Layer Of Magnetic Tunnel Junction Research Articles

Performance enhancement in spin transfer torque magnetic random access memory through in situ cap layer optimization

Magnetic tunnel junctions (MTJs), a key component of spin transfer torque magnetic random access memory, are typically fabricated using two main processes: plasma etching and in situ protective cap layer deposition. It has been found that while the etching process predominantly affects MTJ performance, the cap layer process can further enhance electrical and magnetic properties. In this study, we achieved performance improvements in MTJs by optimizing the cap layer deposition process through various experimental methods, such as modifying the gas mixtures used in the deposition process and incorporating a novel post-plasma treatment. During the deposition of the silicon nitride (SiNx) cap layer, N-rich dissociated compounds can induce passivation of the MTJ layer, leading to additional loss of tunneling magnetoresistance (TMR) and coercive field (Hc). To circumvent this challenge, we prioritized modifying the gas ratio in the SiNx deposition process. Additionally, hydrogen introduced during SiNx deposition can penetrate the MTJ pillars and degrade their properties. To mitigate this, we developed a novel post-nitrogen plasma treatment in a plasma-enhanced chemical vapor deposition chamber, which effectively desorbed the excess hydrogen from the MTJ film stack. As a result of these optimized processes, the TMR loss, compared to a blanket wafer, was reduced from 25% to 8%, and Hc increased by up to 33% for the same stack, achieving significant performance enhancements.

Effect of MgO Grain Boundaries on the Interfacial Perpendicular Magnetic Anisotropy in Spin-Transfer Torque Magnetic Random Access Memory: A First-Principles Study

Spin-transfer torque magnetic random access memory (STT-MRAM) requires large interfacial perpendicular magnetic anisotropy (iPMA) to be maintained even after many microfabrication steps. The data retention of STT-MRAM depends on the quality of the interface between the ferromagnetic layer and the insulating layer in magnetic tunnel junction (MTJ). We have investigated the effects of the grain boundary (GB) in the MgO layer on the iPMA using first-principles calculations. The results show that the iPMA is reduced by the presence of GBs, even though some iPMA remains. This is explained by a decrease in the peak of density of states (DOS) just above the Fermi energy, and that the DOS at high energy is increased in the down spin channel for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{xz}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$d_{yz}$ </tex-math></inline-formula> orbitals. Moreover, we found that the bond structure of Fe-d orbitals around the GB is a key factor controlling the iPMA, in addition to the well-known role of interfacial Fe–O bonds.

Unconventional Seedless Multilayers with Large Perpendicular Anisotropy for Back-End-of-Line Compatible Spintronic Devices

An unconventional class of hard and out-of-plane magnetized seedless multilayers (SL-MLs) of the form [Co/insertion layer/Pt]n, grown as a top reference layer in magnetic tunnel junctions (MTJ), was investigated. Among different non-ferrous insertion layers (Ta, Cu, Mg, W, Ru, and Al), Ta produces the highest perpendicular magnetic anisotropy. A back-end-of-line (BEOL) compatible hard top reference layer using Ta-inserted SL-MLs was achieved, exhibiting more than twice effective perpendicular anisotropy compared to a conventional top reference layer. Using this top reference layer, we developed three types of spintronic memory stacks: (a) top-pinned MTJ stacks that can withstand annealing up to 425 °C, (b) BEOL compatible double-MTJ memory cells with read/write mode control layer enabling a faster readout and low-voltage writing, and (c) 2-bit MTJ stacks combining spin-transfer torque and spin-orbit torque writing. The magnetic properties of these three types of memory stacks are discussed.

