Search

Two-dimensional (2D) π-conjugated metal–organic frameworks (πMOFs) are a new class of designer electronic materials that are porous and tunable through the constituent organic molecules and choice of metal ions. Unlike typical MOFs, 2D πMOFs exhibit high conductivity mediated by delocalized π-electrons and have promising applications in a range of electrical devices as well as exotic physical properties. Here, we develop a growth method that generates single-crystal plates with lateral dimensions exceeding 10 μm, orders of magnitude bigger than previous methods. Synthesis of large single crystals eliminates a significant impediment to the fundamental characterization of the materials, allowing determination of the intrinsic conductivity and mobility along the 2D plane of πMOFs. A representative 2D πMOF, Ni-CAT-1, exhibits a conductivity of up to 2 S/cm, and Hall measurement reveals the origin of the high conductivity. Characterization of crystalline 2D πMOFs creates the foundation for developing electronic applications of this promising and highly diverse class of materials., A new growth method that generates the first large planar crystals of 2D conductive metal−organic frameworks enables electrical characterization along the 2D plane.

Toward magnonic devices The field of magnonics aims to use spin waves (SWs) and their associated quasiparticles—magnons—as carriers of information. Compared with the movement of charge in conventional electronics, a major advantage of SWs is reduced Joule heating. However, SWs are trickier to direct and control. Two groups now go a step further toward magnon-based devices. Han et al. show that in multilayer films, domain walls can be used to change the phase and magnitude of a spin wave. Wang et al. demonstrate how magnon currents can be used to switch the magnetization of an adjacent layer. Science , this issue p. 1121 , p. 1125

Magnetic domain walls are information tokens in both logic and memory devices, and hold particular interest in applications such as neuromorphic accelerators that combine logic in memory. Here, we show that devices based on the electrical manipulation of magnetic domain walls are capable of implementing linear, as well as programmable nonlinear, functions. Unlike other approaches, domain-wall-based devices are ideal for application to both synaptic weight generators and thresholding in deep neural networks. Prototype micrometer-size devices operate with 8 ns current pulses and the energy consumption required for weight modulation is < 16 pJ. Both speed and energy consumption compare favorably to other synaptic nonvolatile devices, with the expected energy dissipation for scaled 20 nm devices close to that of biological neurons., 24 pages, 5 figures

Magnetic field-driven domain wall motion is investigated in closely spaced sub-100 nm wide Co nanowires. Anticorrelations appear in the spatial distribution of pinning sites within weakly interacting nanowires, with a reduced probability of pinning adjacent to another pinning site over a mean correlation length similar to the line width roughness. In contrast, strong magnetostatic interactions between domain walls in adjacent nanowires eliminate the correlations, reducing the domain wall propagation distance for a given applied magnetic field.

Antiferromagnetic oxides have recently gained much attention because of the possibility to manipulate electrically and optically the Néel vectors in these materials. Their ultrafast spin dynamics, long spin diffusion length and immunity to large magnetic fields make them attractive candidates for spintronic applications. Additionally, there have been many studies on spin wave and magnon transport in single crystals of these oxides. However, the successful applications of the antiferromagnetic oxides will require similar spin transport properties in thin films. In this work, we systematically show the sputtering deposition method for two uniaxial antiferromagnetic oxides, namely Cr2O3 and α-Fe2O3, on A-plane sapphire substrates, and identify the optimized deposition conditions for epitaxial films with low surface roughness. We also confirm the antiferromagnetic properties of the thin films. The deposition method developed in this article will be important for studying the magnon transport in these epitaxial antiferromagnetic thin films.

The influence of the size of anti-notches on the domain wall propagation in Permalloy nanowires with edge taper is investigated. The critical magnetic fields and current pulses required for a domain wall to pass a symmetrical circular anti-notch obstacle were estimated by high-resolution in-situ Kerr microscopy experiments and by micromagnetic simulations. The nanowires, made using electron beam lithography and ion beam etching, had an average width of 220 nm and the anti-notches consisted of circular features that increased the wire width from 5% to 35%. The critical magnetic flux densities for domain walls to pass the obstacles increased with anti-notch diameter, from 0.6 mT to 3.4 mT in the simulations and 0.3 mT to 1.5 mT in the experiment. The critical current densities ranged from $0.5 \times {10^{12}}\ \text{A/m}^{2}$ to $10 \times {10^{12}}\ \text{A/m}^{2}$ in the simulations, with a strong dependence on the domain wall type, but the experiment yielded higher critical current densities of $6 \times {10^{12}}\ \text{A/m}^{2}$ to $25 \times {10^{12}}\ \text{A/m}^{2}$ .

