Layer Switching Research Articles

Anisotropic Spin-Orbit Torque through Crystal-Orientation Engineering in Epitaxial Pt

One of the main objectives of spintronics is to provide power-efficient switching of magnetic layers through electrical means, and in order to achieve this goal, alternate material systems with enhanced spin-orbit torque (SOT) must be engineered. In this work we provide evidence of anisotropy in the SOT and spin Hall effect (SHE) in epitaxial $\mathrm{Pt}$(110) grown on $\mathrm{Mg}\mathrm{O}$(110) single-crystal substrates, and find that the spin Hall angle and the dampinglike torque are 20% larger when current is applied along the [001] crystallographic direction as compared to [$1\overline{1}0$], leading to an equivalent reduction in switching current density along [001]. The anisotropy in SOT is attributed to the bulk contributions of the SHE in the $\mathrm{Pt}$ layer through its anisotropic resistance in this specific orientation. Measurements additionally suggest that the Rashba-Edelstein effect at the $\mathrm{Pt}$/$\mathrm{Ti}$ interface due to the $\mathrm{Pt}$(110) surface has a non-negligible effect on the spin diffusion length and SOT. By providing experimental evidence of the crystal orientation dependence of SOT-induced magnetization switching, this work helps to establish a path for energy-efficient magnetization switching through the alignment of devices with crystallographic directions of enhanced SOT generation.

Tunable Stochastic Oscillator Based on Hybrid VO₂/TaOₓ Device for Compressed Sensing

Compressed sensing with a tunable random sampling strategy has great potential in reducing the energy/time consumption during the constant acquisition of external information, thereby it is considered one of the most promising sensing strategies in edge-terminal. In this letter, a novel tunable stochastic oscillator (TSO) based on the VO2/TaOx structure was proposed and employed as the core controlling device for compressed sensing. The TSO is jointly governed by the metal-insulator-transition (IMT) based threshold switching of the VO2 layer and the stochastic resistive switching by the TaOx layer. The tunable memristor/resistor in series can modulate the oscillation frequency of the TSO. The orthogonal matching pursuit (OMP) algorithm is implemented to reconstruct the compressed sparse signals, demonstrating a good performance of TSO in compressed sensing.

Strain-induced creation and switching of anion vacancy layers in perovskite oxynitrides

Perovskite oxides can host various anion-vacancy orders, which greatly change their properties, but the order pattern is still difficult to manipulate. Separately, lattice strain between thin film oxides and a substrate induces improved functions and novel states of matter, while little attention has been paid to changes in chemical composition. Here we combine these two aspects to achieve strain-induced creation and switching of anion-vacancy patterns in perovskite films. Epitaxial SrVO3 films are topochemically converted to anion-deficient oxynitrides by ammonia treatment, where the direction or periodicity of defect planes is altered depending on the substrate employed, unlike the known change in crystal orientation. First-principles calculations verified its biaxial strain effect. Like oxide heterostructures, the oxynitride has a superlattice of insulating and metallic blocks. Given the abundance of perovskite families, this study provides new opportunities to design superlattices by chemically modifying simple perovskite oxides with tunable anion-vacancy patterns through epitaxial lattice strain.

Evaluation of spot size using volumetric repainting technique on a ProteusPLUS PBS Proton Therapy System

Volumetric repainting is considered as one of the techniques for motion mitigation in proton therapy. Faster layer switching time to deliver a volumetric repainting proton plan is very critical to reduce the overall treatment time. Recently, IBA (proton therapy vendor at the Miami Cancer Institute) has implemented a “field regulation” – a new feature to reduce the switching time between layers by applying a magnetic field setpoint to specific groups of magnets. In order to investigate the impact of field regulation and volumetric repainting technique on the spot size, several spot maps were generated. The spot sizes were measured at the isocenter and four off-axis points using the Lynx 2D scintillation detector. The average difference in spot size between two delivery sequences (“down” vs. “up” directions) for given energy at all five locations was 0.6±0.5%. The measurement results from the current study demonstrated that the impact of field regulation on the spot size was very minimal, and this was true for both the volumetric and non-volumetric techniques on a ProteusPLUS proton system with a PBS dedicated nozzle.

Driving the polar spin reorientation transition of ultrathin ferromagnets with antiferromagnetic\u2013ferromagnetic phase transition of nearby FeRh alloy film

We show that in-plane to out-of-plane magnetization switching of a ferromagnetic layer can be driven by antiferromagnetic–ferromagnetic phase transition in a nearby FeRh system. For FeRh/Au/FeAu trilayers, the impact of the magnetic phase transition of FeRh onto the perpendicular magnetization of monoatomic FeAu superlattices is transferred across the Au spacer layer via interlayer magnetic coupling. The polar spin reorientation process of the FeAu spins driven by the magnetic phase transition in the FeRh reveals its major features; namely it is reversible and displays hysteresis.

