K. Galatsis

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.

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.

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.

Voltage-induced switching of nanoscale magnetic tunnel junctions

We demonstrate voltage-induced (non-STT) switching of nanoscale, high resistance voltage-controlled magnetic tunnel junctions (VMTJs) with pulses down to 10 ns. We show ~10x reduction in switching energies (compared to STT) with leakage currents <; 105 A/cm2. Switching dynamics, from quasi-static to the nanosecond regime, are studied in detail. Finally, a strategy for eliminating the need for external magnetic-fields, where switching is performed by set/reset voltages of different amplitudes but same polarity, is proposed and verified experimentally.

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.

Effect of resistance-area product on spin-transfer switching in MgO-based magnetic tunnel junction memory cells

We use ultrafast current-induced switching measurements to study spin-transfer switching performance metrics, such as write energy per bit (EW) and switching current density (Jc), as a function of resistance-area product (RA) (hence MgO thickness) in magnetic tunnel junction cells used for magnetoresistive random access memory (MRAM). EW increases with RA, while Jc decreases with increasing RA for both switching directions. The results are discussed in terms of RA optimization for low write energy and current drive capability (hence density) of the MRAM cells. Switching times <2 ns and write energies <0.3 pJ are demonstrated for 135 nm×65 nm CoFeB/MgO/CoFeB devices.

A room temperature polyaniline nanofiber hydrogen gas sensor

Electro-conductive polyaniline (PANI) nanofiber based surface acoustic wave (SAW) gas sensors have been investigated with hydrogen (H 2) gas. A template-free, rapidly mixed method was employed to synthesize polyaniline nanofibers using chemical oxidative polymerization of aniline. The nanofibers were deposited onto a layered ZnO/64deg YX LiNbO3 SAW transducer for gas sensing applications. The novel sensor was exposed to various concentrations of H2 gas at room temperature. The sensor response, defined as the relative variation in operating frequency of oscillation due to the introduction of the gas, was 3.04 kHz towards a 1% H2 concentration. A relatively fast response time of 8 sec and a recovery time of 60 sec with good repeatability were observed at room temperature. Due to room temperature operation, the novel gas sensor is promising for environmental and industrial applications

A Material Framework for Beyond-CMOS Devices

Beyond-CMOS devices concepts are greatly dependent on new functional materials to provide inspiration and innovation beyond the silicon status quo. Here, we propose a material framework specifically for beyond-CMOS devices. In doing so, material system examples and data points presented are taken from the Center on Functional Accelerated Nanomaterials Engineering, the STARnet Center of Excellence.

Ultrafast spin torque memory based on magnetic tunnel junctions with combined in-plane and perpendicular polarizers

Since the initial prediction and experimental demonstration of magnetization reversal by spin transfer torque (STT), there has been continuous progress toward the development of nonvolatile magnetic random access memory based on STT switching (STT-RAM) in nanoscale magnetic tunnel junctions (MTJs). In the most common STT-RAM configuration shown in Fig. 1(a), the magnetic moments of the free layer and the pinned polarizing layer of an MTJ lie collinear to one another in the plane of the junction. In this configuration (in-plane STT-RAM or IST-RAM), STT is small during the initial stages of the free layers magnetic moment reversal, resulting in a relatively long nanosecond-scale switching time. Switching can be greatly accelerated in an alternative STT-RAM configuration, in which a second polarizer with magnetic moment perpendicular to the MTJ plane is added to the magnetic multilayer (orthogonal STT-RAM or OST-RAM). The initial STT from the perpendicular polarizer is large and has been predicted to induce ultrafast precessional switching of the free layers magnetization on a time scale of 100 ps. The differences in the reversal modes expected for the IST-RAM and OST-RAM devices are illustrated in Figs. 1(c) and 1(d), wherein magnetization switching trajectories are shown for the two types of memory.

The study of InO/sub x//ZnO/XZ LiNbO/sub 3/ layered SAW devices for ozone sensing

An investigation of layered surface acoustic wave (SAW) devices based on an InO/sub x//ZnO/XZ LiNbO/sub 3/ structure for sensing ozone (O/sub 3/) is presented. Device structures are based on an XZ lithium niobate (LiNbO/sub 3/) substrate with a 1.2 /spl mu/m zinc oxide (ZnO) guiding layer. 40 and 200 nm thin films of indium oxide (InO/sub x/) provide the selectivity towards O/sub 3/ gas. The sensor response features are analyzed in terms of response time, recovery time and response magnitude as a function of operating temperature. A high sensitivity with fast and stable responses towards O/sub 3/ was achieved. Exceptionally large frequency shifts as high as 78.5 kHz were observed for O/sub 3/ concentrations as low as 25 parts per billion (ppb) in air. Microstructural characterization of the InO/sub x/ films by scanning electron microscopy (SEM) is also presented.