Due to the difficulty in detecting and manipulating magnetic states of antiferromagnetic materials, studying their switching dynamics using electrical methods remains a challenging task. In this work, by employing heavy metal/rare earth-transition metal alloy bilayers, we experimentally studied current-induced domain wall dynamics in an antiferromagnetically coupled system. We show that the current-induced domain wall mobility reaches a maximum close to the angular momentum compensation. With experiment and modelling, we further reveal the internal structures of domain walls and the underlying mechanisms for their fast motion. We show that the chirality of the ferrimagnetic domain walls remains the same across the compensation points, suggesting that spin orientations of specific sublattices rather than net magnetization determine Dzyaloshinskii-Moriya interaction in heavy metal/ferrimagnet bilayers. The high current-induced domain wall mobility and the robust domain wall chirality in compensated ferrimagnetic material opens new opportunities for high-speed spintronic devices., Comment: 13 pages, 3 figures

Magnetic nanowires are the foundation of several promising nonvolatile computing devices, most notably magnetic racetrack memory and domain wall logic. Here, we determine the analog information capacity in these technologies, analyzing a magnetic nanowire containing a single domain wall. Although wires can be deliberately patterned with notches to define discrete positions for domain walls, the line edge roughness of the wire can also trap domain walls at dimensions below the resolution of the fabrication process, determining the fundamental resolution limit for the placement of a domain wall. Using a fractal model for the edge roughness, we show theoretically and experimentally that the analog information capacity for wires is limited by the self-affine statistics of the wire edge roughness, a relevant result for domain wall devices scaled to regimes where edge roughness dominates the energy landscape in which the walls move.

Analog implementations of neuromorphic circuits within digital systems are increasingly becoming attractive due to the high throughput and low energy per operation they offer. Magnetic logic devices based on spin-orbit torque offer a pathway to low-power resistive analog circuits. We present the magnetic tunnel junction (MTJ) function evaluator, a design for a logic device that evaluates nonlinear and linear functions for neural networks. We model the device and extend it into a functional design implementation in a logic-in-memory architecture in a hybrid process with magnetic device layers and 45 nm CMOS.

Recent studies on the magneto-transport properties of topological insulators (TI) have attracted great attention due to the rich spin-orbit physics and promising applications in spintronic devices. Particularly the strongly spin-moment coupled electronic states have been extensively pursued to realize efficient spin-orbit torque (SOT) switching. However, so far current-induced magnetic switching with TI has only been observed at cryogenic temperatures. It remains a controversial issue whether the topologically protected electronic states in TI could benefit spintronic applications at room temperature. In this work, we report full SOT switching in a TI/ferromagnet bilayer heterostructure with perpendicular magnetic anisotropy at room temperature. The low switching current density provides a definitive proof on the high SOT efficiency from TI. The effective spin Hall angle of TI is determined to be several times larger than commonly used heavy metals. Our results demonstrate the robustness of TI as an SOT switching material and provide a direct avenue towards applicable TI-based spintronic devices.

Spintronic computing promises superior energy efficiency and nonvolatility compared to conventional field-effect transistor logic. But, it has proven difficult to realize spintronic circuits with a versatile, scalable device design that is adaptable to emerging material physics. Here we present prototypes of a logic device that encode information in the position of a magnetic domain wall in a ferromagnetic wire. We show that a single three-terminal device can perform inverter and buffer operations. We demonstrate one device can drive two subsequent gates and logic propagation in a circuit of three inverters. This prototype demonstration shows that magnetic domain wall logic devices have the necessary characteristics for future computing, including nonlinearity, gain, cascadability, and room temperature operation., National Science Foundation (U.S.) (Contract ECCS-1101798), United States. Dept. of Energy. Office of Science. Graduate Fellowship Program

The electrical and magnetic properties of polycrystalline as-sintered and annealed Co0.3Mn0.2Zn0.5Fe2O4 prepared by using conventional electroceramic processing steps in the laboratory are presented. The microstructures are examined via scanning electron micrographs and energy dispersive X-ray (EDX) analysis. The ac electrical behavior represented via complex plane plots in the frequency range 160–211 MHz reveals inductive contribution via resonance originated within the material system. The ac permeability is presented in the frequency range 50–300 MHz affirmed resonance behavior. The inductive response in the complex planes (impedance Z* and modulus M*) reflecting resonance behavior due to magnetic softness is obvious around 140 MHz whereas in the permeability dispersion negative domain is observed at about 150 MHz attributing to the spin rotation.

Band to band tunneling current of strained InxGa1-xAs (x=0.47,0.65, 0.75) surface channel double gate TFET has been calculated using a simple numerical model. This proposed model provides reasonably accurate result for planner TFETs in low voltage (below 0.5V) region. However, it has been found that the increase of strain reduces the tunneling current as well as transconductance of the device. But it exhibits almost no roll-off for gate threshold voltage. Moreover, it has been found that increasing temperature increases both on and off current, so the Ion/Ioff is affected weakly by temperature change. Besides, gate threshold voltage exhibits almost no roll-off with change in strain in the channel.

