Tunnel Magnetoresistance Ratio Research Articles

First Demonstration of 25-nm Quad Interface p-MTJ Device With Low Resistance-Area Product MgO and Ten Years Retention for High Reliable STT-MRAM

We successfully developed 25-nm quad CoFeB/MgO-interfaces perpendicular magnetic tunnel junction (quad-MTJ) with enough thermal stability. To fabricate the quad-MTJ, a physical vapor deposition (PVD) process for depositing novel free layer and low resistance-area (RA) product MgO layer and low-damage fabrication processes were developed. The developed quad-MTJ technology and advanced process bring better tunnel magneto resistance (TMR) ratio and RA to quad-MTJ than those of double-interface MTJ (double-MTJ), even though quad-MTJ has an additional MgO layer. Scaling down the MTJ size to 25 nm, we demonstrated the advantages of quad-MTJ compared with double-MTJ as follows: 1) two times larger thermal stability factor (Δ), which results in over ten years retention; 2) superiority of large Δ in the measuring temperature range up to 200 °C; 3) ~1.5 times higher write efficiency; 4) lower write current at short write pulse regions at less than 100 ns; and e) excellent endurance of over 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> thanks to higher write efficiency, which results from the reduced voltage owing to low RA and the low damage integration process technology. As a result, the developed quad-MTJ technologies will open the way for the realization of high-density STT-MRAM with low power, high speed, high reliability, and excellent scalability down to 2× nm node.

Body-biasing-based Latch Offset Cancellation Sensing Circuit for Deep Submicrometer STT-MRAM

Even though spin-transfer-torque magnetoresistive random access memory (STT-MRAM) is considered to be a leading candidate for next generation memory, designing a sensing circuit (SC) that achieves sufficient read yield is challenging because of the increased process variation, decreased read current (I<SUB>read</SUB>), and small tunnel magnetoresistance (TMR) ratio. In this paper, a novel body-biasing-based latch offset cancellation SC (BBLOC-SC) that is capable of canceling the offset voltage caused by the latch sense amplifier is proposed. Monte Carlo HSPICE simulation results using industry-compatible 28-nm model parameters show that the proposed BBLOC-SC achieves a much higher read yield compared to the state-of-the-art SCs, regardless of TMR and I<SUB>read</SUB>.

Giant tunneling magnetoresistance in van der Waals magnetic tunnel junctions formed by interlayer antiferromagnetic bilayer CoBr2

The discovery of two-dimensional (2D) van der Waals (vdW) intrinsic magnets has opened a promising avenue to design high-performance magnetic tunnel junctions (MTJs) based on 2D materials. In this work, using first-principles calculations, it is demonstrated that bilayer $\mathrm{Co}{\mathrm{Br}}_{2}$ is intrinsically a magnetic semiconductor with intralayer ferromagnetic (FM) and interlayer antiferromagnetic (AFM) couplings and the interlayer AFM coupling in bilayer $\mathrm{Co}{\mathrm{Br}}_{2}$ is independent on the stacking orders. Moreover, using the nonequilibrium Green's function combined with density functional theory, it is found that due to the large difference between interlayer AFM and FM states of the $\mathrm{Co}{\mathrm{Br}}_{2}$ barrier, the conductance of spin filter (SF) vdW MTJs based on the graphene/bilayer $\mathrm{Co}{\mathrm{Br}}_{2}$/graphene heterostructure for the interlayer FM state of the $\mathrm{Co}{\mathrm{Br}}_{2}$ barrier is about 25 times that for the interlayer AFM state of the $\mathrm{Co}{\mathrm{Br}}_{2}$ barrier. Consequently, a high tunneling magnetoresistance (TMR) ratio of $2420%$ is achieved in this SF-vdW MTJ at zero bias. In particular, because the current for the interlayer FM state of the $\mathrm{Co}{\mathrm{Br}}_{2}$ barrier rapidly increases with the increase of bias voltage, a giant TMR ratio of up to about $38\phantom{\rule{0.16em}{0ex}}000%$ can be achieved in this SF-vdW MTJ at 0.2-V bias. Our results suggest that SF-vdW MTJs formed by the interlayer AFM barrier with variable conductivity hold great potential for developing vdW MTJs with a high TMR ratio.

