Magnetic Memory Research Articles

Electric field control of 180° magnetization reversal in a spin-valve multiferroic heterostructure

Electrical tuning of magnetism, rather than a spin current or magnetic field, has attracted much attention due to its great potential for designing energy-efficient spintronic devices. However, pure electric field (E-field) control of 180° magnetization reversal is still challenging. Thus, we report an E-field-controlled 180° magnetization reversal in a spin valve (SV)/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) multiferroic heterostructure. Via the converse magnetoelectric coupling effect in CoFe/PMN-PT heterostructures, the magnetic anisotropy and coercive field of the CoFe film can be tremendously modulated by an E-field. We fabricated an optimized SV grown on the PMN-PT substrate, in which the magnetic moments of the free and pinned layers are parallel to each other at the initial state. By applying an E-field, the coercive field of the free layer was enhanced, exhibiting an antiparallel configuration between the free and pinned layers. Based on the theoretical and experimental results, a complete 180° magnetization reversal of the free layer can be obtained without a magnetic field. Accordingly, an E-field-controlled giant, reversible and repeatable magnetoresistance modulation can be achieved. This work proposes a feasible strategy to achieve E-field-controlled 180° magnetization reversal, which is critical for exploring ultralow power consumption magnetic memory devices.

Publisher's Note: “Strain-mediated voltage controlled magnetic anisotropy based switching for magnetic memory applications” [J. Appl. Phys. 134, 123905 (2023)

Publisher's Note: “Strain-mediated voltage controlled magnetic anisotropy based switching for magnetic memory applications” [J. Appl. Phys. 134, 123905 (2023)

Solidification of Ni-Mn-Ga magnetic shape memory alloy built via laser powder bed fusion on a single crystal substrate

Ni-Mn-Ga-based magnetic shape memory (MSM) alloys have significant potential as fast actuation materials due to their ability to undergo large magnetic-field-induced strains in an external applied magnetic field. This study explored the manufacture of Ni-Mn-Ga via the laser powder bed fusion (PBF-LB/M) additive manufacturing process, aiming to determine the parameter combinations that enhance the development of a crystallographically oriented coarse grain structure. In the first part of the study, the single tracks were melted on pre-heated single-crystalline Ni-Mn-Ga substrates by varying the applied laser power and scanning speed. This experiment revealed the existence of a parameter window where epitaxial solidification of the single-track can be achieved. Additionally, the results demonstrated that preheating prevents cracking of the single-crystal substrate. Subsequent experiments using a thin-walled 3D geometry demonstrated that this epitaxial structure was replicated across multiple layers, with the built samples exhibiting crystallographically oriented oligocrystalline structures following a high-temperature homogenization treatment. PBF-LB/M shows high potential for facilitating the additive manufacturing of oriented coarse grain oligocrystals for the production of microscale actuators.

High frequency enhancement of dielectric constant, crystal structure analysis, and magnetic properties of Pr3+- Zn2+ substituted Co2Y-type barium hexagonal ferrites

The effect of Pr3+-Zn2+ doping on the magnetic and dielectric properties of Co2Y-type barium hexaferrites with the chemical composition Ba2Co2Fe12-x-yPrxZnyO22 (x = 0.00, 0.10, 0.20, and y = 0.00, 0.15, 0.25), was investigated in this study. The prepared Pr3+-Zn2+ substituted Co2Y-type barium hexaferrites were calcined for 5 h at 1100 °C in a digital heating chamber. The XRD analysis confirms the presence of a crystalline phase in the undoped Co2Y-type barium hexaferrite samples, with a space group identified as R-3m. Characteristic absorption bands of Co2Y-type barium hexaferrites were observed at 540 and 422 cm−1 due to the asymmetric stretching vibration of Fe3+ and O2− at both, octahedral and as well as tetrahedral crystallographic sites. The magnetic interaction between the particles causes slight agglomeration. Dielectric and impedance spectroscopy were performed using Koop's phenomenological theory and the Maxwell-Wagner two-layer dielectric relaxation model. The ε′ was found to decrease with an increase in applied frequency due to the space-charge polarization phenomenon. The rise in ε′ at higher frequencies enhances the material's suitability for high-frequency applications. The magnetic parameters were extracted from the M − H hysteresis loop, and it was observed that they were affected by the site preferences of Pr3+ and Zn2+. The prepared Y-type hexaferrite's optimized magnetic parameters make it a good option for magnetic memory applications.

