W.D. Yu

Influence of pressures on the crystallization process of an amorphous Fe73.5Cu1Nb3Si13.5B9 alloy

Amorphous Fe73.5Cu1Nb3Si13.5B9 alloys, prepared by a melt-spinning technique, were annealed at a temperature of 823 K under pressures in the range of 1–5 GPa and ambient pressure. The high pressure experiments were carried out in a belt-type pressure apparatus. The microstructure of the annealed alloys has been investigated by x-ray diffraction, electron diffraction, and transmission electron microscopy. Experimental results show that the initial crystalline phase in these annealed alloys is α-Fe solid solution (named as α-Fe phase below), and high pressure has a great influence on the crystallization process of the α-Fe phase. The grain size of the α-Fe phase decreases with the increase of pressure (P). The volume fraction of the α-Fe phase increases with increasing the pressure as the pressure is below 2 GPa, and then decreases (P>2 GPa). The mechanism for the effects of the high pressure on the crystallization process of amorphous Fe73.5Cu1Nb3Si13.5B9 alloy and latent applications of high-pressure anne...

ZnO based oxide system with continuous bandgap modulation from 3.7 to 4.9 eV

ZnO based oxide system Zn1−x−yBexMgyO has been prepared by pulsed laser deposition. By incorporating different amounts of beryllium and magnesium into ZnO, the bandgap of ZnBeMgO has been modulated from 3.7 to 4.9 eV continuously. The crystal quality of ZnBeMgO film has been improved significantly comparing with that of either ZnMgO or BeZnO. These ZnBeMgO films are promising for fabricating high-efficiency optoelectronic devices such as solar-blind UV detectors.

Microstructure and magnetic properties of Sm2(Fe, Si)17Cx/α-Fe nanocomposite magnets prepared under high pressure

We present microstructure and magnetic properties of Sm2(Fe, Si)17Cx/α-Fe nanocomposite magnets prepared under a pressure of 4 GPa at a temperature of 923 K. A high-pressure experiment was carried out in a belt-type pressure apparatus. Analyses of x-ray diffraction and transmission electron microscopy show that the nanocomposite magnets have a grain size of 6–8 nm, which provides a strong exchange coupling between hard and soft magnetic phases. As a result, the magnets have, compared with nanocomposite magnets prepared under normal pressure, a significant increase in both coercivity, from 132 to 500 kA/m, and remanent magnetization, from 0.68 to 0.83Ms.

Zero-biased solar-blind photodetector based on ZnBeMgO/Si heterojunction

An n-type Zn1−x−yBexMgyO thin film was deposited on a p-type Si substrate by pulsed laser deposition to obtain a solar-blind photodetector. The spectral response characteristic with a cutoff wavelength of 280 nm was demonstrated to realize the photodetection of the solar-blind wave zone. The responsivity of the device was improved by inserting an Al-doped ZnO (AZO) contact layer, which was expected to enhance the carrier collection efficiency significantly. Correspondingly, the peak responsivity was improved from 0.003 to 0.11 A W−1 at zero bias, and a high external quantum efficiency of 53% at 270 nm was achieved. The fast rise time reached 20 ns. This work demonstrated the possibility of a wurtzite ZnO based oxide system to realize high performance zero-biased solar-blind photodetectors.

Electrical conduction transition and largely reduced leakage current in aluminum-doped barium strontium titanate thin films heteroepitaxially grown on Ir∕MgO∕Si(100)

Ba0.6Sr0.4Ti1−xAlxO3 (BSTA, x=0, 3 at. %, 6 at. %) thin films have been prepared on Ir∕MgO-buffered silicon substrates by pulsed-laser deposition. All-epitaxial growth of BSTA∕Ir∕MgO∕Si heterostructures has been evidenced by x-ray diffraction and reflection high-energy electron diffraction. A large reduction in the leakage current density of BSTA thin films was observed by aluminum doping. For 3 at. % Al-doped BSTA thin films, the dominant conduction mechanism shows space-charge-limited current behavior at a low electric field, where the trap-filled limit field is determined as ETFL=10KV∕cm, while at a high electric field the Poole–Frenkel emission is operative. In contrast, the conduction mechanism for 6 at. % Al-doped BSTA thin film is dominated by field-enhanced Schottky emission.