Enhanced tunnel magnetoresistance in Mn-based perpendicular magnetic tunnel junctions utilizing antiferromagnetically coupled bcc-Co-based interlayer

High tunnel magnetoresistance (TMR) in perpendicular magnetic tunnel junctions (p-MTJs) with tetragonal Mn-based electrodes is expected to play a key role in the realization of practical high-density magnetic memories, advanced THz devices, and magnetic sensors. In this study, we demonstrated the use of bcc-Co-based alloys, such as bcc-Co and bcc-CoMn binary alloys, as antiferromagnetically coupled interlayers for MnGa-based p-MTJs. The interlayer of bcc-Co enhanced the TMR ratio of MnGa-based MTJs by 70% at 300 K and 145% at 10 K. Furthermore, the TMR ratio of the MTJ with the bcc-CoMn interlayer was enhanced up to 85 (209)% at 300 (10) K at a thickness of 0.8 nm. The enhancement in the TMRs can be attributed to the electronic band with the Δ1 symmetry of the bcc-Co-based alloy. In addition, the Co-based interlayer supported the growth of a high-quality MgO barrier sufficient for sustaining the coherency of the tunnel electrons from the Bloch state in the ferromagnetic electrode. These results suggest that bcc-Co-based interlayers are promising interlayer materials for MnGa-based p-MTJs.

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.

Interfacial giant tunnel magnetoresistance and bulk-induced large perpendicular magnetic anisotropy in (111)-oriented junctions with fcc ferromagnetic alloys: A first-principles study

We study the tunnel magnetoresistance (TMR) effect and magnetocrystalline anisotropy in a series of magnetic tunnel junctions (MTJs) with $L1_1$-ordered fcc ferromagnetic alloys and MgO barrier along the [111] direction. Considering the (111)-oriented MTJs with different $L1_1$ alloys, we calculate their TMR ratios and magnetocrystalline anisotropies on the basis of the first-principles calculations. The analysis shows that the MTJs with Co-based alloys (CoNi, CoPt, and CoPd) have high TMR ratios over 2000$\%$. These MTJs have energetically favored Co-O interfaces where interfacial antibonding between Co $d$ and O $p$ states is formed around the Fermi level. We find that the resonant tunneling of the antibonding states, called the interface resonant tunneling, is the origin of the obtained high TMR ratios. Our calculation of the magnetocrystalline anisotropy shows that many $L1_1$ alloys have large perpendicular magnetic anisotropy (PMA). In particular, CoPt has the largest value of anisotropy energy $K_{\rm u} \approx 10\,{\rm MJ/m^3}$. We further conduct a perturbation analysis of the PMA with respect to the spin-orbit interaction and reveal that the large PMA in CoPt and CoNi mainly originates from spin-conserving perturbation processes around the Fermi level.

Effect of surface modification treatment on top-pinned MTJ with perpendicular easy axis

We investigated the effects of surface modification treatment (SMT) on the perpendicular magnetic anisotropy (PMA) and the thermal tolerance of top-pinned magnetic tunnel junctions (MTJs) with a Co/Pt synthetic ferrimagnetic coupling reference layer. Applying an SMT to the bottom Pt layer increased the PMA of the overlying Co/Pt multilayer. X-ray diffraction spectrum analysis revealed that the SMT resulted in a higher crystallinity and smaller lattice spacing in the Co/Pt multilayer in the thinner bottom Pt layer, which may have increased the PMA in the Co/Pt multilayer. The tunnel magnetoresistance (TMR) ratio of the MTJ with SMT increased as the annealing temperature was increased up to 400 °C; conversely, the TMR ratio of the MTJ without SMT decreased at an annealing temperature of 400 °C. Evaluation of the m-H loops revealed that, after annealing at 400 °C, the reference layers in the MTJs after SMT possessed better magnetic properties than those in the MTJs without an SMT; this is attributable to the higher PMA of the reference layers with SMT. EDX line analysis revealed that SMT suppressed Pt diffusion to the MgO barrier, resulting in a higher thermal tolerance and larger PMA of the reference layer.

Researchers double room temperature tunnel magnetoresistance ratios of Fe/MgO/Fe

By carefully tuning the growing condition for each individual layer, researchers close the gap between the theoretically predicted value for the tunnel magnetoresistance ratio and the experimental reality.

