Ten States of Nonvolatile Memory through Engineering Ferromagnetic Remanent Magnetization

Emerging nonvolatile multilevel memory devices have been regarded as a promising solution to meet the increasing demand of high‐density memory with low‐power consumption. In particular, decimal system of the new computers instead of binary system could be developed if ten nonvolatile states are realized. Here, a general remanent magnetism engineering method is proposed for realizing multiple reliable magnetic and resistance states, not depending on a specific material or device structure. Especially, as a proof‐of‐concept demonstration, ten states of nonvolatile memory based on the manipulation of ferromagnetic remanent magnetization have been revealed in both Co/Pt magnetic multilayers with strong perpendicular magnetic anisotropy and MgO‐based magnetic tunneling junctions at room temperature. Considering ferromagnets have been one of the key factors that enabled the information revolution from its inception, this state‐of‐the‐art remanent magnetism engineering approach has a very broad application prospect in the field of spintronics.     

Tailoring Multilevel‐Stable Remanence States in Exchange‐Biased System through Spin‐Orbit Torque

Multilevel remanence states have potential applications in ultra‐high‐density storage and neuromorphic computing. Continuous tailoring of the multilevel remanence states by spin‐orbit torque (SOT) is reported in perpendicularly magnetized Pt/Co/IrMn heterostructures. Double‐biased hysteresis loops with only one remanence state can be tuned from the positively or negatively single‐biased loops by SOT controlled sign of the exchange‐bias field. The remanence states associated with the heights of the sub‐loops are continually changed by tuning the ratio of the positively and negatively oriented ferromagnetic domains. The multilevel storage cells are demonstrated by reading the remanent Hall resistance through changing the sign and/or the magnitude of current pulse. The synaptic plasticity behaviors for neuromorphic computing are also simulated by varying the remanent Hall resistance under the consecutive current pulses. This work demonstrates that SOT is an effective method to tailor the remanence states in the double‐biased heavy metal/ferromagnetic/antiferromagnetic system. The multilevel‐stable remanence states driven by SOT show potential applications in future multilevel memories and neuromorphic computing devices.     

Tuning a Binary Ferromagnet into a Multistate Synapse with Spin–Orbit‐Torque‐Induced Plasticity

Ferromagnets with binary states are limited for applications as artificial synapses for neuromorphic computing. Here, it is shown how synaptic plasticity of a perpendicular ferromagnetic layer (FM1) can be obtained when it is interlayer exchange‐coupled by another in‐plane ferromagnetic layer (FM2), where a magnetic field‐free current‐driven multistate magnetization switching of FM1 in the Pt/FM1/Ta/FM2 structure is induced by spin–orbit torque. Current pulses are used to set the perpendicular magnetization state, which acts as the synapse weight, and spintronic implementation of the excitatory/inhibitory postsynaptic potentials and spike timing‐dependent plasticity are demonstrated. This functionality is made possible by the action of the in‐plane interlayer exchange coupling field which leads to broadened, multistate magnetic reversal characteristics. Numerical simulations, combined with investigations of a reference sample with a single perpendicular magnetized Pt/FM1/Ta structure, reveal that the broadening is due to the in‐plane field component tuning the efficiency of the spin–orbit torque to drive domain walls across a landscape of varying pinning potentials. The conventionally binary FM1 inside the Pt/FM1/Ta/FM2 structure with an inherent in‐plane coupling field is therefore tuned into a multistate perpendicular ferromagnet and represents a synaptic emulator for neuromorphic computing, demonstrating a significant pathway toward a combination of spintronics and synaptic electronics.     

