both phases according to the stability criteria for the cubic and tetragonal phases. The elastic parameters demonstrate the mechanically anisotropic and ductile nature of the alloy system. Due to the HM-like behavior of the cubic phase and considering practical aspects, we probed the effect of strain and the substrate on various physical properties of the alloy. The transmission profile was calculated for the Fe2CrTe/MgO/Fe2CrTe heterojunction to investigate its possible use as a magnetic tunneling junction material in both the cubic and tetragonal phases. A large tunneling magnetoresistance ratio of ≈103 % was determined for the tetragonal phase and it was one order of magnitude larger than that for the cubic phase.

Excellent spin-filtering and giant tunneling magnetoresistance in a dual-electrode van der Waals magnetic tunnel junction based on ferromagnetic CrSe2

Magnetic tunnel junctions (MTJs) both with efficient spin-filtering effect and large tunneling magnetoresistance (TMR) ratio are extremely attractive and highly desirable in the field of spintronics. Inspired by recent successful preparation of a stable two-dimensional (2D) layered ferromagnet 1T-CrSe2 on a 2H-WSe2 substrate via chemical vapor deposition (CVD) synthesis, herein, employing first-principles calculations, we investigate the spin-dependent transport properties of van der Waals (vdW) MTJs built with a monolayer 2H-WSe2 tunnel barrier and a 1T-CrSe2 single-electrode or a 1T-MoSe2/1T-CrSe2 dual-electrode. Compared to low TMR (67.3%) in single-electrode MTJ, dual-electrode MTJ exhibits excellent spin-filtering effect (with 99.96% spin polarization) and a giant TMR ratio (up to 2.29 × 105%), which mainly originates from the half-metallicity of CrSe2 induced by charge transfer at MoSe2/CrSe2 interface. Relatively high TMR values (over 1 × 103%) are always maintained at small bias voltages (|Vb| ≤ 0.35 V). Our work have for the first time theoretically verified feasibility of realizing the transition of 2D CrSe2 from metallicity to half-metallicity in the vdW MTJ and yielding excellent spin-filtering and giant magnetoresistance via electrode interface engineering, thereby providing important theoretical guidance for the experimental design of high-performance vdW MTJs in spintronic devices.

Large tunneling magnetoresistance and low resistance-area product in CrSe2/NiCl2/CrSe2 van der Waals magnetic tunnel junction

The emergence of two-dimensional van der Waals (vdW) intrinsic magnets offers exciting opportunities to explore high-performance vdW magnetic tunnel junctions (vdW MTJs) and miniaturization of devices beyond Moore's law for future energy efficient nano-electronic devices. In this work, using first principles calculations, we investigate the spin-dependent transport of vdW MTJs formed by two vdW ferromagnetic (FM) CrSe2 electrodes and an interlayer antiferromagnetic bilayer NiCl2 barrier (CrSe2/NiCl2/CrSe2 vdW MTJ). We find that in contrast to the large resistance-area (RA) products higher than several kilohms square micrometer in crystalline MgO based MTJs with high tunneling magnetoresistance (TMR) ratios, the large TMR ratio of about 5200% and the low RA products ranging from 0.11 to 6 Ω μm2 are simultaneously achieved in the CrSe2/NiCl2/CrSe2 vdW MTJ at zero bias due to the spin-dependent tunnel transport associated with the FM CrSe2 electrode and the significant spin filtering effect associated with the half-metallic bilayer NiCl2 barrier. Moreover, the TMR ratio of the CrSe2/NiCl2/CrSe2 vdW MTJ increases first with increasing bias voltage and then decreases with the further increase in the bias voltage after reaching the highest value, and the optimized bias voltage can significantly promote the TMR ratio up to 12 000%. Our results pave the way to develop high-performance vdW MTJs with both large TMR ratios and low RA products for future spintronic applications.

