MgO-based Magnetic Tunnel Junctions Research Articles

Switching characteristics of MgO-based MTJ with intermediate state

The resistance change of magnetic tunnel junctions (MTJs) is used to store digital information in spin-transfer-torque magnetoresistive random-access memory. In some MTJs, intermediate (IM) states occur between two stable resistance states just after the application of a switching pulse. In this study, the effect of IM states on switching is investigated. IM states increase the switching voltage, and their probability is significantly high at a narrow pulse width. For no-error switching, MTJs with IM states require higher voltages and wider pulse widths compared with MTJs without IM states. A 300 nm MTJ has multiple IM states, whereas a 120 nm MTJ has only one IM state. Overall, a large MTJ’s free layer has multiple regions with different switching characteristics, and switching occurs stepwise through IM states.

Large voltage-controlled magnetic anisotropy effect in magnetic tunnel junctions prepared by deposition at cryogenic temperatures

We investigated the influence of the buffer material and a cryogenic temperature deposition process on the voltage-controlled magnetic anisotropy (VCMA) effect for an ultrathin CoFeB layer in bottom-free type MgO-based magnetic tunnel junctions prepared by a mass production sputtering process. We used Ta and TaB buffers and compared the differences between them. The TaB buffer enabled us to form a flat and less-contaminated CoFeB/MgO interface by suppressing the diffusion of Ta with maintaining a stable amorphous phase. Furthermore, the introduction of cryogenic temperature deposition for the ultrathin CoFeB layer on the TaB buffer improved the efficiency of the VCMA effect and its annealing tolerance. Combining this with interface engineering employing an Ir layer for doping and a CoFe termination layer, a large VCMA coefficient of −138 ± 3 fJ/Vm was achieved. The developed techniques for the growth of ultrathin ferromagnet and oxide thin films using cryogenic temperature deposition will contribute to the development of high-performance spintronic devices, such as voltage-controlled magnetoresistive random access memories.

Electromagnetic Radiation Effects on MgO-Based Magnetic Tunnel Junctions: A Review.

Magnetic tunnel junctions (MTJs) have been widely utilized in sensitive sensors, magnetic memory, and logic gates due to their tunneling magnetoresistance. Moreover, these MTJ devices have promising potential for renewable energy generation and storage. Compared with Si-based devices, MTJs are more tolerant to electromagnetic radiation. In this review, we summarize the functionalities of MgO-based MTJ devices under different electromagnetic irradiation environments, with a focus on gamma-ray radiation. We explore the effects of these radiation exposures on the MgO tunnel barriers, magnetic layers, and interfaces to understand the origin of their tolerance. This review enhances our knowledge of the radiation tolerance of MgO-based MTJs, improves the design of these MgO-based MTJ devices with better tolerances, and provides information to minimize the risks of irradiation under various irradiation environments. This review starts with an introduction to MTJs and irradiation backgrounds, followed by the fundamental properties of MTJ materials, such as the MgO barrier and magnetic layers. Then, we review and discuss the MTJ materials and devices' radiation tolerances under different irradiation environments, including high-energy cosmic radiation, gamma-ray radiation, and lower-energy electromagnetic radiation (X-ray, UV-vis, infrared, microwave, and radiofrequency electromagnetic radiation). In conclusion, we summarize the radiation effects based on the published literature, which might benefit material design and protection.