Ferroelectric Domain Wall Memristor

A domain wall‐enabled memristor is created, in thin film lithium niobate capacitors, which shows up to twelve orders of magnitude variation in resistance. Such dramatic changes are caused by the injection of strongly inclined conducting ferroelectric domain walls, which provide conduits for current flow between electrodes. Varying the magnitude of the applied electric‐field pulse, used to induce switching, alters the extent to which polarization reversal occurs; this systematically changes the density of the injected conducting domain walls in the ferroelectric layer and hence the resistivity of the capacitor structure as a whole. Hundreds of distinct conductance states can be produced, with current maxima achieved around the coercive voltage, where domain wall density is greatest, and minima associated with the almost fully switched ferroelectric (few domain walls). Significantly, this “domain wall memristor” demonstrates a plasticity effect: when a succession of voltage pulses of constant magnitude is applied, the resistance changes. Resistance plasticity opens the way for the domain wall memristor to be considered for artificial synapse applications in neuromorphic circuits.     

低矫顽力GMR磁传感器及其单畴模型的研究

在硅片上制备结构为Ta/NiFeCr/NiFe/CoFe/Cu/CoFe/IrMn/Ta的IrMn顶钉扎自旋阀薄膜,并最终制成了一组基于此自旋阀结构的GMR磁传感器芯片。利用弱磁场下的退火工艺,改变薄膜易磁化轴的方向,当退火温度为150℃、外加磁场为120Oe时,GMR芯片的矫顽力可以降至0.2Oe以下。同时建立了一种自旋阀自由层的单畴模型,用以解释这一退火效应。利用Mat-lab计算GMR芯片的Meff-H曲线,所得到的计算结果与实验结果一致。所以,自旋阀自由层易磁化轴的方向与GMR磁传感器的性能有着密切的关系。     

Enhanced Magnetic Anisotropy for Reprogrammable High-Force-Density Microactuators

The precise control of magnetic properties at the microscale has transformative potential in healthcare and human-robot interaction. This research focuses on understanding the magnetic interactions in nanostructure assemblies responsible for microactuation. By combining experimental measurements and micromagnetic simulations, the interactions in both nanocube and nanochain assemblies are elucidated. Hysteresis measurements and first-order reversal curves (FORC) reveal that the spatial arrangement of these assemblies governs their collective magnetism. A critical concentration threshold is observed where a transition from ferromagnetic-like to antiferromagnetic-like coupling occurs. Leveraging the high uniaxial anisotropy of 1D nanochains, the remanent magnetization of assembled chain structures is maximized for efficient magneto-mechanical energy transduction. By utilizing an optimized magnetic nanostructure concentration, a flexible film is fabricated, and its significantly enhanced mechanical deformation response to a small magnetic field, surpassing conventional particle-based samples by a factor of five, is demonstrated. Demonstrating excellent transduction efficiency, visible deformations such as bending and S-shaped twisting modes are achieved with an applied field of less than 400 Oe. Furthermore, the reprogrammability of the actuator, achieving a U-shaped bending mode by altering its magnetization profile, is showcased. This research provides valuable insights for designing reconfigurable and effective microactuators and devices at significantly smaller scales than previously possible.     

Anti‐Ferromagnet Controlled Tunneling Magnetoresistance

The requirement for high‐density memory integration advances the development of newly structured spintronic devices, which have reduced stray fields and are insensitive to magnetic field perturbations. This could be visualized in magnetic tunnel junctions incorporating anti‐ferromagnetic instead of ferromagnetic electrodes. Here, room‐temperature anti‐ferromangnet (AFM)‐controlled tunneling anisotropic magnetoresistance in a novel perpendicular junction is reported, where the IrMn AFM stays immediately at both sides of AlO_x tunnel barrier as the functional layers. Bi‐stable resistance states governed by the relative arrangement of uncompensated anti‐ferromagnetic IrMn moments are obtained here, rather than the traditional spin‐valve signal observed in ferromagnet‐based tunnel junctions. The experimental observation of room‐temperature tunneling magnetoresistance controlled directly by AFM is practically significant and may pave the way for new‐generation memories based on AFM spintronics.     

A novel magnetic random access memory design using square ring elements for the hard layer

The behavior of narrow permalloy square rings under the influence of a magnetic field was studied using magnetic force microscopy (MFM). Two stable states of opposite polarity at remanence and simple switching were observed. We propose a design for the hard layer of magnetic random access memory (MRAM) that uses these states in square rings for data storage.     