Demonstration of reconfigurable magnetic tunnel diode and giant tunnel magnetoresistance in magnetic tunnel junctions made with spin gapless semiconductor and half-metallic Heusler alloy

Here, we report fabrication of a high quality exchanged biased trilayer magnetic tunnel junction (MTJ) utilizing a magnetron sputtering system. The MTJ is composed of a type-II spin gapless semiconductor (SGS) Ti2CoSi inverse Heusler alloy (used as a lower electrode), a thin MgO layer (used as a tunnel barrier), and a half-metallic ferromagnet (HMF) Co2MnSi Heusler alloy (used as an upper electrode). Spin dependent transport properties reveal that the micro-fabricated MTJ can act as a reconfigurable magnetic tunnel diode, which lets the electric current to flow in either the forward or reverse path relying on the relative alignment of the magnetization direction of the upper and lower magnetic electrodes. A considerably high on/off current ratio (∼103) and a significantly low turn on voltage (VT) of 0.09 V have been achieved at 5 K for both parallel and antiparallel configurations. Another important characteristic shown by our fabricated MTJ is that it exhibits extremely large tunnel magnetoresistance ratios of 892% at 5 K and 197% at room temperature, which brings to light the utmost importance of using the combination of HMF and SGS materials as magnetic electrodes in a tunnel junction for potential applications in modern spintronic devices. All these exceptional features can undoubtedly nominate CMS/MgO/TCS MTJs as a promising candidate to serve as memory or logic elements in next generation ultra-high density magnetoresistive random access memories together with contemporary spin based electronic devices.

Enhanced tunnel magnetoresistance in Fe/Mg4Al-Ox/Fe(001) magnetic tunnel junctions

Spinel MgAl2O4 and family oxides are emerging barrier materials useful for magnetic tunnel junctions (MTJs). We report large tunnel magnetoresistance (TMR) ratios up to 429% at room temperature (RT) and 1034% at 10 K in a Fe/Mg-rich spinel/Fe(001) MTJ prepared using electron-beam evaporation of Mg4Al-Ox. Resistance oscillations with a MTJ barrier thickness of 0.3 nm were significantly enhanced compared to those of a Fe/MgO/Fe(001) MTJ, resulting in a large TMR oscillation peak-to-valley difference of 125% at RT. The differential conductance (dI/dV) spectra were symmetric with bias polarity, and the spectrum in the parallel magnetization state at low temperature demonstrates significant peaks within broad local minima at |V| = 0.2–0.6 V, indicating improved barrier interfaces by the Mg4Al-Ox barrier. This study demonstrates that TMR ratios in Fe(001)-MTJs can still be improved.

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.

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.

Controlling oxygen distribution of an MgAl2O4 barrier for magnetic tunnel junctions by two-step process

An MgAl2O4 barrier with an ordered spinel structure for magnetic tunnel junctions (MTJs) was prepared via a two-step process by repeating Mg–Al alloy deposition and post-oxidation to tune its oxidation state. The obtained Fe/MgAl2O4/Fe(001) epitaxial MTJs showed a large tunnel magnetoresistance (TMR) ratio (>150%) in a wide resistance × area (RA) range; this behavior was in contrast with that of MTJs prepared through a conventional one-step process, which exhibited a large TMR ratio only in a narrow RA range. The bias voltage at which the TMR is halved from the zero-bias value increased up to 1.20 and 1.47 V for the positive and negative bias polarities, respectively, when optimizing the two-step process. The nanostructure analysis revealed an improved oxygen distribution on the atomic scale in the MgAl2O4 barrier with the two-step process, providing a coherent barrier suitable for various practical applications.

Large tunnel magnetoresistance in a fully epitaxial double-barrier magnetic tunnel junction of Fe/MgO/Fe/γ-Al2O3/Nb-doped SrTiO3

We report large tunnel magnetoresistance (TMR) ratios of up to 219% at 300 K and 366% at 3.7 K obtained for a high-quality fully epitaxial double-barrier magnetic tunnel junction (MTJ) composed of Fe/MgO/Fe/γ-Al2O3/Nb-doped SrTiO3. The obtained TMR ratios are among the highest values reported in Fe/MgO/Fe structures. This result may be attributed to the small in-plane wave vectors of the tunneling electrons injected from the Nb-doped SrTiO3 electrode with a small carrier density, demonstrating good compatibility between the Fe-based MTJ and SrTiO3.