Second harmonic injection locking of coupled spin torque vortex oscillators with an individual phase access

The synchronisation of magnetic tunnel junctions in the high frequency domain has attracted significant interest in the context of novel computation paradigms, specifically neuromorphic spintronics and probabilistic computing. In this work, a design for the coupling and synchronization of spin torque vortex oscillators (STVOs) is implemented. The geometry comprises the fabrication of adjacent pairs of STVO nanopillars (MgO-based magnetic tunnel junctions), with an edge-to-edge distance down to 100 nm, together with individual top contacts that allow an independent electrical access to each device. In this geometry, the magneto-dipolar coupling promotes the synchronization of the two oscillators, at the same time as the access to the frequency and phase of each individual oscillator is possible. Both frequency and time domain measurements confirm a successful synchronization, with the coupling being controlled by the relative DC bias in each oscillator. As a proof-of-concept towards an oscillator-based Ising machine, it is also shown that the second harmonic injection locking of an STVO can be controlled by tuning the magneto-dipolar coupling to its correspondent STVO pair. These results represent a step forward for the implementation of magneto-dipolar coupled magnetic tunnel junctions, specifically in the field of unconventional computing hardware.

Tremendous tunneling magnetoresistance effects based on van der Waals room-temperature ferromagnet Fe3GaTe2 with highly spin-polarized Fermi surfaces

Recently, van der Waals (vdW) magnetic heterostructures have received increasing research attention in spintronics. However, the lack of room-temperature magnetic order of vdW materials has largely impeded its development in practical spintronic devices. Inspired by the lately discovered vdW ferromagnet Fe3GaTe2, which has been shown to have magnetic order above room temperature and sizable perpendicular magnetic anisotropy, we investigate the basic electronic structure and magnetic properties of Fe3GaTe2 as well as tunneling magnetoresistance effect in magnetic tunnel junctions (MTJs) with structure of Fe3GaTe2/insulator/Fe3GaTe2 by using first-principles calculations. It is found that Fe3GaTe2 with highly spin-polarized Fermi surface ensures that such magnetic tunnel junctions may have prominent tunneling magnetoresistance effect at room temperature even comparable to existing conventional AlOx and MgO-based MTJs. Our results suggest that Fe3GaTe2-based MTJs may be the promising candidate for realizing long-waiting full magnetic vdW spintronic devices.

Low magnetic damping in an ultrathin CoFeB layer deposited on a 300 mm diameter wafer at cryogenic temperature

We deposited an ultrathin CoFeB(1.1 nm) layer, which functions as a storage layer of MgO-based magnetic tunnel junctions for spin-transfer-torque (STT) magnetoresistive random-access memory (MRAM), on ϕ300 mm wafers at 100 K and investigated its effect on the magnetization dynamics of CoFeB. We observed clear reductions in both the inhomogeneous linewidth and total magnetic damping parameter for the CoFeB layer deposited at 100 K compared to those deposited at 300 K through the improvement in the interfacial quality. The results show that deposition at cryogenic temperatures is an effective manufacturing process for high-quality magnetic thin films with low magnetic damping.

CoFeBX layers for MgO-based magnetic tunnel junction sensors with improved magnetoresistance and noise performance

Magnetoresistive sensors have been enthusiastically selected for applications requiring magnetic field detection with small footprint sensors. The optimisation of the sensor response includes using soft magnetic free layers, based on CoFeB and NiFe alloys. Here we report the TMR and noise performance of magnetically saturated in-plane MTJ sensors including CoFeBTa and CoFeSiB soft magnetic films as free layers (FL). Assessing magneto-crystalline anisotropy μ0Hk values of 2.1 and 0.7 mT in CoFeB 2.5 (nm)/Ru 0.2/CoFeBTa 4 and CoFeB 3/Ru 0.2/CoFeSiB 4 compared to 1.7 mT in CoFeB 2.5/Ru 0.2/NiFe 4, together with an improved magnetoresistance of 230% in CoFeBSi comparing with 170% (NiFe) with superior noise characteristics, with Hooge parameter of αH = 7 × 10−11 μm2.

Tunneling anisotropic magnetoresistance in MgO-based magnetic tunnel junctions induced by spin-orbit coupling

We performed a first-principles study of the tunneling anisotropic magneto-resistance (TAMR) in Ag(Ir,Pt)$/$MgO$/$Fe junctions. Enhanced TAMR with ideal and skewed fourfold angular dependence is found in-plane and out-of-plane TAMR of the system, respectively, which shows simple barrier thickness dependency with number around 10\% in some junctions. The complex angular dependency of the interfacial resonant states due to the spin-orbit coupling should be responsible to the complex and enhanced TAMR found in these junctions.