Reducing Dzyaloshinskii-Moriya interaction and field-free spin-orbit torque switching in synthetic antiferromagnets

Perpendicularly magnetized synthetic antiferromagnets (SAF), possessing low net magnetization and high thermal stability as well as easy reading and writing characteristics, have been intensively explored to replace the ferromagnetic free layers of magnetic tunnel junctions as the kernel of spintronic devices. So far, utilizing spin-orbit torque (SOT) to realize deterministic switching of perpendicular SAF have been reported while a large external magnetic field is typically needed to break the symmetry, making it impractical for applications. Here, combining theoretic analysis and experimental results, we report that the effective modulation of Dzyaloshinskii-Moriya interaction by the interfacial crystallinity between ferromagnets and adjacent heavy metals plays an important role in domain wall configurations. By adjusting the domain wall configuration between Bloch type and Néel type, we successfully demonstrate the field-free SOT-induced magnetization switching in [Co/Pd]/Ru/[Co/Pd] SAF devices constructed with a simple wedged structure. Our work provides a practical route for utilization of perpendicularly SAF in SOT devices and paves the way for magnetic memory devices with high density, low stray field, and low power consumption.

Magnetic switching behavior of each magnetic layer in perpendicular magnetic tunnel junctions

Magnetization of the reference layer (RL) in a normal magnetic tunnel junction (MTJ) has to be fixed below the switching field of the free layer. However, in the fabrication of magnetic random access memory (MRAM) chip consisting of huge number of MTJs, there often exist a few poor MTJ devices which show an abnormal magnetoresistance (RH) behavior. To eliminate chip fails and reduce bit error rate, in this paper magnetic switching behavior of each layer in perpendicular MTJs was studied through detailed RH measurements as a function of maximum applied field (Hmax). Switching behavior of each layer is evaluated through detailed analysis on the magnetostatic interactions of all magnetic layers in MTJs. As a result, three types of abnormal magnetic behaviors of RL and their related physical mechanisms were explored: 1), magnetization of RL tilts in elevated fields due to weak perpendicular magnetic anisotropy (PMA) below the switching field of free layer; 2), RL switches at a field below switching field of the free layer due to weak exchange coupling field (Hex) in the synthetic antiferromagnetic (SAF) structure; 3), RL switch takes place due to so-called flip flop behavior in the SAF structure. Relevant methods to overcome these abnormal magnetic switching behaviors are suggested and used in our successful fabrication of MRAM chips.

Magnetic and structural properties of CoFeB thin films grown by pulsed laser deposition

The emergence of thin film CoFeB has driven research and industrial applications in the past decades, with the magnetic random access memory (MRAM) the most prominent example. Because of its beneficial properties, it fulfills multiple functionalities as information-storing, spin-filtering, and reference layer in magnetic tunnel junctions. In future, this versatility can be exploited beyond the traditional applications of spintronics by combining with advanced materials, such as oxide-based materials. Pulsed laser deposition (PLD) is their predominant growth-method, and thus the compatibility of CoFeB with this growth technique will be tested here. This encompasses a comprehensive investigation of the structural and magnetic propoperties. In particular, we find a substantial ‘dead’ magnetic layer and confirm that it is caused by oxidation employing the x-ray magnetic circular dichroism (XMCD) effect. The low damping encountered in vector network analyzer-based ferromagnetic resonance (VNA-FMR) renders them suitable for magnonics applications. These findings demonstrate that CoFeB thin films are compatible with emergent, PLD-grown materials, ensuring their relevance for future applications.

Tunable magnetic low-frequency noise in magnetic tunnel junctions: effect of shape anisotropy

The intrinsic magnetic low-frequency noise (LFN) is of fundamental scientific interest to the study of magnetic tunnel junctions (MTJs). To gain insight into its mechanism, the fluctuation-dissipation theorem, which describes the linear relation between magnetic LFN and magnetic sensitivity product, has been utilized. However, deviation from the linear correlation has been reported in some studies. To understand and effectively control the magnetic LFN, a more elaborate analytical description and further experimental validation are required. In this work, the magnetic LFN contributed from the magnetization fluctuation in the pinned layer of MTJs with various shape anisotropies was investigated. The MTJs with different shape anisotropies, achieved by altering their aspect ratios, possessed distinct demagnetizing factors. Large magnetic noise was correlated with the increase of magnetic phase loss of ferromagnetic layers during magnetization reversal at which magnetization fluctuation was enhanced. Upon increasing the shape anisotropy, a notable reduction of the magnetic phase loss in the antiparallel (AP) state was observed while it exhibited a slight decrease in the parallel (P) state, revealing that the increase of the shape anisotropy caused a more pronounced suppression of the equilibrium magnetization fluctuation in the AP state. These phenomena were computationally validated by constructing a macrospin model to describe the thermally-induced magnetization fluctuation in the pinned layer. This work reveals the physical relation between MTJ shape anisotropy and magnetic LFN. The effect of the shape anisotropy on the magnetic LFN can be extended to other types of in-plane uniaxial anisotropies.