Exceeding 400% tunnel magnetoresistance at room temperature in epitaxial Fe/MgO/Fe(001) spin-valve-type magnetic tunnel junctions

Giant tunnel magnetoresistance (TMR) ratios of 417% at room temperature (RT) and 914% at 3 K were demonstrated in epitaxial Fe/MgO/Fe(001) exchange-biased spin-valve magnetic tunnel junctions (MTJs) by tuning growth conditions for each layer, combining sputter deposition for the Fe layers, electron-beam evaporation of the MgO barrier, and barrier interface tuning. Clear TMR oscillation as a function of the MgO thickness with a large peak-to-valley difference of ∼80% was observed when the layers were grown on a highly (001)-oriented Cr buffer layer. Specific features of the observed MTJs are symmetric differential conductance (dI/dV) spectra for the bias polarity and plateau-like deep local minima in dI/dV (parallel configuration) at |V| = 0.2–0.5 V. At 3 K, fine structures with two dips emerge in the plateau-like dI/dV, reflecting highly coherent tunneling through the Fe/MgO/Fe. We also observed a 496% TMR ratio at RT by a 2.24 nm-thick-CoFe insertion at the bottom-Fe/MgO interface.

Influence of Total Ionizing Dose on Magnetic Tunnel Junctions With Perpendicular Anisotropy

Spin-transfer torque magnetic random access memory (STT-MRAM) is a promising solution for onboard, radiation-tolerant memory for space environments. This is because magnetic tunnel junctions (MTJs) have demonstrated outstanding resilience to ionizing radiation. In this study, we examine the effect of total ionizing dose (TID) on the MTJ's thermal stability factor, intrinsic critical switching voltage, and write energy. These parameters were calculated from switching probability distribution curves taken at three different pulsewidths. Additionally, dc current hysteresis plots were obtained to measure the MTJ's tunnel magnetoresistance ratio, its dependence on voltage, and the dc current required to switch states. Tests were carried out at two TID exposure levels up to 1 Mrad(Si) on a total of 24 MTJs of varying cross section dimensions in order to identify the influence of scaling as well as to obtain statistical variation for any changes observed. Results indicate that TID exposure had small effects on the MTJ's thermal stability and critical switching voltage, which affected the write energy, especially for switching from the antiparallel (AP) to parallel (P) direction. For switching from P to AP, these changes fluctuated, suggesting that TID has multiple effects, which have competing influences on the MTJ's switching properties. The work presented here shows that, while MTJs are highly resistant to ionizing radiation, a subtle influence on some properties for the STT-MRAM write operation should be considered in radiation environment assessments for both terrestrial and space applications.

Perpendicular magnetic anisotropy and its electrical control in FeNiB ultrathin films

We study the perpendicular magnetic anisotropy in (Fe100−xNix)80B20 (FeNiB) films with various Ni contents. Perpendicularly magnetized films are achieved when the Ni content is in the range of 30 at. %–70 at. %. An effective perpendicular magnetic anisotropy (PMA) constant of 1.1× 105 J/m3 is achieved for the (Fe50Ni50)80B20 film. We also fabricate magnetic tunnel junction devices containing FeNiB films, and electrical measurements show that a tunneling magnetoresistance ratio of more than 20% can be achieved for devices having an orthogonal magnetization configuration. The PMA of the FeNiB film clearly changes by varying the bias voltage applied along the FeNiB/MgO junction, and a voltage-controlled magnetic anisotropy (VCMA) efficiency of over 30 fJ/Vm is demonstrated. From systematic investigations, there is no clear correlation between PMA and VCMA efficiency in the FeNiB/MgO junction. These experimental results should facilitate the development of energy-efficient magnetic random-access memory.

Tunable Tunneling Magnetoresistance in van der Waals Magnetic Tunnel Junctions with 1T-CrTe2 Electrodes.