Research on the defect depth detection for pipeline steel with double defects using metal magnetic memory method

Metal magnetic memory testing is an effective method to detect the stress state of ferromagnetic materials, and shows great application potential in the field of defect detection of oil and gas pipelines recently. In this paper, the correlation between the normal component of residual magnetic field (RMF) and the defect depth, as well as the applied loading were investigated by tensile tests. The X70 pipeline steels were processed into specimens with double defects of six types of depths and three types of spacings. Some magnetic parameters are defined to characterize the distortion of the RMF in the defect area. The effect of defect depth and applied loading on the magnetic parameters is discussed. The RMF signals measured on the smooth side of the specimens were compared with those on the defective side. Results show that the normal parameters, obtained from the front and back of the specimens, are capable of detecting the defect depth. The slope of the linear relationship between the two increases with the increase of defect spacing. The expressions of normal magnetic parameters were derived based on the Jiles model and magnetic dipole model. The simulation results of defective side and smooth side were compared, as well as the simulation results of different defect spacing. The experimental and numerical simulation results show that the defect depth of pipeline steel can be evaluated by the characteristic parameters of the normal RMF signal under a certain stress level.

Single current control of magnetization in vertical high-aspect-ratio nanopillars on in-plane magnetization layers

Ferromagnetic pillars standing on a substrate hold promise for use in recording segments of multibit nonvolatile memories. These pillars exhibit high thermal stability in their magnetization owing to the influence of shape and perpendicular magnetic anisotropies. Recent micromagnetic simulations have demonstrated the feasibility of magnetization control in these pillars. Such control was achieved through the spin-transfer torque induced by the current flowing within the pillar and the spin-orbit torque generated by the current flowing through the heavy-metal lead at the bottom of the pillars. However, the presence of two current paths complicates circuit design, posing challenges in device integration. To solve this problem, we propose a new structure wherein a pillar is placed on a thin film with in-plane magnetization. When current flows through this structure, a torque is applied to the magnetization of the pillar, similar to that of the three-terminal structure. Magnetization reversal and control in the proposed structure were demonstrated using micromagnetic simulations. Specifically, magnetization reversal was achieved in a 100 nm-long permalloy pillar, whereas the magnetization corresponding to a three-bit sequence was generated in a 250 nm-long permalloy pillar. We propose two methods to control the magnetization of multibit memory. One method uses two different current intensities, whereas the other uses constant and pulsed currents of identical intensity. Notably, in the proposed structure, magnetization was controlled using only a unidirectional current. In particular, magnetization can be controlled with a pulsed current using a single current strength. This advancement will simplify the circuitry required to control magnetic memory, bringing the realization of magnetic memory devices closer to reality.

Evaluation of residual load-bearing capacity for corroded steel strands via MMM technique

The quantitative evaluation of the residual load-carrying capacity of corroded cables is critical to the safe service of bridge structures. In this study, the locally corroded steel strands with different corrosion degrees were obtained through electrochemical experiments and the metal magnetic memory (MMM) signals were collected by the TSC-9 M-12 tester. The tensile failure tests of corroded steel strands were conducted by the WAW-1000 tensile testing machine. The distribution laws of the normal component Hz signals for steel strands in different corrosion cases were detailed and a three-dimensional (3D) magnetic dipole model of corroded steel strands was developed considering the end effects. The linear relationships between the magnetic characteristic quantity Hn and the corrosion cross-section loss rate As were revealed and verified by the theoretical and finite element simulation results. Based on the above relationships, a quantitative linear relationship between the residual load-carrying capacity F and the corrosion cross-section loss rate As of the corroded steel strand was proposed. The relative errors of the residual load-carrying capacity F calculated by the proposed quantitative method were overall within 20%. This study provides a basis for evaluating the residual load-carrying capacity of corroded cables using the MMM technique.

Field-free magnetization switching through large out-of-plane spin–orbit torque in the ferromagnetic CoPt single layers

Spin–orbit torque (SOT) induced magnetization switching in an energy-efficient and fast way has exhibited great application potential in next generation magnetic memories. However, a complicated layer structure is usually needed to break the mirror symmetry for achieving SOT induced field-free magnetization switching. Here, we report a sizeable field-free magnetization switching through large out-of-plane SOT in the chemically disordered A1-CoxPt100−x single layers within a Co composition range from 40 to 70. The largest absolute out-of-plane SOT efficiency is found at its equiatomic concentration (Co50Pt50), in which the absolute in-plane SOT efficiency also reaches the maximum value, 22.7 Oe/107 A cm−2. We further demonstrate that the symmetry dependence of field-free magnetization switching might arise from the chemically ordered L11-CoPt nano-scaled platelets formed during the sample deposition. We expect that the experimental identification of the field-free magnetization switching in the ferromagnetic CoPt single layer is desirable to simplify the applications of spin logic devices.