Current-in-plane spin-valve magnetoresistance in ferromagnetic semiconductor (Ga,Fe)Sb heterostructures with high Curie temperature

We demonstrate spin-valve magnetoresistance (MR) with a current-in-plane (CIP) configuration in (Ga,Fe)Sb/InAs (thickness tInAs nm)/(Ga,Fe)Sb trilayer heterostructures, where (Ga,Fe)Sb is a ferromagnetic semiconductor (FMS) with high Curie temperature (TC). An MR curve with an open minor loop is clearly observed at 3.7 K in a sample with tInAs = 3 nm, which originates from the parallel-antiparallel magnetization switching of the (Ga,Fe)Sb layers and spin-dependent scattering at the (Ga,Fe)Sb/InAs interfaces. The MR ratio increases (from 0.03 to 1.6%) with decreasing tInAs (from 9 to 3 nm) due to the enhancement of the interface scattering. This demonstration of the spin-valve effect in Fe-doped FMS heterostructures paves the way for device applications of the high-TC FMS.

Ultralow power spin\u2013orbit torque magnetization switching induced by a non-epitaxial topological insulator on Si substrates

The large spin Hall effect in topological insulators (TIs) is very attractive for ultralow-power spintronic devices. However, evaluation of the spin Hall angle and spin–orbit torque (SOT) of TIs is usually performed on high-quality single-crystalline TI thin films grown on dedicated III-V semiconductor substrates. Here, we report on room-temperature ultralow power SOT magnetization switching of a ferrimagnetic layer by non-epitaxial BiSb TI thin films deposited on Si/SiO2 substrates. We show that non-epitaxial BiSb thin films outperform heavy metals and other epitaxial TI thin films in terms of the effective spin Hall angle and switching current density by one to nearly two orders of magnitude. The critical SOT switching current density in BiSb is as low as 7 × 104 A/cm2 at room temperature. The robustness of BiSb against crystal defects demonstrate its potential applications to SOT-based spintronic devices.

Multilevel Bipolar Electroforming-Free Resistive Switching Memory Based on Silicon Oxynitride

Resistive random-access memory (RRAM) devices are fabricated by utilizing silicon oxynitride (SiOxNy) thin film as a resistive switching layer. A SiOxNy layer is deposited on a p+-Si substrate and capped with a top electrode consisting of Au/Ni. The SiOxNy-based memory device demonstrates bipolar multilevel operation. It can switch interchangeably between all resistance states, including direct SET switching from a high-resistance state (HRS) to an intermediate-resistance state (IRS) or low-resistance state (LRS), direct RESET switching process from LRS to IRS or HRS, and SET/RESET switching from IRS to LRS or HRS by controlling the magnitude of the applied write voltage signal. The device also shows electroforming-free ternary nonvolatile resistive switching characteristics having RHRS/RIRS > 10, RIRS/RLRS > 5, RHRS/RLRS > 103, and retention over 1.8 × 104 s. The resistive switching mechanism in the devices is found to be combinatory processes of hopping conduction by charge trapping/detrapping in the bulk SiOxNy layer and filamentary switching mode at the interface between the SiOxNy and Ni layers.

A novel energy sequence optimization algorithm for efficient spot-scanning proton arc (SPArc) treatment delivery