Machine learning analysis of tunnel magnetoresistance of magnetic tunnel junctions with disordered MgAl2O4

Through Bayesian optimization and the least absolute shrinkage and selection operator (LASSO) technique combined with first-principles calculations, we investigated the tunnel magnetoresistance (TMR) effect of Fe/disordered-MgAl2O4(MAO)/Fe(001) magnetic tunnel junctions (MTJs) to determine structures of disordered-MAO that give large TMR ratios. The optimal structure with the largest TMR ratio was obtained by Bayesian optimization with 1728 structural candidates, where the convergence was reached within 300 structure calculations. Characterization of the obtained structures suggested that the in-plane distance between two Al atoms plays an important role in determining the TMR ratio. Since the Al-Al distance of disordered MAO significantly affects the imaginary part of complex band structures, the majority-spin conductance of the {\Delta}1 state in Fe/disordered-MAO/Fe MTJs increases with increasing in-plane Al-Al distance, leading to larger TMR ratios. Furthermore, we found that the TMR ratio tended to be large when the ratio of the number of Al, Mg, and vacancies in the [001] plane was 2:1:1, indicating that the control of Al atomic positions is essential to enhancing the TMR ratio in MTJs with disordered MAO. The present work reveals the effectiveness and advantage of material informatics combined with first-principles transport calculations in designing high-performance spintronic devices based on MTJs.

Density functional theory study on the effect of Cu- and Na-substituted layers on spin-dependent transport and TMR in the Fe/ZnO/Fe MTJ

Using density functional theory, effects of Na- and Cu-substituted layers on the spin-dependent electronic transport properties of Fe/ZnO/Fe magnetic tunnel junction based on zinc oxide barrier tunnel, with rock-salt crystalline structure, have been studied. In zero-bias voltage, conductance and tunneling magneto-resistance (TMR) ratio of structures are calculated. It is showed that substituted layers in the pristine junction greatly affect conductance and TMR ratio of this junction. The results indicated that Cu-substituted layer with reducing conductance of pristine structure in the antiparallel alignment configuration, and increasing its conductance in the parallel alignment, leads to a large TMR ratio, up to 1800%. Due to the large conductance of pristine and Cu-substituted devices in the parallel alignment, these structures would be very beneficial for experimental applications that require the spin-polarized current.

Half-metallic fully compensated ferrimagnetism and multifunctional spin transport properties of Mn3Al

The complete (100%) spin polarization, zero net magnetic moment and high Curie temperature (605 K) make the recently fabricated half-metallic fully compensated ferrimagnet Mn3Al a promising spintronic material. In order to explore the potential applications in spintronic devices, in this work, we give a theoretical analysis for the Mn3Al/GaAs(0 0 1) heterostructure and the Mn3Al/GaAs/Mn3Al(0 0 1) magnetic tunnel junction. Using the first-principles calculations combined with nonequilibrium Green’s function method, we demonstrate from the calculated bias-dependent spin transport properties that the heterostructure exhibits perfect spin filtering effect and spin diode effect, and the magnetic tunnel junction behaves a large tunnel magnetoresistance ratio (up to 10 900%). The physical origins of these versatile spin transport properties are discussed in terms of the half-metallic band structure, the spin-dependent transmission spectra and the band-to-band transmission theory.

Effects of Gamma Irradiation on Magnetic Properties of Double-Interface CoFeB/MgO Multifilms

Perpendicular-anisotropy magnetic tunnel junction (p-MTJ), a type of magnetic storage element, has shown outstanding performances in terms of high density, good scalability, and low power. In particular, the double-interface CoFeB/MgO p-MTJ with W spacer and bridging layers was recently demonstrated. This device shows a large tunnel magnetoresistance ratio, low junction resistance, and high robustness against high-temperature annealing. To consider this type of p-MTJ for the space environment, the gamma irradiation effects need to be evaluated. In this paper, the double-interface CoFeB/MgO multifilms are exposed to the gamma irradiation at different doses. The gamma irradiation effects on double-interface CoFeB/MgO multifilms are experimentally evaluated based on the comparison between the results of prerad and postrad magnetic properties. In the case of a low irradiation dose, the coercivity gradually increases with the increasing dose, whereas no variation is observed for the saturation magnetization. However, the magnetism is destroyed if the irradiation dose is sufficiently high. Some possible mechanisms are discussed to explain the experimental results.

