Template nanowires for spintronics applications: nanomagnet microwave resonators functioning in zero applied magnetic field

Low-cost spintronic devices functioning in zero applied magnetic field are required for bringing the idea of spin-based electronics into the real-world industrial applications. Here we present first microwave measurements performed on nanomagnet devices fabricated by electrodeposition inside porous membranes. In the paper, we discuss in details a microwave resonator consisting of three nanomagnets, which functions in zero external magnetic field. By applying a microwave signal at a particular frequency, the magnetization of the middle nanomagnet experiences the ferromagnetic resonance (FMR), and the device outputs a measurable direct current (spin-torque diode effect). Alternatively, the nanodevice can be used as a microwave oscillator functioning in zero field. To test the resonators at microwave frequencies, we developed a simple measurement setup.     

Switching of magnetization by nonlinear resonance studied in single nanoparticles

Magnetization reversal in magnetic particles is one of the fundamental issues in magnetic data storage. Technological improvements require the understanding of dynamical magnetization reversal processes at nanosecond time scales. New strategies are needed to overcome current limitations. For example, the problem of thermal stability of the magnetization state (superparamagnetic limit) can be pushed down to smaller particle sizes by increasing the magnetic anisotropy. High fields are then needed to reverse the magnetization, which are difficult to achieve in current devices. Here we propose a new method to overcome this limitation. A constant applied field, well below the switching field, combined with a radio-frequency (RF) field pulse can reverse the magnetization of a nanoparticle. The efficiency of this method is demonstrated on a 20-nm-diameter cobalt particle by using the microSQUID (superconducting quantum interference device) technique. Other applications of this method might be nucleation or depinning of domain walls.     

Substantial reduction of critical current for magnetization switching in an exchange-biased spin valve

Great interest in current-induced magnetic excitation and switching in a magnetic nanopillar has been caused by the theoretical predictions of these phenomena. The concept of using a spin-polarized current to switch the magnetization orientation of a magnetic layer provides a possible way to realize future 'current-driven' devices: in such devices, direct switching of the magnetic memory bits would be produced by a local current application, instead of by a magnetic field generated by attached wires. Until now, all the reported work on current-induced magnetization switching has been concentrated on a simple ferromagnet/Cu/ferromagnet trilayer. Here we report the observation of current-induced magnetization switching in exchange-biased spin valves (ESPVs) at room temperature. The ESPVs clearly show current-induced magnetization switching behaviour under a sweeping direct current with a very high density. We show that insertion of a ruthenium layer between an ESPV nanopillar and the top electrode effectively decreases the critical current density from about 10(8) to 10(7) A cm(-2). In a well-designed 'antisymmetric' ESPV structure, this critical current density can be further reduced to 2 x 10(6) A cm(-2). We believe that the substantial reduction of critical current could make it possible for current-induced magnetization switching to be directly applied in spintronic devices, such as magnetic random-access memory.     

Magnetostatic interactions in antiferromagnetically coupled patterned media

In an array of closely spaced magnetic islands as in patterned media, magnetostatic interactions play a major role in widening the switching field distribution and reducing the thermal stability. Patterned antiferromagnetically coupled (AFC) media provide interesting systems for studying the effect of magnetostatic interactions on the reversal of closely spaced AFC bits in an array, as AFC structure helps to reduce the remanent magnetization (M(r)), leading to reduced magnetostatic interactions. Here, we study the magnetic reversal of single domain-patterned AFC CoCrPt:oxide bilayer system with perpendicular magnetic anisotropy, by imaging the remanence state of the bits after the application of a magnetic field with magnetic force microscopy (MFM). The influence of magnetostatic fields from the neighboring bits on the switching field distribution (SFD) for an entity in a patterned media is studied by varying the stabilizing layer thickness of the AFC structure and bit spacing. We observe a distinct increase in stability and coercivity with an increase in stabilizing layer thickness for the 40 nm spaced bits. This demonstrates the effectiveness of the AFC structure for reducing magnetostatic interactions in patterned media, such that high thermal stability can be achieved by the reduced M(r), without writability issues.     