Background Spot-scanning proton arc therapy (SPArc) has been proposed to improve dosimetric outcome and to simplify treatment workflow. To efficiently deliver a SPArc plan, it’s crucial to minimize the number of energy layer switches (ELS) a sending because of the magnetic hysteresis effect. In this study, we introduced a new SPArc energy sequence optimization algorithm (SPArc_seq) to reduce ascended ELS and to investigate its impact on the beam delivery time (BDT). Method and materials An iterative energy layer sorting and re-distribution mechanism following the direction of the gantry rotation was implemented in the original SPArc algorithm (SPArc_orig). Five disease sites, including prostate, lung, brain, head neck cancer (HNC) and breast cancer were selected to evaluate this new algorithm. Dose-volume histogram (DVH) and plan robustness were used to assess the plan quality for both SPArc_seq and SPArc_orig plans. The BDT evaluations were analyzed through two methods: 1. fixed gantry angle delivery (BDTfixed) and 2. An in-house dynamic arc scanning controller simulation which considered of gantry rotation speed, acceleration and deceleration (BDTarc). Results With a similar total number of energy layers, SPArc_seq plans provided a similar nominal plan quality and plan robustness compared to SPArc_orig plans. SPArc_seq significantly reduced the number of ascended ELS by 83% (19 vs.115), 70% (16 vs. 64), 82% (19 vs. 104), 80% (19 vs. 94) and 70% (9 vs. 30), which effectively shortened the BDTfixed by 65% (386 vs. 1091 s), 61% (235 vs. 609 s), 64% (336 vs. 928 s), 48% (787 vs.1521 s) and 25% (384 vs. 511 s) and shortened BDTarc by 54% (522 vs.1128 s), 52% (310 vs.645 s), 53% (443 vs. 951 s), 49% (803 vs.1583 s) and 26% (398 vs. 534 s) in prostate, lung, brain, HNC and breast cancer, respectively. Conclusions The SPArc_seq optimization algorithm could effectively reduce the BDT compared to the original SPArc algorithm. The improved efficiency of the SPArc_seq algorithm has the potential to increase patient throughput, thereby reducing the operation cost of proton therapy.

QoS-Aware Multicast for Scalable Video Streaming in Software-Defined Networks

Scalable Video Coding (SVC) is a promising coding technique that enables flexible video transmission to support heterogeneous devices. However, the current best-effort delivery model characterized by the Internet cannot fully exploit the scalability feature of SVC because the network nodes are transparent for multimedia streaming applications. Software-defined networking (SDN) has emerged as an innovative network paradigm that decouples the control and forwarding planes while routing. This architecture with the OpenFlow protocol enables network operators to obtain per-flow QoS control in a more scalable, flexible, and finely granular manner compared to the conventional network architecture. Inspired by these facts, this paper designs a comprehensive QoS-aware multicast scheme for scalable video streaming over SDN networks. First, we present the designed multimedia streaming multicast system architecture and formulate the QoS-aware multicast routing problem as a non-linear programming model. We then propose a fundamental tree construction algorithm that decomposes the video streaming requests into subrequests and serves them in a bottom-up manner. Furthermore, we design a layer switching strategy based on the video distortion model to mitigate network-induced video quality distortion. Finally, the experimental results are presented to validate the performance of our methods in terms of scalability, network utility, video playout quality, and playback interruption ratio.

Proportional-Fair Multi-User Scalable Layered Wireless Video Streaming Powered by Energy Harvesting

The problem of adaptive multi-user scalable layered video transmission is considered in energy harvesting (EH) aided wireless communication systems. With the goal of improving the quality of video services while providing fairness amongst the users despite the random nature of both energy harvesting and the channel quality, we formulate our Scalable Video Coding (SVC) design as a Constrained Utility Function Maximization (CUFM) problem. The proportional fairness and playback smoothness of our design is guaranteed by maximizing the log-sum of the users’ video qualities, while satisfying the battery fullness constraint and video layer (quality) fluctuation constraint. By invoking the classical Lyapunov drift based optimization technique, we further decompose the CUFM problem into two parallel subproblems, i.e., a dynamic transmission power allocation problem and a dynamic layer selection problem. By solving these two subproblems, we derive a joint power allocation and video layer selection strategy for multi-user SVC video transmission. The theoretical performance bound of the proposed solution is also presented. Numerical simulations are conducted with real H.264 SVC video traces and the experimental results demonstrate the reduced playback interruption rate and layer switching rate compared to a heuristic algorithm ProNTO. The results also illustrate a tradeoff between the system's utility function and the playback smoothness experienced by the users.

A novel energy layer optimization framework for spot-scanning proton arc therapy.

