Direction Of Electron Flow Research Articles

Study of A Phenomenon STT (Spin Transfer Torque) on the Material La0.7Sr0.3MnO3 Shaped Nanowire Using Micromagnetic Simulation

STT (Spin Transfer Torque) can be referred to as a process of manipulation and control of spin current in the field of spintronics. When the material is ferromagnetic nanowire La0.7Sr0.3MnO3injected currents will move the domain wall with accompanying changes of spin currents. In mikromagnetik simulation shows that the application is capable of producing flow velocity or pressure of domain wall in the direction of electron flow. The domain wall pressure generating magnetization changes with increasing current density occurs. To that end, the simulation research was done in order to obtain the effect of the injection of electric current to the magnetization of the material. This phenomenon is simulated by modeling the material into the 3D geometry. The greater the current density is given the domain wall velocity or pressure on the nanowire faster so that the magnetization process is also faster. Changes in the velocity of the fastest domain wall is obtained when the material is injected with a current density as well as M-t get a graph showing oscillation pattern that is denser when the current is increased. Furthermore, the total energy analysis with variations in size diameter of 10 nm, 20 nm and 30 nm. The results show that with increasing diameter, total energy tends to increase. Keywords: spin transfer torque, La0.7Sr0.3MnO3, magnetisation, domain wall, ferromagnetic

Effects of Grain Orientation on Cu6Sn5 Growth Behavior in Cu6Sn5-Reinforced Composite Solder Joints During Electromigration

Electromigration is a major reliability problem in composite solder joints. Due to the anisotropy of the β-Sn crystal structure, the Sn grain orientations present in the solder matrix dominate the principal failure mechanism in solder joints under electric current stressing. In this work, the Cu6Sn5 growth behavior in Cu6Sn5-reinforced composite solder joints with three different Sn grain orientations was investigated at current density of 104 A/cm2 at room temperature. Micron-sized Cu particles were added to Sn-3.5Ag solder at 2% volume fraction using an in situ method. After current stressing for 528 h, the polarity effect in the composite solder joint was greatest for an angle (θ) between the c-axis and electron flow direction of 30°, resulting in higher growth rate of Cu6Sn5 in the solder matrix compared with composite solder joints with θ of 60° or 90°. There were no noticeable changes in the composite solder joint with θ of 90°. The growth behavior of Cu6Sn5, Cu atomic motion, and Cu diffusivity in the composite solder joints with different Sn grain orientations were analyzed in detail.

Effect of the Angle Between Sn Grain c-Axis and Electron Flow Direction on Cu-Reinforced Composite Solder Joints Under Current Stressing

With a body-centered tetragonal crystal structure, Sn grains were demonstrated to have highly anisotropic behaviors in various properties. The electromigration behavior of lead-free solder was impacted by the grain orientations. In this paper, the angle between the c-axis and the electron flow direction in composite solder joints (angle θ) was proven to be an important factor during electromigration. The effects of angle θ on the electromigration of composite solder joints were investigated in this paper. Cu particle-reinforced Sn3.5Ag solder joints were stressed under a current density of 104 A/cm2 at room temperature. After 336 h current stressing time, different electromigration phenomena occurred at the two sides of the grain boundary in the composite solder joint which contained two Sn grains with different angle θ. The Sn grains with the larger angle θ had a smaller growth rate of Cu6Sn5. In addition, a composite solder joint with a single Sn grain was set as the contrast and its angle θ was smaller than that of the composite solder joint with two Sn grains. The growth rate of Cu6Sn5 in the composite solder joint with a single grain was faster than that of the composite solder joint with two Sn grains.

Observation of Three‐Dimensional Magnetic Reconnection in the Terrestrial Magnetotail

AbstractStudy of magnetic reconnection has been focused on two‐dimensional geometry in the past decades, whereas three‐dimensional structures and dynamics of reconnection X line are poorly understood. In this paper, we report Cluster multispacecraft observations of a three‐dimensional magnetic reconnection X line with a weak guide field (~25% of the upstream magnetic field) in the Earth's magnetotail. We find that the X line not only retreated tailward but also expanded across the tail following the electron flow direction with a maximum average speed of (0.04–0.15) VA,up, where VA,up is the upstream Alfvén speed, or (0.14–0.57) Vde, where Vde is the electron flow speed in the out‐of‐plane direction. An ion diffusion region was observed by two spacecraft that were separated about 10 ion inertial lengths along the out‐of‐plane direction; however, these two spacecraft observed distinct magnetic structures associated with reconnection: one spacecraft observed dipolarization fronts, while the other one observed flux ropes. This indicates that reconnection proceeds in drastically different ways in different segments along the X line only a few ion inertial lengths apart.