Nanoscale Spin Injector Driven by a Microwave Voltage

We propose an electrically driven spin injector into normal metals and semiconductors, which is based on a magnetic tunnel junction (MTJ) subjected to a microwave voltage. Efficient functioning of such an injector is provided by electrically induced magnetization precession in the "free" layer of MTJ, which generates the spin pumping into a metallic or semiconducting overlayer. We theoretically describe the spin and charge dynamics in the CoFeB/MgO/CoFeB/Au(GaAs) heterostructures. First, the magnedynamics in the free CoFeB layer is quantified with the account of a spin-transfer torque and a voltage-controlled magnetic anisotropy. By numerically solving the magnetodynamics equation, we determine dependences of the precession amplitude on the frequency $f$ and magnitude $V_\mathrm{max}$ of the ac voltage applied to the MTJ. It is found that the frequency dependence changes drastically above the threshold amplitude $V_\mathrm{max} \approx 200$mV, exhibiting a break at the resonance frequency $f_\mathrm{res}$ due to nonlinear effects. The results obtained for the magnetization dynamics are used to describe the spin injection and pumping into the Au and GaAs overlayers. Since the generated spin current creates additional charge current owing to the inverse spin Hall effect, we also calculate distribution of the electric potential in the thick Au overlayer. The calculations show that the arising transverse voltage becomes experimentally measurable at $f = f_\mathrm{res}$. Finally, we evaluate the spin accumulation in a long n$^+$-GaAs bar coupled to the MTJ and determine its temporal variation and spatial distribution along the bar. It is found that the spin accumulation under resonant excitation is large enough for experimental detection even at micrometer distances from the MTJ. This result demonstrates high efficiency of the described nanoscale spin injector.

Enhanced electric control of magnetic anisotropy via high thermal resistance capping layers in magnetic tunnel junctions

We studied nonlinear magnetic anisotropy changes to the DC bias voltage of magnetic tunnel junctions (MTJs) with capping layers of different thermal resistances. We found that increasing the thickness of MgO capping layers (in the range 0.3–0.5 nm) in MTJs enhances the Joule heating-induced magnetic anisotropy change, which indicates an enhancement of the interfacial thermal resistance at the FeB|MgO capping layer interface. This enhanced interfacial thermal resistance may be attributed to roughness at the FeB|MgO interface. Moreover, we observed a larger power-driven magnetic anisotropy change of 3.21 µJ W−1m−1 in the MTJ with a composite MgO (0.3 nm)|W (2 nm)|MgO (0.4 nm) capping layer. This research supports methods of efficient spin manipulation of spintronic devices such as microwave devices and magnetic memories.

Record thermopower found in an IrMn-based spintronic stack

The Seebeck effect converts thermal gradients into electricity. As an approach to power technologies in the current Internet-of-Things era, on-chip energy harvesting is highly attractive, and to be effective, demands thin film materials with large Seebeck coefficients. In spintronics, the antiferromagnetic metal IrMn has been used as the pinning layer in magnetic tunnel junctions that form building blocks for magnetic random access memories and magnetic sensors. Spin pumping experiments revealed that IrMn Néel temperature is thickness-dependent and approaches room temperature when the layer is thin. Here, we report that the Seebeck coefficient is maximum at the Néel temperature of IrMn of 0.6 to 4.0 nm in thickness in IrMn-based half magnetic tunnel junctions. We obtain a record Seebeck coefficient 390 (±10) μV K−1 at room temperature. Our results demonstrate that IrMn-based magnetic devices could harvest the heat dissipation for magnetic sensors, thus contributing to the Power-of-Things paradigm.