Ferrimagnetic Particles in the Lung Part 1: The Magnetizing Process

The magnetization process of ferrimagnetic particles in the lung takes place when an external magnetic field is applied to the lung; the particles both become magnetized and rotate toward alignment with the applied field. The purpose here is to consolidate previous and new measurements of this process, for Fe3O4 particles in the human lung, and to interpret the measurements. The particular quantity measured was Bn, the remanent magnetic field at the chest produced by the particles after removal of the applied field. Bn was measured as a function of three variables. The first two are the strength and duration of the applied field, yielding the remanent magnetization curve and the viscosity curve respectively; these were measured both for particles initially unmagnetized and initialiy magnetized. The curves are qualitatively explained by a simple physics model consisting of single-domain and multidomain particles; these reside in a viscous fluid and experience physiological impulses (relaxation) which oppose the alignment during magnetization. The third variable is the state of breathing; Bn was measured when normal breathing was switched to exhale-and-hold during either magnetization or measurement. The effects on Bn of this switch are explained by considering the alveolar walls; forced exhalation is assumed to "crumple" the walls, thereby realigning the particles contained in the alveoli.     

Catalytic materials manufactured by the polyol process for monolithic dye‐sensitized solar cells

Three types of screen‐printable catalytic pastes were successfully prepared to be used as counterelectrode for monolithic dye solar cells encapsulated with glass frit. The electroless bottom‐up method or so‐called polyol process has been applied to fabricate thermally stable SnO₂:Sb/Pt and carbon black/Pt nanocomposites. The catalytic and electric properties of these materials were compared with a new platinum‐free type of carbon counterelectrode. The layers containing low platinum amounts (less than 5 µg/cm²) exhibit a very low charge transfer resistance of about 0·4 Ω · cm². Also the conductive carbon layer shows an acceptable charge transfer resistance of 1·6 Ω · cm². Additionally the catalytic layer containing porous carbon black reveals excellent sheet resistance below 5 Ω/□; this feature has enabled to work out a low cost counterelectrode which combined suitable catalytic and conductive properties. The layers have been characterized using following methods: electrochemical impedance spectroscopy (EIS), field emission scanning electron microscopy (FE‐SEM), energy filter transmission electron microscopy (EF‐TEM) and inductively coupled plasma mass spectroscopy (ICP‐MS). Copyright © 2008 John Wiley & Sons, Ltd.     

Giant magnetoresistance materials for read heads

The sensitivity of magnetoresistive read heads can be increased by using layered magnetic materials showing the giant magnetoresistance effect, instead of a single magnetic film showing the anisotropic magnetoresistance effect. For this purpose, exchange-biased spin-valve layered structures are very suitable. For well-chosen compositions and nanometer-scale layer thicknesses these materials combine a fair giant magnetoresistance effect with a very small field interval in which the resistance change takes place. In this paper we give an overview of aspects which determine the functioning of materials of this class in read heads, including their preparation, magnetotransport properties and the magnetic interactions which determine the magnetization reversal process.     

超细Co/Cu纳米多层膜磁性研究

利用磁控溅射方法制备了纳米Co/Cu多层膜。利用扫描探针显微镜(SPM)观测了其表面形貌和磁畴结构,并通过振动样品磁强计(VSM)测量了磁性。结果表明,薄膜的微结构和磁性随非磁性层厚度的变化有着非常显著的变化。超细Co颗粒构造的多层膜样品,颗粒尺寸逐渐增大,磁畴尺寸先减小后增大,最后发生明显的聚集。磁性金属和非磁性金属的比例对多层膜之间的交换耦合相互作用有显著影响。平行膜面方向上的饱和场明显小于垂直膜面方向。当体积比约为1∶80时,平行膜面方向饱和场为95.9 kA/m,垂直方向饱和场为328.1 kA/m。此时两个方向上的饱和场、剩磁、矫顽力和磁滞损耗均为最小值。     