Two-dimensional (2D) van der Waals (vdW) heterostructures have opened new avenues for spintronic applications with novel properties. Here, by density functional theory calculations, we investigated the spin-dependent transport in vdW magnetic tunnel junctions (MTJs) composed of 1T-CrTe2 ferromagnetic electrodes. Meanwhile, graphene and h-BN are employed as tunnel barriers. It has been found that the tunneling magnetoresistance (TMR) effects of two types of vdW MTJs present analogous trends: thicknesses of barriers have a great influence on the TMR ratios, which reach up to the maximum when barriers increase to five monolayers. However, despite the similarity, the graphene-barrier junction is more promising for optimization. Through observing the energy-resolved transmission spectra of vdW MTJs, we noticed that TMR ratios of graphene-barrier junctions are tunable and could be enhanced through tuning the position of Fermi energy. Therefore, we successfully realized the TMR optimization by substitutional doping. When substituting one carbon atom with one boron atom in the graphene barrier, TMR ratios are drastically improved, and a TMR ratio as high as 6962% could be obtained in the doped seven-monolayer-barrier junction. Our results pave the way for vdW MTJ applications in spintronics.

Magnetic, magnetoresistive and low-frequency noise properties of tunnel magnetoresistance sensor devices with amorphous CoFeBTa soft magnetic layers

Magnetic field sensors using the tunnel magnetoresistance (TMR) effect require linear resistance–magnetic field (R–H) response curves with small hysteresis, for which the soft magnetic property of the free layer (FL) is critical. In this work, we investigated amorphous CoFeBTa (CFBT) as a soft magnetic layer of the FL of CoFeB/MgO/CoFeB-based magnetic tunnel junctions in view of magnetic, TMR, and low-frequency noise properties. A two-step annealing process enabled an orthogonal magnetization configuration between the FL and the reference layer, by which linear R–H curves with small hysteresis were realized. The change in the shape of the R–H curve depending on annealing temperature is explained by the Stoner–Wohlfarth model. The highest TMR ratio of ∼160% and sensitivity of ∼70%/mT were obtained with a CFBT (20 nm)/Ta (0.3 nm)/CoFeB (3 nm) FL. The noise of the TMR devices are dominated by 1/f noise below ∼10 kHz in frequency, which limits the detectivity (D) of the magnetic field of the sensor. The sensor devices patterned to 50 µm diameter circular shapes showed a minimum D of ∼2 nT/ at 10 Hz, which is superior to the previously reported values for the TMR sensors with NiFe soft magnetic layers.

Field- and Current-Driven Magnetization Reversal and Dynamic Properties of CoFeB-MgO-Based Perpendicular Magnetic Tunnel Junctions* *Supported by the National Key Research and Development Program of China (Grant No. 2016YFA0300803), the Open Research Fund of Jiangsu Provincial Key Laboratory for Nanotechnology, the National Natural Science Foundation of China (Grant Nos. 11774150 and 11874135), and the Natural Science Foundation of Jiangsu Province of

We report a perpendicular magnetic tunnel junction (pMTJ) cell with a tunnel magnetoresistance (TMR) ratio of nearly 200% at room temperature based on CoFeB/Ta/CoFeB as the free layer (FL) and a synthetic antiferromagnetic (SAF) multilayer [Pt/Co]/Ru/[Pt/Co]/Ta/CoFeB as the reference layer (RL). The field-driven magnetization switching measurements show that the pMTJs exhibit an anomalous TMR hysteresis loop. The spin-polarized layer CoFeB of SAF-RL has a lower critical switching field than that of FL. The reason is related to the interlayer exchange coupling (IEC) through a moderately thick Ta spacer layer among SAF-RLs, which generates a moderate and negative bias magnetic field on CoFeB of RL. However, the IEC among RLs has a negligible influence on the current-driven magnetization switching of FL and its magnetization dynamics.

Perpendicular magnetic anisotropy of a Ta/NiFeB/MgO system and its magnetic coupling with [Pt/Co] multilayers

The perpendicular magnetic anisotropy (PMA) and crystalline structure of Ta/NiFeB/MgO stacks are investigated. When the stack is annealed, PMA is induced and the magnetic moment increases due to the crystallization of bcc NiFe from amorphous NiFeB. This structure is combined with [Pt/Co]6 multilayers to study the change in the magnetic coupling and PMA behavior with the change in the Ta thickness inserted in-between and the annealing temperature. Among the [Pt/Co]6/Ta/NiFeB/MgO stacks, the stack with a 0.5-nm-thick Ta layer shows the strongest PMA with an effective PMA energy density of 4.4 × 106 erg/cm3 upon annealing at 400 °C. Further, the X-ray diffraction study of the crystalline structures confirms the formation of the NiFe (001) phase, which is essential for achieving a high tunneling magnetoresistance ratio.