Fabrication of soft-magnetic FeAlSi thin films with nm-order thickness for the free layer of magnetic tunnel junction based sensors

FeAlSi epitaxial films were fabricated on single crystalline MgO(100) substrates and their structural and magnetic properties were investigated to apply them to the free layer in magnetic tunnel junction (MTJ) based sensors. We found that the film composition must be precisely controlled, and the post-annealing temperature was varied to obtain a D03-ordered structure and soft magnetic property. By adjustment of the film composition and optimizing annealing temperature, we succeeded in obtaining D03-ordered FeAlSi thin films with a small surface roughness and low coercivity. The fabricated FeAlSi film is greatly useful as a free layer material in MTJ based sensor devices.

Effect of second-order magnetic anisotropy on nonlinearity of conductance in CoFeB/MgO/CoFeB magnetic tunnel junction for magnetic sensor devices

We studied the effect of second-order magnetic anisotropy on the linear conductance output of magnetic tunnel junctions (MTJs) for magnetic-field-sensor applications. Experimentally, CoFeB/MgO/CoFeB-based MTJs were fabricated, and the nonlinearity, NL was evaluated for different thicknesses, t of the CoFeB free layer from the conductance. As increasing t from 1.5 to 2.0 nm, maximum NL, NLmax was found to decrease from 1.86 to 0.17% within the dynamic range, Hd = 1.0 kOe. For understanding the origin of such NL behavior, a theoretical model based on the Slonczewski model was constructed, wherein the NL was demonstrated to be dependent on both the normalized second-order magnetic anisotropy field of Hk2/|Hkeff| and the normalized dynamic range of Hd/|Hkeff|. Here, Hkeff, Hk2, are the effective and second-order magnetic anisotropy field of the free layer in MTJ. Remarkably, experimental NLmax plotted as a function of Hk2/|Hkeff| and Hd/|Hkeff|, which were measured from FMR technique coincided with the predictions of our model. Based on these experiment and calculation, we conclude that Hk2 is the origin of NL and strongly influences its magnitude. This finding gives us a guideline for understanding NL and pioneers a new prospective for linear-output MTJ sensors to control sensing properties by Hk2.

Stabilization of the easy-cone magnetic state in free layers of magnetic tunnel junctions

The influence of temperature, FeCoB layer thickness, and insertion of ultrathin metal spacers (Ta and W) on the magnetic anisotropy of MgO/FeCoB/MgO free layers has been explored by means of ferromagnetic resonance. The second-order contribution to the perpendicular magnetic anisotropy (PMA), stemming from the fluctuations of the first-order term, accounts for the onset of an easy-cone magnetic state in the course of the transition from the in-plane to out-of-plane magnetization. Since the second-order term is small, the easy-cone state is stable only within a narrow range of free-layer thicknesses and temperatures, where the interfacial first-order PMA term gets counterbalanced by the magnetostatic term. We have found that the insertion of metallic spacers in the middle of the FeCoB layer noticeably widens the range of thicknesses and temperatures for obtaining the easy-cone state. We show that the W spacer outperforms its Ta counterpart in this enhancement, albeit increasing the magnetization damping due to a higher degree of alloying with FeCoB. A physical mechanism of the easy-cone stabilization is proposed. It considers a variation of the local saturation magnetization $({M}_{\mathrm{S}})$ and, notably, Curie temperature (${T}_{\mathrm{C}}$) near the MgO/FeCoB interface versus the distance to the metal spacer (or the capping) layer. The larger ${M}_{\mathrm{S}}$ and ${T}_{\mathrm{C}}$ values near MgO provide more thermal stability to the first-order PMA (${k}_{\mathrm{s}1}$), while the lower ${M}_{\mathrm{S}}$ and ${T}_{\mathrm{C}}$ ones near the spacer layer result in a faster drop of the magnetostatic term with temperature. As a result, the effective PMA field exhibits a thermal stabilization effect that can be exploited to stabilize the easy-cone anisotropy. Alongside the improved conditions for setting an easy cone in the MgO/FeCoB/MgO free layers, we demonstrate that an easy-cone configuration with an almost temperature-independent opening angle can be obtained using a MgO/FeCoB(1.6 nm)/Ta free layer.