Artificial Neuron Based on the Bloch-Point Domain Wall in Ferromagnetic Nanowires.

Nanomagnetism and spintronics are currently active areas of research, with one of the main goals being the creation of low-energy-consuming magnetic memories based on nanomagnet switching. These types of devices could also be implemented in neuromorphic computing by crafting artificial neurons (ANs) that emulate the characteristics of biological neurons through the implementation of neuron models such as the widely used leaky integrate-and-fire (LIF) with a refractory period. In this study, we have carried out numerical simulations of a 120 nm diameter, 250 nm length ferromagnetic nanowire (NW) with the aim of exploring the design of an artificial neuron based on the creation and destruction of a Bloch-point domain wall. To replicate signal integration, we applied pulsed trains of spin currents to the opposite faces of the ferromagnetic NW. These pulsed currents (previously studied only in the continuous form) are responsible for inducing transitions between the stable single vortex (SV) state and the metastable Bloch point domain wall (BP-DW) state. To ensure the system exhibits leak and refractory properties, the NW was placed in a homogeneous magnetic field of the order of mT in the axial direction. The suggested configuration fulfills the requirements and characteristics of a biological neuron, potentially leading to the future creation of artificial neural networks (ANNs) based on reversible changes in the topology of magnetic NWs.

Optimizing preparation conditions and characterizing for CoxPt1-x alloy cylindrical nanowires fabricated by electrodeposition on nanoporous polycarbonate membranes

This study investigated the influence of the preparation conditions on the Co composition, crystal structure, and magnetic and electric conduction properties of cylindrical CoxPt1-x alloy nanowires having 0.08 ≤ x ≤ 0.90, which were electrodeposited on nanoporous templates. When prepared with an optimal concentration ratio of Co and Pt aqueous solutions and electrodeposition potentials, highly oriented crystal structures of fcc Co-Pt (111) was obtained in CoxPt1-x nanowires (0.58 ≤ x ≤ 0.80). Nanowires with (111)-oriented fcc Co-Pt crystals exhibited average lengths and diameters of 5.8–10.8 μm and 130 nm, respectively, and showed an easy axis of magnetocrystalline anisotropy in the longitudinal direction of the nanowire. These results indicate a direct correlation between the crystal structure and magnetic properties. The concentration ratio of Co and Pt in the electrolytes and the electrodeposition potentials for fabricating CoxPt1-x alloy nanowires using the diluted Co composition (0.08 ≤ x ≤ 0.25) were determined. These nanowires were polycrystalline and had a small saturation magnetization. Furthermore, the anisotropic magnetoresistance (AMR) for a single CoxPt1-x nanowire was measured to estimate the domain wall (DW) width in the single CoxPt1-x nanowire. The value of the magnetoresistance (MR) ratio for the single nanowire (0.58 ≤ x ≤ 0.80) with highly (111)-oriented fcc Co-Pt crystal was 0.082–0.23 %, and the estimated value of DW width was 17–25 nm. These findings provide valuable information for constructing three-dimensional magnetic memory devices.

Spin-Orbit Torque Vector Quantification in Nanoscale Magnetic Tunnel Junctions.

Spin-orbit torques (SOT) allow ultrafast, energy-efficient toggling of magnetization state by an in-plane charge current for applications such as magnetic random-access memory (SOT-MRAM). Tailoring the SOT vector comprising of antidamping (TAD) and fieldlike (TFL) torques could lead to faster, more reliable, and low-power SOT-MRAM. Here, we establish a method to quantify the longitudinal (TAD) and transverse (TFL) components of the SOT vector and its efficiency χAD and χFL, respectively, in nanoscale three-terminal SOT magnetic tunnel junctions (SOT-MTJ). Modulation of nucleation or switching field (BSF) for magnetization reversal by SOT effective fields (BSOT) leads to the modification of SOT-MTJ hysteresis loop behavior from which χAD and χFL are quantified. Surprisingly, in nanoscale W/CoFeB SOT-MTJ, we find χFL to be (i) twice as large as χAD and (ii) 6 times as large as χFL in micrometer-sized W/CoFeB Hall-bar devices. Our quantification is supported by micromagnetic and macrospin simulations which reproduce experimental SOT-MTJ Stoner-Wohlfarth astroid behavior only for χFL > χAD. Additionally, from the threshold current for current-induced magnetization switching with a transverse magnetic field, we show that in SOT-MTJ, TFL plays a more prominent role in magnetization dynamics than TAD. Due to SOT-MRAM geometry and nanodimensionality, the potential role of nonlocal spin Hall spin current accumulated adjacent to the SOT-MTJ in the mediation of TFL and χFL amplification merits to be explored.