Strain-mediated magnetoelectric control of tunneling magnetoresistance in magnetic tunneling junction/ferroelectric hybrid structures

Because of the wide selectivity of ferromagnetic and ferroelectric (FE) components, electric-field (E-field) control of magnetism via strain mediation can be easily realized through composite multiferroic heterostructures. Here, an MgO-based magnetic tunnel junction (MTJ) is chosen rationally as the ferromagnetic constitution and a high-activity (001)-Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-0.3PT) single crystal is selected as the FE component to create a multiferroic MTJ/FE hybrid structure. The shape of tunneling magnetoresistance (TMR) versus in situ E-fields imprints the butterfly loop of the piezo-strain of the FE without magnetic-field bias. The E-field-controlled change in the TMR ratio is up to –0.27% without magnetic-field bias. Moreover, when a typical magnetic field (∼ ±10 Oe) is applied along the minor axis of the MTJ, the butterfly loop is changed significantly by the E-fields relative to that without magnetic-field bias. This suggests that the E-field-controlled junction resistance is spin-dependent and correlated with magnetization switching in the free layer of the MTJ. In addition, based on such a multiferroic heterostructure, a strain-gauge factor up to approximately 40 is achieved, which decreases further with a sign change from positive to negative with increasing magnetic fields. This multiferroic hybrid structure is a promising avenue to control TMR through E-fields in low-power-consumption spintronic and straintronic devices at room temperature.

IrCrMnZ (Z = Al, Ga, Si, Ge) Heusler alloys as electrode materials for MgO-based magnetic tunneling junctions: a first-principles study

We study IrCrMnZ (Z = Al, Ga, Si, Ge) systems using first-principles calculations from the perspective of their application as electrode materials of MgO-based magnetic tunnel junctions (MTJs). These materials have highly spin-polarized conduction electrons with a partially occupied Δ1 band, which is important for coherent tunneling in a parallel magnetization configuration. The Curie temperatures of IrCrMnAl and IrCrMnGa are very high (above 1300 K), as predicted from mean-field-approximation. The stability of the ordered phase against various antisite disorders is investigated. We discuss here the effect of ‘spin-orbit-coupling’ on the electronic structure around the Fermi level. Further, we investigate the electronic structure of the IrCrMnZ/MgO heterojunction along the (001) direction. IrCrMnAl/MgO and IrCrMnGa/MgO maintain half-metallicity even at the MgO interface, with no interfacial states at/around the Fermi level in the minority-spin channel. Large majority-spin conductance of IrCrMnAl/MgO/IrCrMnAl and IrCrMnGa/MgO/IrCrMnGa is reported from the calculation of the ballistic spin-transport property for the parallel magnetization configuration. We propose IrCrMnAl/MgO/IrCrMnAl and IrCrMnGa/MgO/IrCrMnGa as promising MTJs with a weaker temperature dependence of tunneling magnetoresistance ratio, owing to their very high Curie temperatures.

The negative interface capacitance and its anisotropy in magnetic tunnel junctions

The capacitance of magnetic tunnel junctions (MTJs) with the tunneling magnetocapacitance (TMC) effect has drawn lots of attention but there is still a lack of research on the capacitance anisotropy. In this work, a study of the capacitance in MgO-based MTJ is performed. The negative capacitance is observed and indicates the importance of interface capacitance in the MTJ. In further research, the interface capacitance depends on the relative orientation of magnetization in the two ferromagnetic layers (FMs), exhibiting a strong anisotropy. The Debye-Fröhlich model combined with the magnetization dependence of relaxation time is introduced to explain the behavior. Theory and experiment results of the relaxation time as a function of the relative angle of the two FM magnetizations are well fitted. It reveals that the TMC originates from the interface charge accumulation and spin-dependent tunneling. All the results suggest that the interface capacitance with apparent anisotropy plays an essential role in the TMC.