Spin valve effect in sputtered FL-MoS2 and ferromagnetic shape memory alloy based magnetic tunnel junction

In the last decade, two-dimensional (2D) transition metal dichalcogenides have been introduced with great significance in the spintronic devices for their extraordinary electrical, optical, and spin-dependent properties. In this work, we have fabricated a few-layer molybdenum disulfide (FL-MoS2) (~6 nm) as a non-magnetic spacer layer in Ni–Mn–In/FL-MoS2/Ni–Mn–In magnetic tunnel junction (MTJ) using DC magnetron sputtering. FL-MoS2 thin film sandwiched between two ferromagnetic shape memory alloy based electrodes exhibit semiconducting behavior, confirmed by current-voltage (I–V) characteristics and temperature dependent resistance measurement. The fabricated MTJ shows spin valve effect in the presence of an external magnetic field. The tunneling magnetoresistance (TMR) has been recorded in 10 K–300 K temperature range. The highest TMR ratio of 0.51% was obtained at a low temperature ~10 K, corresponding to the spin polarization of ~5%. This TMR ratio reduces to a value of 0.032% as the temperature of the device increases up to 300 K, displaying a finite TMR at room temperature. A detailed study of thickness and temperature-dependent magnetization versus magnetic field (M − H) hysteresis loops of Ni–Mn–In thin films has been performed to understand the complex TMR behavior. The present study paves the way for the use of sputtered FL-MoS2 and ferromagnetic shape memory alloy in ultrafast spintronics for advanced magnetic devices application.

Scalability of Quad Interface p-MTJ for 1X nm STT-MRAM With 10-ns Low Power Write Operation, 10 Years Retention and Endurance &gt; 10¹¹

We fabricated a quadruple-interface perpendicular magnetic tunnel junction (MTJ) (Quad-MTJ) down to 33 nm using physical vapor-deposition, reactive ion etching, and damage-control integration process technologies that we developed under a 300-mm process. We demonstrated the greater scalability and higher writing speed of Quad-MTJ compared with double-interface perpendicular MTJ: 1) it has twice the thermal stability factor—1X nm Quad-MTJ can achieve 10 years retention—while maintaining a low resistance-area product and high tunnel magnetoresistance ratio; 2) smaller overdrive ratio of write voltage to obtain a sufficiently low write-error rate; 2) smaller pulsewidth dependence of the switching current; and 4) more than double the write efficiency at 10-ns write operation down to 33-nm MTJ. The effective suppression of the switching current increase for higher write speeds was explained by the spin-transfer-torque model using the Fokker-Planck equation. Our 33-nm Quad-MTJ also achieved excellent endurance (at least 1011) owing to its higher write efficiency and low-damage integration-process technology. It is thus a promising method for low power, high speed, and reliable STT-MRAM with excellent scalability down to the 1X nm node.

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 (&amp;gt;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.

Observation of plateau-like magnetoresistance in twisted Fe3GeTe2/Fe3GeTe2 junction

Controlling the stacking of van der Waals (vdW) materials is found to produce exciting new findings, since hetero- or homo-structures have added the diverse possibility of assembly and manipulated functionalities. However, so far, the homostructure with a twisted angle based on the magnetic vdW materials remains unexplored. Here, we achieved a twisted magnetic vdW Fe3GeTe2 (FGT)/Fe3GeTe2 junction with broken crystalline symmetry. A clean and metallic vdW junction is evidenced by the temperature-dependent resistance and the linear I–V curve. Unlike the pristine FGT, a plateau-like magnetoresistance (PMR) is observed in the magnetotransport of our homojunction due to the antiparallel magnetic configurations of the two FGT layers. The PMR ratio is found to be ∼0.05% and gets monotonically enhanced as temperature decreases like a metallic giant magnetoresistance. Such a tiny PMR ratio is at least three orders of magnitude smaller than the tunneling magnetoresistance ratio, justifying our clean metallic junction without a spacer. Our findings demonstrate the feasibility of the controllable homostructure and shed light on future spintronics using magnetic vdW materials.

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.