Layer In Junctions Research Articles

Electrical control of the switching layer in perpendicular magnetic tunnel junctions with atomically thin Ir dusting

The use of magnetic tunnel junction (MTJ)-based devices constitutes an important basis of modern spintronics. However, the switching layer of an MTJ is widely believed to be an unmodifiable setup, instead of a user-defined option, posing a restriction to the function of spintronic devices. In this study, we realized a reliable electrical control of the switching layer in perpendicular MTJs with 0.1 nm Ir dusting. Specifically, a voltage pulse with a higher amplitude drives the magnetization switching of the MTJ's bottom electrode, while a lower voltage amplitude switches its top electrode. We discussed the origin of this controllability and excluded the possibility of back-hopping. Given the established studies on enhancing the voltage-controlled magnetic anisotropy effect by adopting Ir, we attribute this switching behavior to the significant diffusion of Ir atoms into the top electrode, which is supported by scanning transmission electron microscopy with atomic resolution.

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.

Building acoustic performance prediction - Feedback from Adivbois CLT building mockup

The acoustic performance of a CLT based building mockup was investigated within the scope of AdivBois acoustic technical commission with the objective of defining wood building constructions fulfilling requirements. The CLT based building is a three floor construction, with four rooms on each level. Measurements from junction characterization to air-borne and impact sound insulations have been performed. Furthermore, acoustic measurements have been performed before and after linings and floor coverings were implemented. Moreover, several acoustic teams have carried out measurements in the building. This paper concentrates on the comparison between measured acoustic performance and the predicted one. Predictions are based on the EN ISO 12354-1 and -2 standards, using floor and wall acoustic performances measured in laboratory conditions. The effect of inserting resilient layers in junctions, floor covering and receiving room size is discussed.

Computational Simulations of Fabrication of Aluminum-Based Josephson Junctions: Topological Aspects of the Barrier Structure

Although the performance of qubits has been improved in recent years, the differences in the microscopic atomic structure of the Josephson junctions, the core devices prepared under different preparation conditions, are still underexplored. In this paper, the effects of the oxygen temperature and upper aluminum deposition rate on the topology of the barrier layer in the aluminum-based Josephson junctions have been presented by classical molecular dynamics simulations. We apply a Voronoi tessellation method to characterize the topology of the interface and central regions of the barrier layers. We find that when the oxygen temperature is 573 K and the upper aluminum deposition rate is 4 Å/ps, the barrier has the fewest atomic voids and the most closely arranged atoms. However, if only the atomic arrangement of the central region is considered, the optimal rate of the aluminum deposition is 8 Å/ps. This work provides microscopic guidance for the experimental preparation of Josephson junctions, which helps to improve the performance of qubits and accelerate the practical application of quantum computers.

A generalised double layered model with polytropic and quadratic equations of state

This study generates a new exact model for a massive stellar spheres by utilising the Einstein–Maxwell field equations. The model has two interior regions with distinct equations of state (EoS): the polytropic EoS at the core region, and quadratic EoS at the envelope region. Variation of the physical features in the interior of star affirms that the matter variables, gravitational potentials and other physical quantities are consistent with massive stellar bodies. Interestingly, our model can regain solutions of the two-layered model generated by Mardan, Noureen and Khalid. Matching of the layers in junctions is done with help of the Reissner–Nordstrom exterior spacetime. Using our model, stellar masses, and radii consistent with observations and well known stars are generated.

DFT insights into the origin of d0 ferromagnetism, mechanical stability, elastic, and acoustic anisotropy in AZrO3 (A= K, rb, Cs) cubic perovskites

The d0 ferromagnetism through hole induction can be realized in oxide perovskites. The electronic and magnetic properties of AZrO3 (A = K, Rb, and Cs) cubic perovskites have been computationally analyzed using generalized gradient approximation (GGA) and modified Becke-Johnson (mBJ) potentials. From the evaluation of tolerance factor, volume optimization, formation energy, and phonon dispersion, their structural, magnetic, and dynamical stabilities in the ferromagnetic cubic phase have been verified. Additionally, Blackman's and Every's diagrams ensure their elastic stability without any symmetry-breaking phase transitions. The three-dimensional surfaces of elastic moduli and acoustic wave velocities indicate the degree of anisotropy in these perovskites. The compounds become more ductile for larger alkali cations. The band structures from both functionals confirm that the localized magnetic moments around oxygen atoms arise only from the exchange spin splitting of O-2p orbitals. The integer magnetic moment of 1 μB and complete spin polarization implies the presence of half-metallic ferromagnetism. Among the proposed compounds, the predicted mean-field curie temperature of CsZrO3 is above room temperature and may serve as optimal magnetic layers in spin-valve sensors and magnetic tunnel junctions.