Spot-scanning proton arc therapy (SPAT) is an emerging modality to improve plan conformality and delivery efficiency. A greedy and heuristic method is proposed in the existing SPAT algorithm to select energy layers and sequence energy switching with gantry rotation, which does not promise optimality in either dosimetry or efficiency. We aim to develop a method to solve the energy layer switching and dosimetry optimization problems in an integrated framework for SPAT. In an integrated approach, energy layer optimization for spot-scanning proton arc therapy (ELO-SPAT) is formulated with a dose fidelity term, a group sparsity regularization, a log barrier regularization, and an energy sequencing (ES) penalty. The combination of L2,1/2-norm group sparsity regularization and log barrier function allows one energy layer being selected per control point. The ES regularization term sorts the delivery sequence from high energy to low energy to reduce the total energy layer switching time (ELST) and subsequently the total delivery time. Within the ES penalty, the gradient of layer weights between adjacent beams is first calculated along beam direction and then along energy direction. The gradients indicate energy switch patterns between two adjacent beams. The time-wise costly energy switch-up is more heavily penalized in the ES term. This ELO-SPAT method was tested on one frontal base-of-skull (BOS) patient, one chordoma (CHDM) patient with a simultaneous integrated boost, one bilateral head-and-neck (H&N) patient, and one lung (LNG) patient. We compared ELO-SPAT with intensity-modulated proton therapy (IMPT) using discrete beams and SPArc by Ding et al. For the two arc algorithms, both the plans with and without energy sequencing were created and compared. Energy layer optimization for spot-scanning proton arc therapy reduced the runtime of optimization by 84% on average compared with the greedy SPArc method. In both the ELO-SPAT plans with and without ES, one energy layer per control point was selected. Without ES regularization, the energy sequence was arbitrary, with around 40-60 switch-up for the tested cases. After adding ES regularization, the number of energy switch-up was reduced to less than 20. Compared with the energy sequenced SPArc plans, the ELO-SPAT plans with ES led to 24% less total ELST for synchrotron plans and 14% less for cyclotron plans. Both the ELO-SPAT and SPArc plans achieved better sparing compared with the IMPT plans for most Organs-at-risks (OARs), with or without ES. Without ES, the ELO-SPAT plans achieved further improvement of the OARs compared with the SPArc plans, with an averaged reduction of OAR [Dmean, Dmax] by [1.57, 3.34] GyRBE. Adding the ES regularization degraded the plan quality, but the ELO-SPAT plans still had comparable or slightly better sparing than the SPArc plans with ES, with an averaged reduction of OAR [Dmean, Dmax] by [1.42, 2.34] GyRBE. We developed a computationally efficient spot-scanning proton arc optimization method, which solved energy layer selection and sequencing in an integrated framework, generating plans with good dosimetry and high delivery efficiency.

Dithienylpyrrole Electrografting on a Surface through the Electroreduction of Diazonium Salts

The control of the interface and the adhesion process are key issues for the development of new application based on electrochromic materials. In this work the functionalization of an electrode’s surface through electroreduction of diazonium generated in situ from 4-(2,5-di-thiophen-2-yl-pyrrol-1-yl)-phenylamine (SNS-An) has been proposed. The synthesis of the aniline derivative SNS-An was performed and the electrografting was investigated by cyclic voltammetry on various electrodes. Then the organic thin film was fully characterized by several techniques and XPS analysis confirms the presence of an organic film based on the chemical composition of the starting monomer and allows an estimation of its thickness confirmed by AFM scratching measurements. Depending on the number of electrodeposition cycles, the thickness varies from 2 nm to 10 nm, which corresponds to a few grafted oligomers. In addition, the grafted film showed a good electrochemical stability depending on the scan rates up to 400 V/s and the electrochemical response of the modified electrode towards several redox probes showed that the attached layer acts as a conductive switch. Therefore, the electrode behaves as a barrier to electron transfer when the standard redox potential of the probe is below the layer switching potential, whereas the layer can be considered as transparent towards the electron transfer for redox probes with a redox potential above it.

Impact of Spin-Orbit Torque on Spin-Transfer Torque Switching in Magnetic Tunnel Junctions

The paper presents our simulated results showing the substantial improvement of both switching speed and energy consumption in a perpendicular magnetic tunnel junction (p-MTJ), a core unit of Spin-Transfer-Torque Magnetic Random Access Memory (STT-MRAM), by the help of additional Spin-Orbit-Torque (SOT) write pulse current (WPSOT). An STT-SOT hybrid torque module for OOMMF simulation is implemented to investigate the switching behavior of a 20 nm cell in the p-MTJ. We found that the assistance of WPSOT to STT write pulse current (WPSTT) have a huge influence on the switching behavior of the free layer in the p-MTJ. For example, we could dramatically reduce the switching time (tSW) by 80% and thereby reduce the write energy over 70% as compared to those in the absence of the WPSOT. Even a very tiny amplitude of WPSOT (JSOT of the order of 102 A/m2) substantially assists to reduce the critical current density for switching of the free layer and thereby decreases the energy consumption as well. It is worth to be pointed out that the energy can be saved further by tuning the WPSOT parameters, i.e., amplitude and duration along at the threshold WPSTT. Our findings show that the proposed STT-SOT hybrid switching scheme has a great impact on the MRAM technology seeking the high speed and low energy consumption.