Transport of skyrmions by surface acoustic waves

Magnetic skyrmions in thin films with perpendicular magnetic anisotropy are promising candidates for magnetic memory and logic devices, making the development of ways to transport skyrmions efficiently in a desired trajectory of significant interest. Here, we investigate the transport of skyrmions by surface acoustic waves (SAWs) via several modalities using micromagnetic simulations. We show skyrmion pinning sites created by standing SAWs at anti-nodes and skyrmion Hall-like motion without pinning driven by traveling SAWs. We also show how orthogonal SAWs formed by combining a longitudinal traveling SAW and a transverse standing SAW can be used for the 2D positioning of skyrmions. Our results also suggest SAWs offer a viable approach to the transport of multiple skyrmions along a multichannel racetrack.

Magnetic domain structures in ultrathin Bi2Te3/CrTe2 heterostructures

Abstract Chromium tellurium compounds are important two-dimensional van der Waals ferromagnetic materials with high Curie temperature and chemical stability in air, which is promising for applications in spintronic devices. Here, high-quality spin-orbital-torque (SOT) device, Bi2Te3/CrTe2 heterostructure was epitaxially grown on Al2O3 (0001) substrates. Anomalous Hall measurements indicate the existence of strong ferromagnetism in this device with the CrTe2 thickness down to 10 nm. In order to investigate its micromagnetic structure, cryogenic magnetic force microscope (MFM) was utilized to measure the magnetic domain evolutions at various temperatures and magnetic fields. The virgin domain state of the device shows a worm-like magnetic domain structure with the size around 0.6 ~ 0.8 µm. Larger irregular-shape magnetic domains (＞1µm) can be induced and pinned, after the field is increased to coercive field and ramped back to low fields. The temperature-dependent MFM signals exhibit a nice mean-field-like ferromagnetic transition with Curie temperature around 201.5 K, indicating a robust ferromagnetic ordering. Such device can be potentially implemented in future magnetic memory technology.

Unconventional magnetism mediated by spin-phonon-photon coupling

Magnetic order typically emerges due to the short-range exchange interaction between the constituent electronic spins. Recent discoveries have found a crucial role for spin-phonon coupling in various phenomena from optical ultrafast magnetization switching to dynamical control of the magnetic state. Here, we demonstrate theoretically the emergence of a biquadratic long-range interaction between spins mediated by their coupling to phonons hybridized with vacuum photons into polaritons. The resulting ordered state enabled by the exchange of virtual polaritons between spins is reminiscent of superconductivity mediated by the exchange of virtual phonons. The biquadratic nature of the spin-spin interaction promotes ordering without favoring ferro- or antiferromagnetism. It further makes the phase transition to magnetic order a first-order transition, unlike in conventional magnets. Consequently, a large magnetization develops abruptly on lowering the temperature which could enable magnetic memories admitting ultralow-power thermally-assisted writing while maintaining a high data stability. The role of photons in the phenomenon further enables an in-situ static control over the magnetism. These unique features make our predicted spin-spin interaction and magnetism highly unconventional paving the way for novel scientific and technological opportunities.

Highly Reliable Magnetic Memory-Based Physical Unclonable Functions.

Magnetic random-access memory (MRAM), which stores information through control of the magnetization direction, offers promising features as a viable nonvolatile memory alternative, including high endurance and successful large-scale commercialization. Recently, MRAM applications have extended beyond traditional memories, finding utility in emerging computing architectures such as in-memory computing and probabilistic bits. In this work, we report highly reliable MRAM-based security devices, known as physical unclonable functions (PUFs), achieved by exploiting nanoscale perpendicular magnetic tunnel junctions (MTJs). By intentionally randomizing the magnetization direction of the antiferromagnetically coupled reference layer of the MTJs, we successfully create an MRAM-PUF. The proposed PUF shows ideal uniformity and uniqueness and, in particular, maintains performance over a wide temperature range from -40 to +150 °C. Moreover, rigorous testing with more than 1584 challenge-response pairs of 64 bits each confirms resilience against machine learning attacks. These results, combined with the merits of commercialized MRAM technology, would facilitate the implementation of MRAM-PUFs.

