Zhongming Zeng

Switching current reduction using perpendicular anisotropy in CoFeB–MgO magnetic tunnel junctions

We present in-plane CoFeB–MgO magnetic tunnel junctions with perpendicular magnetic anisotropy in the free layer to reduce the spin transfer induced switching current. The tunneling magnetoresistance ratio, resistance-area product, and switching current densities are compared in magnetic tunnel junctions with different CoFeB compositions. The effects of CoFeB free layer thickness on its magnetic anisotropy and current-induced switching characteristics are studied by vibrating sample magnetometry and electrical transport measurements on patterned elliptical nanopillar devices. Switching current densities ∼4 MA/cm2 are obtained at 10 ns write times.

Synthesis and photovoltaic effect of vertically aligned ZnO/ZnS core/shell nanowire arrays

A vertically aligned ZnO/ZnS core/shell nanowire array with type II band alignment was directly synthesized on an indium-tin-oxide glass substrate and the photovoltaic effect of the nanowire array was investigated. The epitaxial relationship, wurtzite (0001) matching zinc-blende (111), was observed in the ZnO/ZnS nano-heterostructure. ZnS coating is found to quench the photoluminescence of ZnO nanowires but enhance the photocurrent with faster response in the photovoltaic device, indicating improvement in charge separation and collection in the type II core/shell nanowire.

Ultralow-current-density and bias-field-free spin-transfer nano-oscillator.

The spin-transfer nano-oscillator (STNO) offers the possibility of using the transfer of spin angular momentum via spin-polarized currents to generate microwave signals. However, at present STNO microwave emission mainly relies on both large drive currents and external magnetic fields. These issues hinder the implementation of STNOs for practical applications in terms of power dissipation and size. Here, we report microwave measurements on STNOs built with MgO-based magnetic tunnel junctions having a planar polarizer and a perpendicular free layer, where microwave emission with large output power, excited at ultralow current densities, and in the absence of any bias magnetic fields is observed. The measured critical current density is over one order of magnitude smaller than previously reported. These results suggest the possibility of improved integration of STNOs with complementary metal-oxide-semiconductor technology, and could represent a new route for the development of the next-generation of on-chip oscillators.

Flexible All-Solid-State Supercapacitors based on Liquid-Exfoliated Black-Phosphorus Nanoflakes.

Flexible all-solid-state supercapacitors are fabricated with liquid-exfoliated black-phosphorus (BP) nanoflakes as an electrode material. These devices deliver high specific volumetric capacitance, power density, and energy density, up to 13.75 F cm(-3) , 8.83 W cm(-3) , and 2.47 mW h cm(-3) , respectively, and an outstanding long life span of over 30 000 cycles, demonstrating the excellent performance of the BP nanoflakes as a flexible electrode material in electrochemical energy-storage devices.

Low writing energy and sub nanosecond spin torque transfer switching of in-plane magnetic tunnel junction for spin torque transfer random access memory

This work investigated in-plane MgO-based magnetic tunnel junctions (MTJs) for the application of spin torque transfer random access memory (STT-RAM). The MTJ in this work had an resistance area product (RA) = 4.3 Ω·μm2, tunneling magnetoresistance ratio ∼135%, thermal stability factor Δ(H)=68 (by field measurement), and Δ(I) = 50 (by current measurement). The optimal writing energy was found to be 0.286 pJ per bit at 1.54 ns for antiparallel (AP) state to parallel (P) state switching, and 0.706 pJ per bit at 0.68 ns for P state to AP state switching. Ultra fast spin torque transfer (STT) switching was also observed in this sample at 580 ps (AP to P) and 560 ps (P to AP). As a result, 0.6–1.3 GHz was determined to be the optimal writing rate from writing energy consumption of view. These results show that in-plane MgO MTJs are still a viable candidate as the fast memory cell for STT-RAM.

High-Power Coherent Microwave Emission from Magnetic Tunnel Junction Nano-oscillators with Perpendicular Anisotropy

The excitation of the steady-state precessions of magnetization opens a new way for nanoscale microwave oscillators by exploiting the transfer of spin angular momentum from a spin-polarized current to a ferromagnet, referred to as spin-transfer nano-oscillators (STNOs). For STNOs to be practical, however, their relatively low output power and their relatively large line width must be improved. Here we demonstrate that microwave signals with maximum measured power of 0.28 μW and simultaneously narrow line width of 25 MHz can be generated from CoFeB-MgO-based magnetic tunnel junctions having an in-plane magnetized reference layer and a free layer with strong perpendicular anisotropy. Moreover, the generation efficiency is substantially higher than previously reported STNOs. The results will be of importance for the design of nanoscale alternatives to traditional silicon oscillators used in radio frequency integrated circuits.

Deep subnanosecond spin torque switching in magnetic tunnel junctions with combined in-plane and perpendicular polarizers

We show that adding a perpendicular polarizer to a conventional spin torque memory element with an in-plane free layer and an in-plane polarizer can significantly increase the write speed and decrease the write energy of the element. We demonstrate the operation of such spin torque memory elements with write energies of 0.4 pJ and write times of 0.12 ns.

The detection of H2S at room temperature by using individual indium oxide nanowire transistors

In(2)O(3) nanotransistors for gas sensor applications were fabricated using individual In(2)O(3) nanowires prepared by chemical vapor deposition. The nanosensors demonstrate characteristics of high sensitivity to H(2)S, and fast response and recovery, with the detection limit at 1 ppm at room temperature. The high sensitivity might be attributed to the strong electron accepting capability of H(2)S to the nanowires and the high surface-to-volume ratio of the nanowires. In addition, the nanosensors show a good selective detection of H(2)S under exposure to NH(3) and CO even at 1000 ppm; they are highly promising for practical applications in detection of low concentration H(2)S at room temperature.

Enhanced stability of black phosphorus field-effect transistors with SiO2 passivation

Few-layer black phosphorus (BP) has attracted much attention due to its high mobility and suitable band gap for potential applic5ations in optoelectronics and flexible devices. However, its instability under ambient conditions limits its practical applications. Our investigations indicate that by passivation of the mechanically exfoliated BP flakes with a SiO2 layer, the fabricated BP field-effect transistors (FETs) exhibit greatly enhanced environmental stability. Compared to the unpassivated BP devices, which show a fast drop of on/off current ratio by a factor of 10 after one week of ambient exposure, the SiO2-passivated BP devices display a high retained on/off current ratio of over 600 after one week of exposure, just a little lower than the initial value of 810. Our investigations provide an effective route to passivate the few-layer BPs for enhancement of their environmental stability.

Low Write-Energy Magnetic Tunnel Junctions for High-Speed Spin-Transfer-Torque MRAM

This letter presents energy-efficient MgO based magnetic tunnel junction (MTJ) bits for high-speed spin transfer torque magnetoresistive random access memory (STT-MRAM). We present experimental data illustrating the effect of device shape, area, and tunnel-barrier thickness of the MTJ on its switching voltage, thermal stability, and energy per write operation in the nanosecond switching regime. Finite-temperature micromagnetic simulations show that the write energy changes with operating temperature. The temperature sensitivity increases with increasing write pulsewidth and decreasing write voltage. We demonstrate STT-MRAM cells with switching energies of <;1 pJ for write times of 1-5 ns.