High-energy-density electron jet generation from an opening gold cone filled with near-critical-density plasma

By using two-dimensional particle-in-cell simulations, we propose a scheme for strong coupling of a petawatt laser with an opening gold cone filled with near-critical-density plasmas. When relevant parameters are properly chosen, most laser energy can be fully deposited inside the cone with only 10% leaving the tip opening. Due to the asymmetric ponderomotive acceleration by the strongly decayed laser pulse, high-energy-density electrons with net laser energy gain are accumulated inside the cone, which then stream out of the tip opening continuously, like a jet. The jet electrons are fully relativistic, with speeds around 0.98−0.998 c and densities at 1020/cm3 level. The jet can keep for a long time over 200 fs, which may have diverse applications in practice.

Endurance improvement of resistance switching behaviors in the La0.7Ca0.3MnO3 film based devices with Ag–Al alloy top electrodes

The resistance switching characteristics of the La0.7Ca0.3MnO3-based devices with the top electrodes of Ag, Ag–Al alloys with the atomic ratios of Ag:Al=2:1 (2AgAl) and Ag:Al=1:2 (Ag2Al), and Al have been investigated. The device with 2AgAl top electrode shows excellent endurance, where more than 1000 cycles of reproducible current-voltage hysteresis with stable high and low resistance states have been observed. Based on Auger electron spectroscopy measurement and the detailed investigation of current-voltage curves of these devices, it is suggested that the oxygen affinity of the metal electrode, which is determined by the chemical component of Ag and Al, has an important influence on the interface structure and the resistance switching endurance. The present work provides a possible way for the improvement of the resistance switching endurance by modulating oxygen affinity of the electrode.

Tunnel current, conductance and magnetoresistance in double-barrier magnetic tunnel junctions

In this paper, we design a new type of magnetic tunnel junction (MTJ), which consists of two ferromagnetic layers separated by two adjacent insulating barriers which have different barrier heights and different dielectric constants. Based on the nearly-free-electron approximation, the tunnel current, conductance and magnetoresistance in the double-barrier MTJ are discussed, respectively, by the treatment of the transfer matrix method. Our numerical results show that there exists an asymmetrical tunneling phenomenon in the double-barrier MTJ under the forward and reverse biases, which is modulated by the configuration of magnetizations of two ferromagnetic layers. We predict that our results will stimulate experimental efforts on fabricating the double-barrier MTJ.

Asymmetrical spin-polarized tunneling and magnetoresistance in ferromagnet/insulator/insulator/ferromagnet junctions

We consider a magnetic tunneling junction (MTJ) composed of two ferromagnetic electrodes separated by two adjacent insulating barriers, which have different dielectric constants and barrier heights. Based on the two-band model and nearly-free-electron approximation, the tunnel current, tunnel conductance, and tunnel magnetoresistance effect of the MTJ under the forward and reverse biases are discussed, respectively. The numerical results are compared with the experimental results of the single-barrier MTJs. We find that there exists a directional and spin-polarized tunneling in this structure. It suggests that this structure will provide additional functions to the traditional MTJs.

A ZnO Nanorod Based 64° YX LiNbO3 Surface Acoustic Wave CO Sensor

Zinc oxide (ZnO) nanorod based surface acoustic wave (SAW) gas sensor has been developed. ZnO nanorods were deposited onto a layered ZnO/64deg YX LiNbO3 substrate using a liquid solution method. Micro-characterization results reveal that the diameter and area density of ZnO nanorods are around 100 nm and 107 cm-2, respectively. The sensor was exposed to different concentrations of CO in synthetic air. The sensor response at operating temperatures between 200degC and 300degC was examined. The study showed that the sensor responded with highest frequency shift at 265degC. At this temperature, stable base-line and fast response and recovery were observed. The developed sensor is promising for industrial applications.