Observation and theoretical calculations of voltage-induced large magnetocapacitance beyond 330% in MgO-based magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) in the field of spintronics have received enormous attention owing to their fascinating spin phenomena for fundamental physics and potential applications. MTJs exhibit a large tunnel magnetoresistance (TMR) at room temperature. However, TMR depends strongly on the bias voltage, which reduces the magnitude of TMR. On the other hand, tunnel magnetocapacitance (TMC), which has also been observed in MTJs, can be increased when subjecting to a biasing voltage, thus exhibiting one of the most interesting spin phenomena. Here we report a large voltage-induced TMC beyond 330% in MgO-based MTJs, which is the largest value ever reported for MTJs. The voltage dependence and frequency characteristics of TMC can be explained by the newly proposed Debye-Fröhlich model using Zhang-sigmoid theory, parabolic barrier approximation, and spin-dependent drift diffusion model. Moreover, we predict that the voltage-induced TMC ratio could reach over 3000% in MTJs. It is a reality now that MTJs can be used as capacitors that are small in size, broadly ranged in frequencies and controllable by a voltage. Our theoretical and experimental findings provide a deeper understanding on the exact mechanism of voltage-induced AC spin transports in spintronic devices. Our research may open new avenues to the development of spintronics applications, such as highly sensitive magnetic sensors, high performance non-volatile memories, multi-functional spin logic devices, voltage controlled electronic components, and energy storage devices.

Sign inversion phenomenon of voltage-induced tunnel magnetocapacitance

Tunnel magnetocapacitance (TMC) in magnetic tunnel junctions (MTJs) has recently attracted interest due to unique properties, such as large magnetic response, thermal stability, and robustness to the bias voltage. In this Letter, we report the sign inversion phenomenon of TMC observed with frequency modulation and dc voltage application to MgO-based MTJs at room temperature. A negative TMC is observed in the frequency region of about kHz due to the appearance of spin capacitance. By applying a dc voltage (a few hundred mV) in this frequency region, the spin flip is promoted in the parallel configuration of MTJs. This results in the observation of the sign inversion of TMC from negative to positive. These physical pictures can be well explained by the calculation based on the modified Debye-Fröhlich model. Our research offers a deeper understanding of AC spin transports, DC spin accumulation, equilibrium and non-equilibrium spin dynamics.

Ab initio study of electronic and magnetic properties of Mn2RuZ/MgO (001) heterojunctions (Z = Al, Ge)

We studied the applicability of Heusler alloys Mn2RuZ (Z = Al, Ga, Ge, Si) to the electrode materials of MgO-based magnetic tunnel junctions. All these alloys possess Hg2CuTi-type inverse Heusler alloy structure and ferrimagnetic ground state. Our study reveals the half-metallic electronic structure with highly spin-polarized Δ1 band, which is robust against atomic disorder. Next we studied the electronic structure of Mn2RuAl/MgO and Mn2RuGe/MgO heterojunctions. We found that the MnAl- or MnGe-terminated interface is energetically more favorable compared to the MnRu-terminated interface. Interfacial states appear at the Fermi level in the minority-spin gap for the Mn2RuGe/MgO junction. We discuss the origin of these interfacial states in terms of local environment around each constituent atom. On the other hand, in the Mn2RuAl/MgO junction, high spin polarization of bulk Mn2RuAl is preserved independent of its termination.