Noise resilient leaky integrate-and-fire neurons based on multi-domain spintronic devices

The capability of emulating neural functionalities efficiently in hardware is crucial for building neuromorphic computing systems. While various types of neuro-mimetic devices have been investigated, it remains challenging to provide a compact device that can emulate spiking neurons. In this work, we propose a non-volatile spin-based device for efficiently emulating a leaky integrate-and-fire neuron. By incorporating an exchange-coupled composite free layer in spin-orbit torque magnetic tunnel junctions, multi-domain magnetization switching dynamics is exploited to realize gradual accumulation of membrane potential for a leaky integrate-and-fire neuron with compact footprints. The proposed device offers significantly improved scalability compared with previously proposed spin-based neuro-mimetic implementations while exhibiting high energy efficiency and good controllability. Moreover, the proposed neuron device exhibits a varying leak constant and a varying membrane resistance that are both dependent on the magnitude of the membrane potential. Interestingly, we demonstrate that such device-inspired dynamic behaviors can be incorporated to construct more robust spiking neural network models, and find improved resiliency against various types of noise injection scenarios. The proposed spintronic neuro-mimetic devices may potentially open up exciting opportunities for the development of efficient and robust neuro-inspired computational hardware.

The influence of the magnetic configuration on the charge current generated by the temperature gradient in the double planar ferromagnetic tunnel junctions

The charge current generated by the temperature gradient applied to the double planar tunnel junctions with ferromagnetic external electrodes and central layer is investigated in the free-electron model. The situation when the bias voltage is applied to the junction is also considered. It has been shown that the analyzed current may depend on the orientation of magnetic moments in the ferromagnetic external electrodes and the central layer. The influence of the magnetic configuration on the tunnel current can be enhanced due to the resonant tunneling by the resonant states. Especially strong modifications of the current flowing through the junction can be observed as a result of changing the orientation of the magnetic moments in the central layer in junctions where the tunneling by the resonant states located below the bottom for the spin-down electrons in the source electrode is observed. The studied effects are strongly depended on the thickness of the central layer, spin splitting of the electron bands in the external electrodes and in the central layer, and the bias voltage applied to the junctions, but are also sensitive to the average temperature of the junction and the height and width of the barriers, whereas they are less sensitive to the temperature difference between the electrodes.

High-Quality Sputtered BiFeO3 for Ultrathin Epitaxial Films

BiFeO3 films were grown by RF magnetron sputtering with various O2 gas flow ratios and substrate temperatures. The optimal sputtering conditions for a slightly excess Bi content produced high-quality parameters: an atomically flat surface (Ra < 0.4 nm), low leakage current (Jc < 10–6 A/cm2), high ferroelectric polarization (72 μC/cm2//[001]pc), and large exchange bias (∼140 Oe). In addition to these typical characterizations, the following two advanced analyses were performed: (i) The lattice constant was identified by Bragg’s diffraction specific to a space group of R3c using X-ray diffraction; it was precisely determined as an expanded a-axis (abulk = 0.568 → aepi = 0.572 nm) and a shrunk c-axis (cbulk = 1.398 → cepi = 1.373 nm). (ii) The ferroelectricity was analyzed by first-order reversal curve (FORC) diagrams, which revealed that ferroelectric switching was packed in a narrow electric field area; an internal electric field in the film body was not observed despite the fact that the BiFeO3 films were as-grown samples. A 3 nm thick BiFeO3 film with a continuous and flat surface/interface was confirmed over a wide area. The crystal symmetry might be identified as a space group of R3c in the case of the 3 nm thick film by comparing the nanobeam selected area electron diffraction patterns with the patterns based on structural calculations. The ferroelectricity might be confirmed by the piezoresponse force microscopy of a 2 nm thick BiFeO3 epitaxial film, owing to the optimal condition of low Jc and uniform ferroelectric switching properties. Furthermore, a 0.4 nm thick ultrathin BiFeO3 film was confirmed to be a continuous one-unit cell perovskite (∼0.4 nm) layer, owing to the optimal condition of low Ra. This study provides a method for investigating the crystal symmetry that affects the multiferroic properties of ultrathin films, which can be used as barrier layers in multiferroic tunnel junctions for highly functional sensors.

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.