Spreading of electron scale magnetic reconnection with a wave number dependent speed due to the propagation of dispersive waves

In this work, we study electron scale spreading of localized magnetic reconnection in the presence of a guide magnetic field, however, without the influence of ions and cross-scale coupling. These fundamental physics studies will help to understand the coupling of the electron scale spreading with the ion scales in real systems. An electron-magnetohydrodynamic (EMHD) model is employed to model the physics at electron scales. Three dimensional EMHD simulations and linear eigen mode analysis are performed for different guide field strengths. The simulations show a wave-like bi-directional spreading of magnetic reconnection at electron scales. The electron scale spreading, however, unlike the ion scale spreading by Alfvén waves, is caused by the uni- and bi-directional propagation of the dispersive flow induced and whistler wave modes, respectively. The dispersive nature of the two wave modes makes the spreading-speed dependent on the wave numbers of the unstable tearing mode, which depend on the thickness of the electron current sheet and the strength of the guide field. A model of the speed of spreading is developed, in which the spreading-speeds parallel and anti-parallel to the guide field are given by linear combinations of the group speeds of the two wave modes. The model prediction of the spreading speeds agrees well with the speeds obtained from the simulation results. For small guide fields, the spreading is asymmetric being faster in the direction of the electron flow. On increasing the guide field, the spreading becomes increasingly symmetric, with the speeds of the order of electron Alfvén speed in the guide magnetic field, due to the dominance of the whistler group speed in determining the speed of the spreading. As a consequence of the asymmetric spreading, the chain of alternate X- and O-points, formed due the growth of oblique tearing modes, extends farther in the direction of electron flow as compared to that in the direction of the guide magnetic field.

Effects of Grain Orientation on the Electromigration of Cu-Reinforced Composite Solder Joints

Grain orientations are a significant factor under high-density current stressing, but there are few literature reports about the influence of grain orientation on electromigration in composite solder joints. Most research relating to the electromigration behavior of composite solders have been carried out by adding different kinds of reinforced particles in order to relieve the electromigration phenomena, while the impacts of grain orientation on the electromigration have been neglected. In this paper, Sn-3.5Ag composite solder joints reinforced by 2 vol.% Cu were fabricated to investigate the effects of Sn grain orientation on electromigration. The one-dimensional single-grained solder joints were stressed under a constant current density of 104A/cm2 for different times at room temperature. The samples, of which the c-axis of the Sn grains was nearly parallel to the electron flow direction, exhibited serious electromigration-induced phenomena after current stressing for 528 h. However, electromigration was not observed in the samples where the Sn grains had their c-axis perpendicular to the electron flow direction. Therefore, the grain orientations have a great influence on the electromigration behavior in composite solders and Cu atoms moved faster along the c-axis of the Sn grains than along the a- and b-axes during current stressing.

Effect of Sn crystallographic orientation on solder electromigration and Ni diffusion in Cu/Ni plating/Sn–0.7Cu joint at low current density

Electromigration (EM) in solder joints has recently been recognized as a serious reliability issue in the field of car electronics. EM in power modules is also of concern for next-generation environmentally-friendly vehicles. The current density of 10 kA/cm2 is well-known as the threshold for EM failure. Few researches have studied the EM behavior of solders at realistic current densities lower than 10 kA/cm2. In the present study, EM in a Cu/Ni plating/Sn–0.7Cu joint was investigated at low current densities of 2.5 and 5.0 kA/cm². It was found that even at a low current density of 2.5 kA/cm2, severe EM damage can be induced depending on Sn crystallographic orientation. When the c-axis of Sn crystals was parallel to the direction of electron flow, the solder detached at the cathode of the joint operated at 2.5 kA/cm2 for 2520 h. Conversely, when the c-axis of Sn crystals was perpendicular to the direction of electron flow, the solder did not detach in the joint until after a much longer time of 8200 h. Thus, it was clarified that the EM lifetime in a Cu/Ni plating/Sn–0.7Cu joint when the c-axis of Sn crystals was parallel to the direction of electron flow at a low current density of 2.5 kA/cm2 was about one-third that of the perpendicular orientation.