We present a prototype of a switch that encodes information in the position of a magnetic domain wall (DW) in a ferromagnetic wire. The information is written using spin transfer torque and/or spin hall effect and read out using a magnetic tunnel junction (MTJ). We build prototypes and show that a single three-terminal device can perform buffer/inverter operations, which can be used as AND/NAND gates. We demonstrate one device can drive two subsequent gates, and we show bit propagation in a circuit of three inverters.

The formation of 360° magnetic domain walls (360DWs) in Co and Ni[subscript 80]Fe[subscript 20] thin film wires was demonstrated experimentally for different wire widths, by successively injecting two 180° domain walls (180DWs) into the wire. For narrow wires (≤ 50 nm wide for Co), edge roughness prevented the combination of the 180DWs into a 360DW, and for wide wires (200 nm for Co) the 360DW was unstable and annihilated spontaneously, but over an intermediate range of wire widths, reproducible 360DW formation occurred. The annihilation and dissociation of 360DWs was demonstrated by applying a magnetic field parallel to the wire, showing that annihilation fields were several times higher than dissociation fields in agreement with micromagnetic modeling. The annihilation of a 360DW by current pulsing was demonstrated., National Science Foundation (U.S.) (Award ECCS 1101798)

Temperature dependency of ballistic double gate tunnel field effect transistors (TFET) have been examined for the first time using Si and GaAs as channel material. I–V characteristics have been simulated using Kane's model of band-to-band tunneling (BTBT). It has been found that off current is more sensitive to temperature variation than on current. Moreover, this temperature dependency is also a function of device scaling which has been discussed in the present work. Besides a simple physical based model has been proposed that can predict the drain current with temperature over wide range (250K – 500 K for Si and 300K– 450K for GaAs).

The effect of temperature and scaling of gate length on the threshold voltage of double gate Ge tunneling FET has been studied. The threshold voltage has been determined from transconductance rather than constant current method. In the 10 nm to 50 nm range scaling has almost no effect on threshold voltage. However, threshold voltage is function of temperature in the range of 250–500 K. Besides, a simple model for threshold voltage incorporating the temperature effect has been developed for ballistic Ge tunneling FET based on the simulated results.

Temperature and length scaling dependence of double gate tunnel FET has been analyzed considering the electric field of junction depletion region. In the ballistic limit I d has been found to be dominated by effective mass and insulator dielectric constant rather than bandgap. Hence, GaAs channel TFET has identified as higher current device than Si counterpart due to lower effective mass. However, off state leakage current has shown comparatively more temperature dependence than on current in GaAs TFET. Besides, transconductance has found to be positively dependent on temperature for this device.

Highly birefringent elliptical hollow core optical fiber under hydrostatic pressure is analyzed in this work using finite element method. The combined effects of thermal stress generated during fabrication and external applied stress have been considered. Birefringence increases while group velocity dispersion decreases when the fiber is under hydrostatic pressure. Higher birefringence, lower material dispersion and moderate effective area can make this fiber attractive for under water communication system.

Thickness modulation at the edges of nanostructured magnetic thin films is shown to have important effects on their perpendicular magnetic anisotropy. Thin film wires with tapered edges were made from [Co/Pd]20 multilayers or L10-FePt films using liftoff with a double-layer resist. The effect of edge taper on the reversal process was studied using magnetic force microscopy and micromagnetic modeling. In [Co/Pd]20, the anisotropy was lower in the tapered edge regions which switched at a lower reverse field compared to the center of the wire. The L10-FePt wires showed opposite behavior with the tapered regions exhibiting higher anisotropy.

A method of patterning magnetic metallic thin films is presented using a bilayer polymethyl methacrylate and hydrogen silsesquioxane electron beam lithography resist mask combined with ion beam etching. The bilayer resist process allows for the combination of a high-resolution resist mask with easy postprocess removal of the mask without damage to the magnetic quality of the film. Co60Fe20B20 and Co/Ni multilayer films were patterned with electron beam lithography at 10–125 keV down to 25 nm wide features with 2 nm average root-mean square edge roughness. Both the in-plane and out-of-plane magnetic anisotropies of the respective film types were preserved after patterning.

Propagation characteristics of an elliptical hollow core optical fiber (EHOF) have been investigated under the application of high lateral and hydrostatic stresses. The simulation results exhibit significant effects of stress on birefringence, polarization mode dispersion (PMD), group velocity dispersion, effective area, and power confinement. High birefringence (~10−3), negative dispersion (−22.07 ps/km-nm), low PMD, small effective area, and ultrahigh value of power density (~103) have been found for EHOF under different stresses. The inclusion of central air hole and asymmetry in geometrical structure of the fiber under stress are responsible for different changes in propagation characteristics.