Understanding and modulation of resistive switching behaviors in PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3/Nb:SrTiO3 multilayer junctions

The insertion layer in the ferroelectric multilayer junctions plays a key role in regulating the energy band structure of interface and their resistive switching behavior. Here, PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3/Nb:SrTiO3 (PZT/LSMO/NSTO) heterostructures with different LSMO thicknesses were prepared by pulsed laser deposition and their ferroelectric properties and resistive switching behaviors were investigated. It is found that the ferroelectric properties of the heterostructures almost do not change with the increasing in the thickness of LSMO, while the resistive switching behaviors are closely related to the LSMO thickness, and the maximum switching ratio of about 103 can be achieved in the PZT/LSMO/NSTO heterostructure with the LSMO thickness of 18 nm. By analyzing the I-V curves and energy band structures, it can be concluded that the resistive switching behaviors depend on the competition between the tunability of depletion layer width and the ability of ferroelectric filed effect. These results provide insights to understand ferroelectric resistive switching in ferroelectric heterostructures, and demonstrate an effective way to improve resistive switching performance by adjusting the band structure through the appropriate thickness of the insertion layer.

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.

Interface application of NiPt alloy nanoparticles decorated rGO nanocomposite to eliminate of contact problem between metal and inorganic/organic semiconductor

In this study, synthesis of monodisperse NiPt alloy NPs, preparation of rGO, decoration of the NPs onto the rGO and interface application of the NiPt/rGO nanocomposite material were presented. Morphological and structural analysis showed that Ni70Pt30 NPs having particle size of 2.2 ± 0.4 nm were obtained and decorated onto the rGO. The ID/IG intensity ratio of the rGO was determined to be 1.02. On the other hand, the Ni70Pt30/rGO nanocomposite was used as interface layer in junctions of the Al/lnP and P3HT:PCBM/Al. In the Al/lnP as inorganic metal-semiconductor junction, the interface layer caused an increase in rectification ratio of up to 103. Moreover, the ideality factor and barrier height enhanced from 1.47 and 0.49 eV to 1.24 and 0.67 eV at 300 K. It is attributed to eliminate of metal diffusion in lnP and passivation of pinning Fermi level. Additionally, the Ni70Pt30/rGO nanocomposite layer was used fabrication of the P3HT:PCBM solar cell to eliminate of S-shaped J-V curve. The reference solar cell having S-shaped J-V curve showed 6.081 mA/cm2 short circuit current density, 0.572 V open circuit voltage, 31.4% fill-factor and then 1.092% power conversion efficiency. The presence of the interface layer has caused an increase in the fill-factor value of more than 30%, while the PCE value increased by 15%. The best cell has 1.244% power conversion efficiency with 5.554 mA/cm2 short circuit current density, 0.544 V open circuit voltage, 41.2% fill-factor.

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.

Impact of Fe80B20 insertion on the properties of dual-MgO perpendicular magnetic tunnel junctions

We explore the impact of Fe80B20 inserted at both Co20Fe60B20/MgO interfaces of dual-MgO free layers (FLs) in bottom-pinned magnetic tunnel junctions (MTJs). MTJ stacks are annealed for 30 min at 350 °C and 400 °C in a vacuum after film deposition. Current-in-plane tunneling measurements are carried out to characterize magnetotransport properties of the MTJs. Conventional magnetometry measurements and ferromagnetic resonance (FMR) are conducted to estimate the saturation magnetization, the effective perpendicular anisotropy field and the Gilbert damping of dual-MgO FLs as a function of the Fe80B20 thickness and annealing temperatures. With ultrathin Fe80B20 (0.2–0.4 nm) inserted, perpendicular magnetic anisotropy (PMA) of FLs increases with similar tunnel magneto-resistance (TMR) and low damping values. As Fe80B20 layer thickness further increases (0.6–1.2 nm), both TMR and PMA degrade, and damping increases dramatically. This study demonstrates a novel approach to tune properties of MTJ stacks with dual-MgO FLs up to 400 °C annealing, which enables MTJ stacks for various applications.

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.

Full voltage manipulation of the resistance of a magnetic tunnel junction.

One of the motivations for multiferroics research is to find an energy-efficient solution to spintronic applications, such as the solely electrical control of magnetic tunnel junctions. Here, we integrate spintronics and multiferroics by depositing MgO-based magnetic tunnel junctions on ferroelectric substrate. We fabricate two pairs of electrodes on the ferroelectric substrate to generate localized strain by applying voltage. This voltage-generated localized strain has the ability to modify the magnetic anisotropy of the free layer effectively. By sequentially applying voltages to these two pairs of electrodes, we successively and unidirectionally rotate the magnetization of the free layer in the magnetic tunnel junctions to complete reversible 180° magnetization switching. Thus, we accomplish a giant nonvolatile solely electrical switchable high/low resistance in magnetic tunnel junctions at room temperature without the aid of a magnetic field. Our results are important for exploring voltage control of magnetism and low-power spintronic devices.

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.