Effect of spin transfer torque on domain wall motion regimes in [Co/Ni] superlattice wires

The combined effect of magnetic field and current on domain wall motion is investigated in epitaxial [Co/Ni] microwires. Both thermally activated and flow regimes are found to be strongly affected by current. All experimental data can be understood by taking into account both adiabatic and nonadiabatic components of the spin transfer torque, the parameters of which are extracted. In the precessional flow regime, it is shown that the domain wall can move in the electron flow direction against a strong applied field, as previously observed. In addition, for a large range of applied magnetic field and injected current, a stochastic domain wall displacement after each pulse is observed. Two-dimensional micromagnetic simulations, including some disorder, show a random fluctuation of the domain wall position that qualitatively matches the experimental results.

Current induced rapid phase transformation in Au/Sn reaction couple

Abstract The phase evolution as well as microstructural change of thin film Au/Sn reaction couple was investigated at room temperature with respect to 1.0 × 10 4 A/cm 2 of current stressing. Transmission electron microscopy (TEM) equipped with nano beam electron diffraction (NBED) and energy dispersive X-ray spectrometer (EDS) revealed the fully conversion of Sn layer into AuSn and Au 5 Sn IMCs after deposition. The rapid phase transformation of Au 5 Sn to Au and AuSn, regardless of direction of electron flow, was evidenced by TEM during short time of current stressing. The electron impingement induced lattice strain of Au 5 Sn higher than the offset yield limit was anticipated through in-situ XRD investigation. The lattice adjustment of Au 5 Sn to AuSn due to escape of Au from Au 5 Sn crystal was responsible for the phase transformation. The polarity migration of produced Au atoms from Au 5 Sn crystal to the anode gave rise to the numerous nano voids formation at the original Au 5 Sn/Au interface with forward electric current and dilution of Sn in Au layer with reverse current stressing.

Drift waves, intense parallel electric fields, and turbulence associated with asymmetric magnetic reconnection at the magnetopause

AbstractObservations of magnetic reconnection at Earth's magnetopause often display asymmetric structures that are accompanied by strong magnetic field (B) fluctuations and large‐amplitude parallel electric fields (E||). The B turbulence is most intense at frequencies above the ion cyclotron frequency and below the lower hybrid frequency. The B fluctuations are consistent with a thin, oscillating current sheet that is corrugated along the electron flow direction (along the X line), which is a type of electromagnetic drift wave. Near the X line, electron flow is primarily due to a Hall electric field, which diverts ion flow in asymmetric reconnection and accompanies the instability. Importantly, the drift waves appear to drive strong parallel currents which, in turn, generate large‐amplitude (~100 mV/m) E|| in the form of nonlinear waves and structures. These observations suggest that turbulence may be common in asymmetric reconnection, penetrate into the electron diffusion region, and possibly influence the magnetic reconnection process.

An Improved Racetrack Structure for Transporting a Skyrmion

Magnetic skyrmions are promising building blocks for next generation data storage due to their stability, small size and extremely low currents to drive them, which can be used instead of traditional magnetic domain walls to store information as data bits in metalic racetrack memories. However, skyrmions can drift from the direction of electron flow due to the Magnus force and thus may annihilate at the racetrack edges, resulting in the loss of information. Here we propose a new skyrmion-based racetrack structure by adding high-K materials (materials with high magnetic crystalline anisotropy) at the edges, which confines the skyrmions in the center region of the metalic racetrack efficiently. This design can overcome both the clogging and annihilation of skyrmions according to our micromagnetic simulation, which occur normally for skyrmions moving on a racetrack under small and large driving currents, respectively. Phase diagrams for skyrmion motion on the proposed racetrack with various values of current density and racetrack edge width have been calculated and given, showing that skyrmions can be driven at a high speed (about 300 m/s) in the racetrack under relatively smaller driving currents. This design offers the possiblity of building an ultrafast and energy-efficient skyrmion transport device.