Heavy ion irradiation induced hard error in MTJ of the MRAM memory array

The radiation effects of magnetic tunnel junction (MTJ) in a 180 nm Magnetoresistive Random Access Memory (MRAM) were investigated at the condition of the high heavy ion fluence (up to 1010 ions·cm−2) and high gamma-ray dose (up to 15 Mrad(Si)), respectively. The Hard Bit Error (HBE) was only observed when the energy of 181Ta ions was larger than 95.8 MeV under the ion fluence of 109 ions·cm−2 (~13 ions·bit−1). For gamma-ray exposure, no obvious effects were observed in devices. The HBE counts induced by 181Ta ions have a strong dependence on ion energy and cumulative fluence. In addition, the annealing characteristics of HBE after heavy ion irradiation at room temperature were also observed. It is found that the high ambient temperature is able to accelerate this annealing process. Monte Carlo simulation by using TRIM program revealed that the displacement damages induced by heavy ions were mainly located in the insulating layer of MTJ, which is responsible for the occurrence of HBE.

Spin-dependent resonant tunneling and magnetoresistance in Ni/graphene/h-BN/graphene/Ni van der Waals heterostructures

Abstract Future development of spintronic devices urgently requires an advanced control method of spin current. Combining graphene with other various characteristic two-dimensional crystals in different ways can produce a new class of functional materials, which offers a unique platform for spintronics. A specific van der Waals heterostructure consisting of graphene and hexagonal boron nitride (h-BN) as the barrier layer in magnetic tunnel junctions (MTJs) is proposed in this paper. This heterostructure device combines the strong spin filtering effect of graphene/ferromagnet interface and the resonant tunneling effect of graphene/h-BN/graphene van der Waals heterostructure. We demonstrate an effective approach to control spin current based on the bias tunability of the spin resonance tunneling transport. Theoretical research unambiguously reveals that spin resonance appears when the electronic spectra of spin electron in two graphene layers are aligned, which results in a large magnetoresistance and negative differential resistance in the device. The efficient bias controllability of spin current, combined with the excellent magnetoresistance behavior, can be the key building blocks in future spin-based magnetic detection and high-frequency logic devices.

Enhanced Annealing Stability of Exchange-Biased Pinned Layer in Magnetic Tunnel Junction Using Ta/Ru/Ta/Ru Underlayer

To realize highly sensitive analog sensor using magnetic tunnel junctions (MTJs), there is a demand for MgO-MTJs with a high signal-to-noise ratio. Thermal annealing is important for improving the crystalline quality of MgO barrier in order to enhance the tunneling magnetoresistance (TMR) ratio and decrease the noise level of MTJs. However, annealing at excessively high temperatures causes degradation of the exchange bias field ( $H_{\text {ex}}$ ) in the exchange-biased pinned layer (EB-PL). In this paper, we aim to develop a MgO-MTJ with high annealing stability by investigating the effect of underlayer structure on the annealing stability of EB-PL in MTJs. We fabricated two types of MTJ, one with a Ta/Ru underlayer (TR-UL) and one with a Ta/Ru/Ta/Ru underlayer (TRTR-UL). The results of evaluating the $H_{\text {ex}}$ of MTJs annealed in the temperature range of 320 °C–420 °C found that the MTJ containing a TRTR-UL (TRTR-MTJ) has higher annealing stability compared with that of the MTJ containing a TR-UL (TR-MTJ). Furthermore, the results of measuring the TMR ratio and 1/ $f$ noise level of the MTJs annealed at 380 °C showed that the TRTR-MTJ exhibits a higher TMR ratio and lower noise level than the TR-MTJ. To determine the mechanism by which the annealing stability improves, we carried out microstructural analysis of both MTJs and found that thermal Mn diffusion in the TRTR-MTJ was smaller than that in the TR-MTJ, and the crystallinity of IrMn on the TRTR-UL was found to be clearly improved compared with that on the TR-UL. As a result of $d$ -spacing analysis of each layer in the TRTR-UL, the second Ta layer deposited on the Ru layer was found to have $d$ -spacing corresponding to the (110) textured $\alpha $ -Ta structure and to be highly crystalline, whereas the first Ta layer was not found to have the components of (110) textured $\alpha $ -Ta structure. These results indicate that the Ta-on-Ru structure is responsible for the high crystallinity of the IrMn layer disposed on the TRTR-UL, resulting in the high annealing stability thanks to small Mn diffusion.