Order parameter of dual-frequency nematic liquid crystals measured over a wide range of thicknesses

ABSTRACT Basic physical properties of liquid crystals strongly depend on the orientational order parameter. Given a fundamental importance of this parameter, it was measured for a variety of liquid crystals by means of various experimental techniques. At the same time, the reported results were limited to a relatively narrow range of thicknesses of the studied liquid crystal cells (typically, within a 1–20 µm interval). This paper presents the values of the orientational order parameter of dual-frequency nematic liquid crystals measured over a wide range of thicknesses (from 5 to 200 µm). The non-monotonous dependence of the order parameter S on the cell thickness obtained for relatively thin layers (< 60 µm) of dual-frequency nematics is followed by a rapid monotonous decay of S for thicker (60–200 µm) liquid crystal samples. The obtained results are important for the development of emerging, non-display, applications of liquid crystals utilising thick cells and fast switching materials such as dual-frequency nematics. In particular, they provide a basis for understanding recently discovered two regimes of electro-optical switching of thin and thick layers of dual-frequency nematics (Crystals, 9(6), pp. 314, 2019).

Spin-valve Josephson junctions with perpendicular magnetic anisotropy for cryogenic memory

We demonstrate a Josephson junction with a weak link containing two ferromagnets with perpendicular magnetic anisotropy and independent switching fields in which the critical current can be set by the mutual orientation of the two layers. Such pseudospin-valve Josephson junctions are a candidate cryogenic memory in an all superconducting computational scheme. Here, we use Pt/Co/Pt/CoB/Pt as the weak link of the junction with dCo=0.6 nm, dCoB=0.3 nm, and dPt=5 nm and obtain a 60% change in the critical current for the two magnetization configurations of the pseudospin-valve. Ferromagnets with perpendicular magnetic anisotropy have advantages over magnetization in-plane systems, which have been exclusively considered at this point, as, in principle, the magnetization and magnetic switching of layers in the junction should not affect the in-plane magnetic flux.

Color switching by polarization effects in phase change materials

Wavelength and polarization-dependent phase shifts on light interacting with a phase change material can lead to coloration shifts. By exploiting polarization selective effects, the range of attainable colors during switching of a phase-change layer is widened. Theoretical and experimental results are shown for the case of VO2, a reversible phase change material that can be excited thermally, electrically or optically.

Reconfigurable Dipole-Induced Resistive Switching of MoS2 Thin Layers on Nb:SrTiO3.

The controllable band gap and charge-trapping capability of MoS2 render it suitable for use in the fabrication of various electrical devices with high-k dielectric oxides. In this study, we investigated reconfigurable resistance states in a MoS2/Nb:SrTiO3 heterostructure by using conductive atomic force microscopy. Low-resistance and high-resistance states were observed in all MoS2 because of barrier height modification resulting from redistribution of charge and oxygen vacancies in the vicinity of interfaces. In a thin layer of the MoS2 film, the carrier density was high, and layer-dependent transport properties appeared because of the charge separation in MoS2. The hysteresis and switching voltage of the MoS2/Nb:SrTiO3 heterostructure could be varied by controlling the number of layers of MoS2.

Two-pulse magnetic field-free switching scheme for perpendicular SOT-MRAM with a symmetric square free layer

A magnetic field-free switching of a symmetric square free layer with perpendicular magnetization by spin–orbit torque is demonstrated based on micromagnetic modeling and numerical simulations. The field-free switching is accomplished by using a two-pulse switching scheme. An appropriate design of the cell structure yields a deterministic and fast switching, about 0.6ns, of the magnetized free layer. It is shown that the switching is robust with respect to fluctuations of the current pulse duration and, furthermore, very robust in case of delays or overlaps between the writing current pulses.

Field-free spin-orbit torque switching through domain wall motion

Deterministic current-induced spin-orbit torque (SOT) switching of magnetization in a heavy transition metal/ferromagnetic metal/oxide magnetic heterostructure with the ferromagnetic layer being perpendicularly-magnetized typically requires an externally-applied in-plane field to break the switching symmetry. We show that by inserting an in-plane magnetized ferromagnetic layer CoFeB underneath the conventional W/CoFeB/MgO SOT heterostructure, deterministic SOT switching of the perpendicularly-magnetized top CoFeB layer can be realized without the need of in-plane bias field. Kerr imaging study further unveils that the observed switching is mainly dominated by domain nucleation and domain wall motion, which might limit the potentiality of using this type of multilayer stack design for nanoscale SOT-MRAM application. Comparison of the experimental switching behavior with micromagnetic simulations reveals that the deterministic switching in our devices cannot be explained by the stray field contribution of the in-plane magnetized layer, and the roughness-caused N\'eel coupling effect might play a more important role in achieving the observed field-free deterministic switching.