HRTEM study of microstructure-coercivity relationship in perpendicular Co25Pd75 thin films

The microstructure of 20 nm thick polycrystalline Co25Pd75 alloy films exhibiting perpendicular magnetic anisotropy (PMA), studied on the atomic scale by analytical transmission electron microscopy (TEM) techniques, is presented. Thin CoPd alloy films are considered promising candidates for pinning MgO-based perpendicular magnetic tunnel junctions (pMTJs) for spin- transfer torque magnetic tunnel junctions (STT-MRAM). The CoPd alloy films studied were grown by DC magnetron sputtering on four different seed layers: MgO, Ta, and Ta/Ru/Ta trilayer and Ta/Pd bilayer. The effect of the seed layers on the Co25Pd75 structural quality, the size and texture of crystallites as well as their compositional homogeneity and structural coherency of the grain boundaries formed between adjacent crystallites, are investigated. The detailed structural analysis reveals that Co25Pd75 layers grown on the Ta/Pd bilayer and Ta/Ru/Ta trilayer seeds consist of larger crystalline columns, with sizes ranging from 25 nm to 15 nm respectively, compared to the other two samples. Correlation of the Co25Pd75 crystalline quality with coercivity is suggested.

Spin-current mediated exchange coupling in MgO-based magnetic tunnel junctions

Heterostructures composed of ferromagnetic layers that are mutually interacting through a nonmagnetic spacer are at the core of magnetic sensor and memory devices. In the present study, layer-resolved ferromagnetic resonance was used to investigate the coupling between the magnetic layers of a Co/MgO/Permalloy magnetic tunnel junction. Two magnetic resonance peaks were observed for both magnetic layers, as probed at the Co and Ni ${L}_{3}$ x-ray absorption edges, showing a strong interlayer interaction through the insulating MgO barrier. A theoretical model based on the Landau-Lifshitz-Gilbert-Slonczewski equation was developed, including exchange coupling and spin pumping between the magnetic layers. Fits to the experimental data were carried out, both with and without a spin pumping term, and the goodness of the fit was compared using a likelihood ratio test. This rigorous statistical approach provides an unambiguous proof of the existence of interlayer coupling mediated by spin pumping.

Experiments and SPICE simulations of double MgO-based perpendicular magnetic tunnel junction* *Project supported by State Grid Corporation of China under the 2018 Science and Technology Project of State Grid Corporation: Research on electromagnetic measurement technology based on EIT and TMR (Grant No. JL71-18-007).

We investigate properties of perpendicular anisotropy magnetic tunnel junctions (pMTJs) with a stack structure MgO/CoFeB/Ta/CoFeB/MgO as the free layer (or recording layer), and obtain the necessary device parameters from the tunneling magnetoresistance (TMR) vs. field loops and current-driven magnetization switching experiments. Based on the experimental results and device parameters, we further estimate current-driven switching performance of pMTJ including switching time and power, and their dependence on perpendicular magnetic anisotropy and damping constant of the free layer by SPICE-based circuit simulations. Our results show that the pMTJ cells exhibit a less than 1 ns switching time and write energies < 1.4 pJ; meanwhile the lower perpendicular magnetic anisotropy (PMA) and damping constant can further reduce the switching time at the studied range of damping constant α < 0.1. Additionally, our results demonstrate that the pMTJs with the thermal stability factor ≃ 73 can be easily transformed into spin-torque nano-oscillators from magnetic memory as microwave sources or detectors for telecommunication devices.

Low frequency 1/f noise in deep submicrometer-sized magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) have been identified as promising candidates for the development of high-performance, ultra-low field magnetometers due to their high sensitivity, low cost, low power consumption, and small size. However, 1/f noise is often quite large at low frequencies and inevitably becomes one of the most difficult issues in developing a magnetic field sensor with ultrahigh sensitivity. Low-frequency 1/f noise can have both electric and magnetic origins, and it is a result of complex non-linear interactions between many degrees of freedom inside a sensor. Therefore, a reduction of the 1/f noise can be expected for the magnetic sensor with very small dimensions. Here, the dependence of the 1/f noise on voltage and strong hard-axis bias field in deep submicrometer-sized MgO-based MTJs is investigated with various junction sizes. The noise spectra were measured by using a home-built low-frequency noise measurement setup with maximum frequency up to 30 kHz. We find that the noise spectral power density is 1/f-like at low frequencies. The experimental results suggest a relative reduction of 1/f noise with respect to the intrinsic thermal noise in small-sized MTJs. The results may open a new approach for reducing the 1/f noise level in MTJ nanosensors.