Controlling Chiral Spin States of a Triangular‐Lattice Magnet by Cooling in a Magnetic Field

Magnetic materials with a non‐collinear and non‐coplanar arrangement of magnetic moments hosting a nonzero scalar spin‐chirality exhibit unique magnetic and spin‐dependent electronic transport properties. The spin chirality often occurs in materials where competing exchange interactions lead to geometrical frustrations between magnetic moments and to a strong coupling between the crystal lattice and the magnetic structure. These characteristics are particularly strong in Mn‐based antiperovskites where the interactions and chirality can be tuned by substitutional modifications of the crystalline lattice. This study presents evidence for the formation of two unequal chiral spin states in magnetically ordered Mn_3.338Ni_0.651N antiperovskite based on density functional theory calculations and supported by magnetization measurements after cooling in a magnetic field. The existence of two scalar spin‐chiralities of opposite sign and different magnitude is demonstrated by a vertical shift of the magnetic‐field dependent magnetization and Hall effect at low fields and from an asymmetrical magnetoresistivity when the applied magnetic field is oriented parallel or antiparallel to the direction of the cooling field. This opens up the possibility of manipulating the spin chirality for potential use in the emerging field of chiral spintronics.     

Spin‐Structured Multilayers: A New Class of Materials for Precision Spintronics

Magnetoelectronic multilayer devices are widely used in today's information and sensor technology. Their functionality, however, is limited by the inherent properties of magnetic exchange or dipolar coupling which constrain possible spin configurations to collinear or perpendicular alignments of adjacent layers. Here, a deposition procedure is introduced that allows for a new class of layered materials in which complex spin structures can be accurately designed to result in a multitude of new and precisely adjustable spintronic and magnetoresistive properties. The magnetization direction and coercivity of each individual layer are determined by the deposition process in oblique incidence geometry and can be completely decoupled from neighboring layers. This applies for layers of any ferromagnetic material down to layer thicknesses of a few nm and lateral dimensions of a few 100 nm, enabling the design of efficient and compact magnetoelectronic devices, encompassing precision magnetoresistive sensors as well as layer systems with multiple addressable remanent states for magnetic memory applications.     

Surfactant‐Free,Self‐Assembled PVA‐Iron Oxide/Silica Core–Shell Nanocarriers for Highly Sensitive,Magnetically Controlled Drug Release and Ultrahigh Cancer Cell Uptake Efficiency

Surfactant‐free, self‐assembled iron oxide/silica core–shell (SAIO@SiO₂) nanocarriers were synthesized as bifunctional magnetic vectors that can be triggered for the controlled release of therapeutic agents by an external magnetic field. In addition, drug release profiles can be well‐regulated through an ultrathin layer of silica shell. The hydrophobic drug molecules were encapsulated within the iron oxide‐PVA core and then further covered with a thin‐layer silica shell to regulate the release pattern. Remote control of drug release from the SAIO@SiO₂ nanocarriers was achieved successfully using an external magnetic field where the core phase being structurally disintegrated to a certain extent while subjected to magnetic stimulus, resulting in a burst release of the encapsulated drug. However, a relatively slow and linear release restored immediately, directly after removal of the stimulus. The nanostructural evolution of the nanocarriers upon the stimulus was examined and the mechanism for controlled drug release is proposed for such a core–shell nanocarrier. Surprisingly, the surfactant‐free SAIO@SiO₂ nanocarriers demonstrated a relatively high uptake efficiency from the HeLa cell line. Together with a well‐regulated controlled release design, the nanocarriers may provide great advantages as an effective cell‐based drug delivery nanosystem for biomedical applications.     

Switching of a superconductive thin-film bridge between the pure superconductive and resistive-superconductive states

Experimental results are presented showing that a superconductive thin-film bridge has two stable states, and it is possible to switch from one to the other with the aid of an applied magnetic field.     