Unusual angular dependence of tunneling magneto-Seebeck effect

We find an unusual angular dependence of the tunneling magneto-Seebeck effect (TMS). The conductance shows normally a cosine-dependence with the angle between the magnetizations of the two ferromagnetic leads. In contrast, the angular dependence of the TMS depends strongly on the tunneling magneto resistance (TMR) ratio. For small TMR ratios we obtain also a cosine-dependence whereas for very large TMR ratios the angular dependence approaches a step-like function. The origin is that the cosine-dependent transmission function enters in the denominator of the definition of the Seebeck coefficient. Although the TMR and TMS are disconnected with respect to their magnitude the size of the TMR effect can be deducted from the angular dependence of the TMS effect.

Effects of annealing temperature on sensing properties of magnetic-tunnel-junction-based sensors with perpendicular synthetic antiferromagnetic Co/Pt pinned layer

We studied the effects of annealing temperature on the sensing properties of linear-output magnetic sensor devices fabricated using a CoFeB/MgO/CoFeB magnetic tunnel junction (MTJ) with a perpendicular synthetic antiferromagnetic (p-SAF) Co/Pt pinned layer. Owing to the large exchange coupling of the p-SAF Co/Pt pinned layer, linear tunnel magnetoresistance (TMR) curves were observed within a wide range of over ±3 kOe. In terms of sensing properties, the MTJ after annealing at 300 °C showed the highest sensitivity of 0.033%/Oe owing to the large TMR ratio of 164%. Moreover, the achieved nonlinearity of 0.061% full scale (FS) is quite low, which is comparable to that of commercialized Hall sensors.

Impact of boron diffusion at MgO grain boundaries on magneto-transport properties of MgO/CoFeB/W magnetic tunnel junctions

Controlling the B diffusion in CoFeB-MgO based magnetic tunnel junctions (MTJs) is important to realize the large tunneling magnetoresistance (TMR) ratio, which is required to realize high density magnetic random access memories (MRAMs). Here we investigate the detailed microstructure of the MgO-CoFeB MTJs having W underlayer and overlayer in comparison with well-studied MTJs with Ta underlayer and overlayer. Detailed microstructure analysis based on transmission electron microscopy and electron energy loss spectroscopy revealed that B diffused into Ta under-/overlayers and crystallization of CoFeB to CoFe efficiently occurred upon annealing at 300 °C and 400 °C in MTJs with Ta underlayers. In contrast, B diffused into the [001] tilt grain boundaries of MgO barrier in the case of MTJs with W under-/overlayers. These structural differences well explain the difference in post annealing temperature dependence of TMR ratio and resistance area product (RA) of MTJs with Ta and W under-/overlayers.

Large tunnel magnetoresistance and temperature-driven spin filtering effect based on the compensated ferrimagnetic spin gapless semiconductor Ti2MnAl

Spin gapless semiconductors, also known a special class of half-metallic magnets, have received more and more attention in spintronic applications due to the complete spin polarization and the electrical conductivity between half-metals and semiconductors. Motivated by the recent synthesis of spin gapless semiconductor Ti2MnAl with compensated ferrimagnetism (zero net magnetic moment) and high Curie temperature (above 650 K), we investigate the bias-voltage- and temperature-driven spin transport properties based on Ti2MnAl by using the density functional theory combined with nonequilibrium Green’s function method. A large tunnel magnetoresistance ratio (up to 2.8 × 106%), spin Seebeck effect, and temperature-driven spin filtering effect are found in the Ti2MnAl/InAs/Ti2MnAl(001) magnetic tunnel junction. These versatile transport properties suggest the potential applications for Ti2MnAl in spintronic devices and spin caloritronic devices.

Quantum size effect in an Fe quantum well detected by resonant tunneling carriers injected from a p-type Ge semiconductor electrode

We report the clear observation of the quantum size effect in an Fe quantum well (QW) detected by resonant tunneling carriers injected from a p-type Ge semiconductor electrode in fully epitaxial double-barrier magnetic tunnel junctions, which are composed of Co/Fe/MgO/Fe QW/MgO/Ge:B grown on a p+-Ge(001) substrate. A large tunnel magnetoresistance (TMR) ratio up to 137% (237%), which is comparable to that in Fe/MgO/Fe, is obtained at 297 K (3.5 K). The quantum oscillations are clearly observed in the dI/dV–V and d2I/dV2–V curves of our devices, and the resonance voltages are in good agreement with the resonant levels calculated by the phase accumulation model. Following these oscillations, the TMR is modulated by the quantum size effect. Our results are promising for realizing future quantum spintronics devices based on semiconductor/metal hybrid heterostructures with advanced functionalities.