Effect of Sn Grain Orientation on Formation of Cu6Sn5 Intermetallic Compound Under Current Stressing

Solder joints with Cu/Sn-Ag/Cu structure and bump height of 15 μm have been used to investigate the electromigration phenomenon at different temperatures and current densities. Moreover, the grain orientation was analyzed using electron backscatter diffraction. It was found that the anisotropic properties of tin affected the formation rate of Cu-Sn intermetallic compounds (IMCs), and that the angle between the electron flow direction and tin grain orientation played an important role in the formation of Cu6Sn5 IMC. With changes in angle, the diffusion rate of copper atoms in tin also varied. When the c-axis of tin was parallel to the electron flux, copper atoms diffused rapidly, resulting in fast formation of Cu-Sn IMCs. On the other hand, if the angle between the c-axis of the grain and the electron flow direction was large, the tin grains were more resistant to Cu diffusion during current stressing, leading to a very slow IMC formation rate.

Hyperfine-controlled domain-wall motion observed in real space and time

We perform real-space imaging of propagating magnetic domains in the fractional quantum Hall system using spin-sensitive photoluminescence microscopy. The propagation is continuous and proceeds in the direction of the conventional current, i.e. opposite to the electron flow direction. The mechanism of motion is shown to be connected to polarized nuclear spins around the domain walls. The propagation velocity increases when nuclei are depolarized, and decreases when the source-drain current generating this nuclear polarization is increased. We discuss how these phenomena may arise from spin interactions along the domain walls.

Ultrafast formation of unidirectional and reliable Cu3Sn-based intermetallic joints assisted by electric current

High-temperature-stable Cu3Sn-based joints were selectively fabricated using electric current-assisted bonding process within an extremely short time (∼200 ms) and under a low pressure of 0.08 MPa in a Cu/Sn/Cu interconnection system at ambient temperature. The experimental results showed that the imposed electric current density (∼104 A/cm2) resulted in sharply increased local temperature as well as accelerated growth of Cu3Sn intermetallic compounds (IMCs). Under the effects of electron wind force-induced electromigration and joule heat-induced temperature, the transient formation of Cu3Sn-based joints can thus be obtained across the interfaces. Furthermore, highly unidirectional 〈1 0 0〉 growth of Cu3Sn IMCs was achieved along the direction of electron flow. By calculating the planar atomic densities of projected images on different planes, the particular growth direction was confirmed to represent the low-scattering path for the traveling electron flow. The oriented Cu3Sn-based joints exhibited more reliable shear properties than the Sn-based joints.

Electromigration induced spontaneous Ag whisker growth in fine Ag-alloy bonding interconnects: Novel polarity effect

Ag-alloy wire has been recognized to be a good candidate for interconnect in light-emitting diode (LED) technology. This study employed fine Ag-alloy wires bonded on Al-Si pad to investigate their failure mechanism under current stressing of 8×104A/cm2 at ambient temperature of 150°C and 175°C, respectively. We report a novel polarity effect that the spontaneous whisker growth of Ag near bonded area only happens when electrons flowed from Al to Ag, while surface of bond joints remains intact with electrons flowed from Ag to Al, suggesting that the nucleation and growth of Ag whisker is strongly affected by the direction of electron flow. We proposed that the different behaviors on the whisker growth is because compressive stress caused by current crowding effect only builds up with electrons flowed from Al to Ag.

Dominant effect of high anisotropy in β-Sn grain on electromigration-induced failure mechanism in Sn-3.0Ag-0.5Cu interconnect

The effect of high diffusivity anisotropy in β-Sn grain on electromigration behavior of micro-bumps was clearly demonstrated using Sn-3.0Ag-0.5Cu solder interconnects with only two β-Sn grains. The orientation of β-Sn grain (θ is defined as the angle between the c-axis of β-Sn grain and the electron flow direction) is becoming the most crucial factor to dominate the different electromigration-induced failure modes: 1) the excessive dissolution of the cathode Cu, blocking at the grain boundary and massive precipitation of columnar Cu6Sn5 intermetallic compounds (IMCs) in the small angle θ β-Sn grain occur when electrons flow from a small angle θ β-Sn grain to a large one; 2) void formation and propagation occur at the cathode IMC/solder interface and no Cu6Sn5 IMCs precipitate within the large angle θ β-Sn grain when electrons flow in the opposite direction. The EM-induced failure mechanism of the two β-Sn grain solder interconnects is well explained in viewpoint of atomic diffusion flux in β-Sn.