High‐Speed Organic Single‐Crystal Transistor Responding to Very High Frequency Band

Although high carrier mobility organic field‐effect transistors (OFETs) are required for high‐speed device applications, improving the carrier mobility alone does not lead to high‐speed operation. Because the cut‐off frequency is determined predominantly by the total resistance and parasitic capacitance of a transistor, it is necessary to miniaturize OFETs while reducing these factors. Depositing a dopant layer only at the metal/semiconductor interface is an effective technique to reduce the contact resistance. However, fine‐patterning techniques for a dopant layer are still challenging especially for a top‐contact solution‐processed OFET geometry because organic semiconductors are vulnerable to chemical damage by solvents. In this work, high‐resolution, damage‐free patterning of a dopant layer is developed to fabricate short‐channel OFETs with a dopant interlayer inserted at the contacts. The fabricated OFETs exhibit high mobility exceeding 10 cm² V^?1 s^?1 together with a reasonably low contact resistance, allowing for high frequency operation at 38 MHz. In addition, a diode‐connected OFET shows a rectifying capability of up to 78 MHz at an applied voltage of 5 V. This shows that an OFET can respond to the very high frequency band, which is beneficial for long‐distance wireless communication.     

Photovoltaic Processes of Singlet and Triplet Excited States in Organic Solar Cells

This article reports the respective photovoltaic processes of singlet and triplet photoexcited states in dissociation and charge reactions based on the studies of magnetic‐field effects of photocurrents. The magnetic‐field effects of photocurrents reveal that weak donor‐acceptor interactions lead to a two‐step photovoltaic process: dissociation in polaron‐pair states evolved from singlet excitonic states and exciton‐charge reactions occurred in triplet excitonic states in the generation of the photocurrent. However, strong donor‐acceptor interactions yield a one‐step photovoltaic process: direct dissociation of both singlet and triplet excitons in bulk‐heterojunction organic solar cells. In addition, the magnetic‐field effects of photocurrents indicate that the dissociated electrons and holes form charge‐transfer complexes with singlet and triplet spin configurations at donor‐acceptor intermolecular interfaces. As a result, the magnetic‐field effects of photocurrents can deliver a critical understanding of singlet and triplet photovoltaic processes to design advanced solar‐energy materials and devices.     

Correction factor for resistivity and Hall mobility measurement of two-layer structures by spreading resistance techniques

The spreading resistance of a circular disc contact located on a two-layer semiconductor structure, and its relative change due to magnetic field has been determined, with the magnetic field perpendicular to the contact face, and with the physical magnetoresistance zero in both layers. The correct value of the spreading resistance differs from the result obtained by the usual approximation, especially if the contact diameter is comparable with the thickness of the upper layer. The conditions are given when the relative change in resistance due to the magnetic field is determined by the mobility in the upper layer only. The results show the applicability of the spreading magnetoresistance technique in determining the Hall mobility in a two-layer structure.     

Photoquantum Hall Effect and Light‐Induced Charge Transfer at the Interface of Graphene/InSe Heterostructures

The transfer of electronic charge across the interface of two van der Waals crystals can underpin the operation of a new class of functional devices. Among van der Waals semiconductors, an exciting and rapidly growing development involves the “post‐transition” metal chalcogenide InSe. Here, field effect phototransistors are reported where single layer graphene is capped with n‐type InSe. These device structures combine the photosensitivity of InSe with the unique electrical properties of graphene. It is shown that the light‐induced transfer of charge between InSe and graphene offers an effective method to increase or decrease the carrier density in graphene, causing a change in its resistance that is gate‐controllable and only weakly dependent on temperature. The charge transfer at the InSe/graphene interface is probed by Hall effect and photoconductivity measurmentes and it is demonstrated that light can induce a sign reversal of the quantum Hall voltage and photovoltaic effects in the graphene layer. These findings demonstrate the potential of light‐induced charge transfer in gate‐tunable InSe/graphene phototransistors for optoelectronics and quantum metrology.