Mechanically induced deterministic 180° switching in nanomagnets

The mechanically induced magnetization switching in nanomagnets is studied by a constraint-free phase field model, which permits exactly constant magnetization magnitude and explicit magneto-mechanical coupling. Depending on the geometry of the nanomagnets, there exist two distinct switching modes: one is the coherent mode where the magnetization vector remains homogeneous during the switching, and the other is the incoherent mode where heterogeneous magnetization distribution occurs. For the application of nanomagnets-based logic and memory devices, the coherent mode is of great interest. Results show that a deterministic 180° switching can happen if mechanical loading is removed once the magnetization rotates to the largest switching angle. The switching time decreases with the magnitude of the applied strain. In addition, the 180° switching under a combination of magnetic field and mechanical strain is also investigated. Simulations demonstrate that an optimum additional strain to reduce the switching time is around 0.2%. The mechanically induced switching is further shown to be damping dependent. A larger damping coefficient is favorable for a faster switching only when the deterministic 180° switching can be guaranteed. This work provides a foundation for the study of mechanically driven/assisted nanomagnets-based logic and memory devices.     

Field‐Free Programmable Spin Logics via Chirality‐Reversible Spin–Orbit Torque Switching

Spin–orbit torque (SOT)‐induced magnetization switching exhibits chirality (clockwise or counterclockwise), which offers the prospect of programmable spin‐logic devices integrating nonvolatile spintronic memory cells with logic functions. Chirality is usually fixed by an applied or effective magnetic field in reported studies. Herein, utilizing an in‐plane magnetic layer that is also switchable by SOT, the chirality of a perpendicular magnetic layer that is exchange‐coupled with the in‐plane layer can be reversed in a purely electrical way. In a single Hall bar device designed from this multilayer structure, three logic gates including AND, NAND, and NOT are reconfigured, which opens a gateway toward practical programmable spin‐logic devices.     

Spin–Orbit Torque Switching in a Nearly Compensated Heusler Ferrimagnet

Ferrimagnetic materials combine the advantages of the low magnetic moment of an antiferromagnet and the ease of realizing magnetic reading of a ferromagnet. Recently, it was demonstrated that compensated ferrimagnetic half metals can be realized in Heusler alloys, where high spin polarization, zero magnetic moment, and low magnetic damping can be achieved at the same time. In this work, by studying the spin–orbit torque induced switching in the Heusler alloy Mn₂Ru_{1? x} Ga, it is found that efficient current‐induced magnetic switching can be realized in a nearly compensated sample with strong perpendicular anisotropy and large film thickness. This work demonstrates the possibility of employing compensated Heusler alloys for fast, energy‐efficient spintronic devices.     

Field dependence of switching currents in an exchange biased spin valve

Current induced magnetic reversal due to spin transfer torque is a promising candidate in advanced information storage technology. It has been intensively studied. This work reports the field-dependence of switching-currents for current induced magnetization switching in a uncoupled nano-sized cobalt-based spin valve of exchange biased type. The dependency is investigated in hysteretic regime at room temperature, in comparison with that of a trilayer simple spin valve. In the simple spin valve, the switching currents behave to the positive and the negative applied magnetic field symmetrically. In the exchange biased type, in contrast, the switching currents respond to the negative field in a quite unusual and different manner than to the positive field. A negative magnetic field then can shift the switching-currents into either negative or positive current range, dependently on whether a parallel or an antiparallel state of the spin valve was produced by that field. This different character of switching currents in the negative field range can be explained by the effect of the exchange bias pinning field on the spin-polarizer (the fixed Co layer) of the exchange biased spin valve. That unidirectional pinning filed could suppress the thermal magnetization fluctuation in the spin-polarizer, leading to a higher spin polarization of the current, and hence a lower switching current density than in the simple spin valve.     

硅纳米管的理论及制备研究进展

硅纳米管在晶体管等纳米电子器件、传感器、场发射显示器件、纳米磁性器件及光电器件、储氢及电化学等领域有着广阔的应用前景.综述了近年来硅纳米管在储氢能力、热稳定性、量子限制行为及力学性能等理论方面的研究成果,以及采用水热法、模板法、电化学溶液沉积过程、化学气相沉积法、热蒸发、电弧法及激光烧蚀等方法制备单晶、多晶及非晶硅纳米管的最新研究进展,并提出了硅纳米管可能的研究方向.     