An effect of capping-layer material on interfacial anisotropy and thermal stability factor of MgO/CoFeB/Ta/CoFeB/MgO/capping-layer structure

We investigate the magnetic properties of a MgO/CoFeB/Ta/CoFeB/MgO/capping-layer (Ru or Ta) structure and properties of a magnetic tunnel junction with the structure as a free layer. By using Ru instead of Ta as the capping layer, interfacial anisotropy Ki increases by a factor of ∼2 and a smaller damping constant is obtained. The increase in Ki results in an enhancement of the thermal stability factor of the free layer with the Ru capping layer compared with that with the Ta capping layer in magnetic tunnel junctions.

Resonate and fire neuron with fixed magnetic skyrmions

In the brain, the membrane potential of many neurons oscillates in a subthreshold damped fashion and fire when excited by an input frequency that nearly equals their eigen frequency. In this work, we investigate theoretically the artificial implementation of such “resonate-and-fire” neurons by utilizing the magnetization dynamics of a fixed magnetic skyrmion in the free layer of a magnetic tunnel junction (MTJ). To realize firing of this nanomagnetic implementation of an artificial neuron, we propose to employ voltage control of magnetic anisotropy or voltage generated strain as an input (spike or sinusoidal) signal, which modulates the perpendicular magnetic anisotropy. This results in continual expansion and shrinking (i.e., breathing) of a skyrmion core that mimics the subthreshold oscillation. Any subsequent input pulse having an interval close to the breathing period or a sinusoidal input close to the eigen frequency drives the magnetization dynamics of the fixed skyrmion in a resonant manner. The time varying electrical resistance of the MTJ layer due to this resonant oscillation of the skyrmion core is used to drive a Complementary Metal Oxide Semiconductor buffer circuit, which produces spike outputs. By rigorous micromagnetic simulation, we investigate the interspike timing dependence and response to different excitatory and inhibitory incoming input pulses. Finally, we show that such resonate and fire neurons have potential application in coupled nanomagnetic oscillator based associative memory arrays.

Ni thickness influence on magnetic properties (Co/Ni/Co/Pt) multilayers with perpendicular magnetic anisotropy

We present a study of perpendicular magnetic anisotropy in [Co/Ni(t)/Co/Pt]×8 multilayers for use as free layers in magnetic tunnel junctions (MTJ) and spin valves. The thickness t of the Ni sub-layer was varied and the resulting magnetic properties were investigated. As determined from magnetic force microscopy and magnetometry measurements, all multilayers exhibited a perpendicular magnetic anisotropy with an increase of saturation magnetization with thickness t. From the temperature dependence of the magnetization, well described by a Bloch law, we find that the spin-wave stiffness constant of the [Co/Ni(t)/Co/Pt]×8 multilayers is larger compared to (Co/Ni) multilayers. These multilayers could be the basis for spintronic devices where the reduction of total Pt content could help to reduce the damping constant while keeping the magnetic anisotropy energy relatively high. These are conflicting requirements needed for high performance magnetic memory devices.

Microscopic theory of the Coulomb based exchange coupling in magnetic tunnel junctions

We study interlayer exchange coupling based on the many-body Coulomb interaction between conduction electrons in magnetic tunnel junction. This mechanism complements the known interaction between magnetic layers based on virtual electron hopping (or spin currents). We find that these two mechanisms have different behavior on system parameters. The Coulomb based coupling may exceed the hopping based exchange. We show that the Coulomb based exchange interaction, in contrast to the hopping based coupling, depends strongly on the dielectric constant of the insulating layer. The dependence of the interlayer exchange interaction on the dielectric properties of the insulating layer in magnetic tunnel junction is similar to magneto-electric effect where electric and magnetic degrees of freedom are coupled. We calculate the interlayer coupling as a function of temperature and electric field for magnetic tunnel junction with ferroelectric layer and show that the exchange interaction between magnetic leads has a sharp decrease in the vicinity of the ferroelectric phase transition and varies strongly with external electric field.