Magnetization switching driven by spin current in a T-type ferromagnetic trilayer

The T-type CoFeB/spacer/CoFeB structure is a promising candidate for the development of perpendicular spin–orbit torque (SOT) magnetic random-access memory and other SOT devices. It consists of an in-plane magnetized layer, a perpendicularly magnetized layer, and a non-magnetic metal spacer that induces interlayer exchange coupling. By engineering the W spacer, this system achieves field-free SOT switching with a nearly 100% switching ratio. Furthermore, it realizes a high exchange coupling field of 255 Oe using a relatively thinner spacer thickness, enhancing the reliability and energy efficiency of SOT switching. Measurement of current switching probability suggests that this perpendicular magnetic anisotropy system may enable the implementation of probability-adjustable true random number generators in future applications. The T-type structures with strong interlayer coupling exhibit great potential for spintronic device applications.

Design and research on sequentially controlled magnetic-driven actuators with high stiffness by changing local EGaIn structural density

Magnetic-driven actuators (MDAs) are attracting increasing attention due to fast response and harmless interaction with humans. However, due to their low stiffness and difficulties in sequential magnetic control, MDAs are difficult to complete high-load tasks, and overcoming these obstacles can enable them to be exploited for a wide range of novel applications. In this work, sequentially controlled MDAs (SC-MDAs) with high stiffness were prepared by embedding a polydimethylsiloxane/liquid metal (PDMS/EGaIn) heating layer between magnetic memory polymer (MSMP) films. The variation of EGaIn structural density between different regions leads to temperature discrepancies, resulting in selective activation of specific regions for sequence control. The modulus of the optimized MSMP film at room temperature is 2.21 GPa, which exceeds that of other materials (about 1 MPa) by three orders of magnitude, demonstrating a substantial increase in stiffness. When the content of NdFeB is adjusted to 80 wt%, driving performance is adjusted by optimizing the length-thickness ratio of SC-MDAs. Each claw of the mechanical gripper exhibits a remarkable increase in load capacity, being able to carry 54 times its own weight, which is significantly higher compared to actuators made from other materials (about its weight). A hand-like actuator realizes the sequential control deformation by combining “fingers” with different heating properties.

Temperature effect on a weighted vortex spin-torque nano-oscillator for neuromorphic computing

In this work, we present fabricated magnetic tunnel junctions (MTJs) that can serve as magnetic memories (MMs) or vortex spin-torque nano-oscillators (STNOs) depending on the device geometry. We explore the heating effect on the devices to study how the performance of a neuromorphic computing system (NCS) consisting of MMs and STNOs can be enhanced by temperature. We further applied a neural network for waveform classification applications. The resistance of MMs represents the synaptic weights of the NCS, while temperature acts as an extra degree of freedom in changing the weights and TMR, as their anti-parallel resistance is temperature sensitive, and parallel resistance is temperature independent. Given the advantage of using heat for such a network, we envision using a vertical-cavity surface-emitting laser (VCSEL) to selectively heat MMs and/or STNO when needed. We found that when heating MMs only, STNO only, or both MMs and STNO, from 25 to 75 °C, the output power of the STNO increases by 24.7%, 72%, and 92.3%, respectively. Our study shows that temperature can be used to improve the output power of neural networks, and we intend to pave the way for future implementation of a low-area and high-speed VCSEL-assisted spintronic NCS.

Manufacturability Evaluation of Magnetic Tunnel Junction-Based Computational Random Access Memory

Manufacturability Evaluation of Magnetic Tunnel Junction-Based Computational Random Access Memory

Current-Induced Magnetization Switching Behavior in Perpendicular Magnetized L10-MnAl/B2-CoGa Bilayer

Rare-earth-free Mn-based binary alloy L10-MnAl with bulk perpendicular magnetic anisotropy (PMA) holds promise for high-performance magnetic random access memory (MRAM) devices driven by spin-orbit torque (SOT). However, the lattice-mismatch issue makes it challenging to place conventional spin current sources, such as heavy metals, between L10-MnAl layers and substrates. In this work, we propose a solution by using the B2-CoGa alloy as the spin current source. The lattice-matching enables high-quality epitaxial growth of 2-nm-thick L10-MnAl on B2-CoGa, and the L10-MnAl exhibits a large PMA constant of 1.04 × 106 J/m3. Subsequently, the considerable spin Hall effect in B2-CoGa enables the achievement of SOT-induced deterministic magnetization switching. Moreover, we quantitatively determine the SOT efficiency in the bilayer. Furthermore, we design an L10-MnAl/B2-CoGa/Co2MnGa structure to achieve field-free magnetic switching. Our results provide valuable insights for achieving high-performance SOT-MRAM devices based on L10-MnAl alloy.