Broad-band noise spectroscopy of giant magnetoresistive read heads

This paper describes wide-band (1 MHz-10 GHz) thermal magnetic fluctuation (TMF) noise measurements conducted on magnetoresistive devices such as giant magnetoresistive (GMR) and tunnel junction magnetoresistive (TMR) heads. The paper discusses the instrumentation, as well as the corrections necessary to separate the TMF noise from the total noise as collected. It shows that the thermal electrical noise component of this total noise is not white, but approximately follows the small-signal resistance of a saturated sensor versus frequency. The paper gives examples of noise imaging using magnetic field sweeps and electrical bias sweeps, and interprets the results. As another application example, it shows that the signal-to-noise ratio of a GMR head does not improve if one replaces the free layer by a synthetic (double) free layer with the same net moment. It presents examples of multimode TMF spectra, together with the causes of these extra resonances: spatial standing spin waves in the free layer and pinned layer resonances. It shows how one can calculate the stiffening field, damping, and free layer dimensions (magnetically active height and width) from the TMF spectra. These parameters can be obtained in a nondestructive fashion. They are difficult to measure in other ways.     

Resistive switching induced by metallic filaments formation through poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate)

We report the design and fabrication of Al/poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/Cu resistive memory devices that utilize the Cu redox reaction and conformational features of PEDOT:PSS to achieve resistive switching. The top Cu electrode acts as the source of the redox ions that are injected through the PEDOT:PSS layer during the forming process. The Cu filament is confirmed directly using the cross-sectional images of transmission electron microscopy and energy-dispersive X-ray spectroscopy. The resultant resistive memory devices can operate over a small voltage range, i.e., the switching-on threshold voltage is less than 1.5 V and the absolute value of the switching-off threshold voltage is less than 1.0 V. The on/off current ratio is as large as 1 × 10(4) and the two different resistance states can be maintained over 10(6) s. Moreover, the devices present good thermal stability that the resistive switching can be observed even at temperature up to 160 °C, at which the oxidation of the Cu top electrode is the failure factor. Furthermore, the cause of failure for Al/PEDOT:PSS/Cu memory devices at higher temperature is confirmed to be the oxidation of Cu top electrode.     

Nanocomputing by field-coupled nanomagnets

Demonstrates through simulations the feasibility of using magnetically coupled nanometer-scale ferromagnetic dots for digital information processing. Microelectronic circuits provide the input and output of the magnetic nanostructure, but the signal is processed via magnetic dot-dot interactions. Logic functions can be defined by the proper placements of dots. We introduce a SPICE macromodel of interacting nanomagnets and use this tool to design and simulate the proposed nanomagnet logic units. This SPICE model allows us to simulate such magnetic information processing devices within the same framework as conventional electronic circuits.     

A direct metal transfer method for cross-bar type polymer non-volatile memory applications

Polymer non-volatile memory devices in 8 × 8 array cross-bar architecture were fabricated by a non-aqueous direct metal transfer (DMT) method using a two-step thermal treatment. Top electrodes with a linewidth of 2?μm were transferred onto the polymer layer by the DMT method. The switching behaviour of memory devices fabricated by the DMT method was very similar to that of devices fabricated by the conventional shadow mask method. The devices fabricated using the DMT method showed three orders of magnitude of on/off ratio with stable resistance switching, demonstrating that the DMT method can be a simple process to fabricate organic memory array devices.     

双沟道层对氮掺杂非晶氧化铟锌薄膜晶体管的影响

采用射频磁控溅射法, 在热氧化p型硅基片上制备了双沟道层非晶氧化铟锌(a-IZO)和氮掺杂氧化铟锌(a-IZON)薄膜晶体管(TFTs), 并研究了双沟道层对器件电学性能和温度稳定性的影响。研究发现, a-IZO/IZON双沟道层TFTs具有较高的场效应迁移率, 为23.26 cm²/(V•s), 并且其阈值电压相较于单层a-IZO-TFTs正向偏移。这是由于氮掺杂可以减少沟道层中的氧空位, 抑制载流子浓度, 使器件具有更好的阈值电压。而a-IZO层避免了由于氮掺杂导致的场效应迁移率和开态电流的下降, 提升了器件的电流开关比。从298 K至423 K的器件转移特性曲线中发现, 双沟道层器件相较于单沟道层器件的温度稳定性更佳, 这可归因于a-IZON层的保护作用。氮掺杂可以减少氧在背沟道层表面的吸收/解吸反应, 改善器件的稳定性。     