Tuning of Hemes b Equilibrium Redox Potential Is Not Required for Cross-Membrane Electron Transfer

In biological energy conversion, cross-membrane electron transfer often involves an assembly of two hemes b. The hemes display a large difference in redox midpoint potentials (ΔEm_b), which in several proteins is assumed to facilitate cross-membrane electron transfer and overcome a barrier of membrane potential. Here we challenge this assumption reporting on heme b ligand mutants of cytochrome bc1 in which, for the first time in transmembrane cytochrome, one natural histidine has been replaced by lysine without loss of the native low spin type of heme iron. With these mutants we show that ΔEm_b can be markedly increased, and the redox potential of one of the hemes can stay above the level of quinone pool, or ΔEm_b can be markedly decreased to the point that two hemes are almost isopotential, yet the enzyme retains catalytically competent electron transfer between quinone binding sites and remains functional in vivo. This reveals that cytochrome bc1 can accommodate large changes in ΔEm_b without hampering catalysis, as long as these changes do not impose overly endergonic steps on downhill electron transfer from substrate to product. We propose that hemes b in this cytochrome and in other membranous cytochromes b act as electronic connectors for the catalytic sites with no fine tuning in ΔEm_b required for efficient cross-membrane electron transfer. We link this concept with a natural flexibility in occurrence of several thermodynamic configurations of the direction of electron flow and the direction of the gradient of potential in relation to the vector of the electric membrane potential.

Effect of magnetic flux density on Sn crystallographic orientation in a solder joint system

Electromigration (EM) in Sn-based solder joints has recently been recognized as a reliability issue in high performance semiconductor devices for high end computing systems. EM in power modules is also of concern for the next generation hybrid electric vehicles. It is well known that the EM lifetime becomes significantly longer when the c axis of the Sn crystals is normal to the direction of electron flow. However, there has been no effective way to realize this alignment for the electronics industry. In the present study, the effect of external magnetic flux density on Sn crystallographic orientation was investigated at 0.5 and 1.3 T. The external magnetic field was applied to a solder joint at the solid–liquid coexistence period during solidification. The c axis of the Sn grains started to align normal to the external magnetic field direction at 0.5 T, and completed at 1.3 T.

Role of diffusion anisotropy in β-Sn in microstructural evolution of Sn-3.0Ag-0.5Cu flip chip bumps undergoing electromigration

Abstract The anisotropy of β-Sn grain is becoming the most crucial factor to dominate the electromigration (EM) behavior with the downsizing of solder bumps. When the c-axis of β-Sn grain is parallel to the electron flow direction, excessive dissolution of cathode Cu occurs due to the large diffusivity of Cu along the c-axis; when the c-axis of β-Sn grain is perpendicular to the electron flow direction, limited dissolution of cathode Cu occurs even in the current crowding regions. However, there is no evident dissolution of cathode Ni regardless of the orientation of β-Sn grains, due to the protection of a stable interfacial (Cu,Ni)6Sn5 intermetallic compound (IMC) layer and the extremely low solubility of Ni in β-Sn. Cu6Sn5-type protrusions selectively precipitated in specific Sn grains with small angle θ (between the c-axis of Sn grain and electron flow direction) but not in the neighbor grains with large angle θ or along the direction of c-axis of β-Sn. Sn hillocks are squeezed out to relieve the compressive stress generated by the formation of Cu6Sn5-type IMCs. The high diffusion anisotropy in β-Sn grains, which is calculated by a proposed model, accounts for the novel diffusion behavior of solute atoms, dissolution of cathode and consequent precipitation of IMCs in Ni/Sn-3.0Ag-0.5Cu/Cu flip chip solder joints.

Microstructure and Grain Orientation Evolution in Sn-3.0Ag-0.5Cu Solder Interconnects Under Electrical Current Stressing

In situ observation was performed on cross-sections of Sn-3.0Ag-0.5Cu solder interconnects to track the evolution of microstructure and grain orientation under electrical current stressing. Cross-sections of Cu/Ni–Sn-3.0Ag-0.5Cu–Ni/Cu sandwich-structured solder interconnects were prepared by the standard metallographic method and subjected to electrical current stressing for different times. The electron backscatter diffraction technique was adopted to characterize the grain orientation and structure of the solder interconnects. The results show that metallization dissolution and intermetallic compound (IMC) migration have close relationships with the grain orientation and structure of the solder interconnects. Ni metallization dissolution at the cathode interface and IMC migration in the solder bulk can be accelerated when the c-axis of the grain is parallel to the electron flow direction, while no observable change was found when the c-axis of the grain was perpendicular to the electron flow direction. IMC can migrate along or be blocked at the grain boundary, depending on the misorientation between the current flow direction and grain boundary.