Bias-field-free high frequency microwave emission of spin-transfer nano-oscillator with magnetizations all in-plane

We reported microwave measurements on a nanoscale MgO-based magnetic tunnel junction having an elliptical shape with large aspect ratios to obtain enough in-plane shape anisotropy to ensure free layer magnetization along the long axis. Combined with the magnetization of a synthetic antiferromagnet pinned layer along the short axis, this results in the perpendicular configuration between the magnetizations of free and pinned layers. A steady high frequency oscillation up to 5 GHz was achieved in such devices at zero magnetic field. Meanwhile, a large frequency tunability of 0.11 Hz·cm2/mA (2.67 GHz/mA) was obtained. The micromagnetic simulations confirm the origin of the high oscillation frequency of our spin transfer nano-oscillators (STNOs). These results suggest the great possibility of improved integration and potential application of STNOs for developing the next-generation of on-chip oscillators.

Theoretical study on temperature-bias phase diagram of MgO-based magnetic tunnel junctions

MgO-based magnetic tunnel junction is a hot issue in the field of spin electronic devices, and its temperature and bias voltage play quite an important role in practical applications. Therefore, it is desiderated to obtain the temperature-bias phase diagram of MgO-based magnetic tunnel junction. This paper develops a theory which is suitable for magnetic tunnel junctions with single crystal barrier. In this theory, the single crystal barrier is regarded as a periodic grating, and the tunneling process is treated by optical diffraction theory, so the coherence of the tunneling electron can be well taken into account. Most importantly, the theory can handle both the temperature effect and bias effect of MgO-based magnetic tunnel junctions. According to the present theory, the temperature-bias phase diagram of MgO-based magnetic tunnel junctions is calculated under different half the exchange splittings, chemical potentials and periodic potentials. The theoretical results show that the extreme phase point of tunneling magnetoresistance (TMR) can move to high temperature region through regulating half the exchange splitting &lt;i&gt;Δ&lt;/i&gt; of ferromagnetic electrode of MgO-based magnetic tunnel junction. This will be beneficial to the applications of magnetic tunnel junctions at room temperature. Moreover, the chemical potential &lt;i&gt;μ&lt;/i&gt; can change the bias corresponding to the maximum phase point of TMR. As is well known, the chemical potential will vary with the material of ferromagnetic electrode. Therefore, if the material of ferromagnetic electrode is chosen with a proper chemical potential, we can obtain a large TMR under high bias voltage. In other words, the output voltage can be considerably increased. This will be favorable for the preparation of high power devices. In addition, it is found that the phase diagram of TMR is significantly dependent on periodic potential &lt;i&gt;v&lt;/i&gt;(&lt;i&gt; &lt;b&gt;K&lt;/b&gt; &lt;/i&gt;&lt;sub&gt;&lt;i&gt;h&lt;/i&gt;&lt;/sub&gt;). As a result, the effects of temperature and bias voltage in the MgO-based magnetic tunnel junctions can be optimized by regulating half the exchange splitting &lt;i&gt;Δ&lt;/i&gt;, chemical potential &lt;i&gt;μ&lt;/i&gt;, and periodic potential &lt;i&gt;v&lt;/i&gt;(&lt;i&gt; &lt;b&gt;K&lt;/b&gt; &lt;/i&gt;&lt;sub&gt;&lt;i&gt;h&lt;/i&gt;&lt;/sub&gt;). The present work provides a solid theoretical foundation for the applications of MgO-based magnetic tunnel junctions.