磁约束阻尼的减振机理

采用在约束层端部上设置永磁体的新方法可使阻尼层获得比传统约束阻尼处理方法更高的剪应变，从而增强粘弹层的阻尼耗能。本文应用Hamilton原理，考虑永磁体的影响，推导了对称双层夹心悬臂梁的运动方程，对模型进行了实验验证；分析了一阶共振时，永磁体对共振振幅、阻尼层剪应变、约束层作用力的影响，阐明了新型的磁机敏约束阻尼方法的减振机理。     

磁记录用磁性颗粒的临界尺寸及其影响因素

信息存储用磁性介质的记录面密度的不断提高,主要途径之一就是减小磁性颗粒的尺寸.但当磁性粒子减小到一定尺寸后,其磁化方向极易改变(超顺磁性),使所记录的信息丢失,不能再用于磁记录.因此降低磁性颗粒的临界尺寸并提高其热稳定性是信息记录高密度化的关键.就磁记录用磁性颗粒的临界尺寸及其主要影响因素进行了论述.     

Heat Capacity and Thermal Damping Properties of Spin-Crossover Molecules: A New Look at an Old Topic

The thermally induced spin-crossover (SCO) phenomenon in transition metal complexes is an entropy-driven process, which has been extensively studied through calorimetric methods. Yet, the excess heat capacity associated with the molecular spin-state switching has never been explored for practical applications. Herein, the thermal damping effect of an SCO film is experimentally assessed by monitoring the transient heating response of SCO-coated metallic microwires, Joule-heated by current pulses. A damping of the wire temperature, up to 10%, is evidenced on a time scale of tens of microseconds due to the spin-state switching of the molecular film. Fast heat-charging dynamics and negligible fatigability are demonstrated, which, together with the solid-solid nature of the spin transition, appear as promising features for achieving thermal energy management applications in functional devices.     

Bulk Spin Torque-Driven Perpendicular Magnetization Switching in L10 FePt Single Layer

Due to its inherent superior perpendicular magnetocrystalline anisotropy, the FePt in L1₀ phase enables magnetic storage and memory devices with ultrahigh capacity. However, reversing the FePt magnetic state, and therefore encoding information, has proven to be extremely difficult. Here, it is demonstrated that an electric current can exert a large spin torque on an L1₀ FePt magnet, ultimately leading to reversible magnetization switching. The spin torque monotonically increases with increasing FePt thickness, exhibiting a bulk characteristic. Meanwhile, the spin torque effective fields and switching efficiency increase as the FePt approaches higher chemical ordering with stronger spin–orbit coupling. The symmetry breaking that generates spin torque within L1₀ FePt is shown to arise from an inherent structural gradient along the film normal direction. By artificially reversing the structural gradient, an opposite spin torque effect in L1₀ FePt is demonstrated. At last, the role of the disorder gradient in generating a substantial torque in a single ferromagnet is supported by theoretical calculations. These results will push forward the frontier of material systems for generating spin torques and will have a transformative impact on magnetic storage and spin memory devices with simple architecture, ultrahigh density, and readily application.     

A link between the coercivity and microstructure of high moment Fe films and their use in magnetic tunnel junctions

Magnetron sputtered single Fe films have been “softened” magnetically by controlled N-doping during the sputter deposition. This technique allows a reduction in grain size and coercivity of the Fe films, without decreasing the saturation magnetization and without the formation of any crystalline FeN phases. We describe this effect through a modification of the random magnetocrystalline anisotropy model, by taking the film thickness into account. The coercivities calculated in this way are in good agreement with those obtained experimentally.It is demonstrated that N-doping can be applied beneficially to control the switching field of the ‘free’ layer in magnetic trilayer films of the MTJ type. It is thus possible to construct an all Fe-electrode magnetic tunnel junction (MTJ) that displays the tunneling magnetoresistance (TMR) effect by altering the switching field of one Fe layer using N-doping. The ability to control the magnetic softness of high magnetic moment materials is important in regard to their incorporation into TMR devices.