Richard A. Webb

Electrical self-healing of mechanically damaged zinc oxide nanobelts.

We report the observation of remarkable electrical self-healing in mechanically damaged ZnO nanobelts. Nanoindentation into intrinsically defect-free ZnO nanobelts induces deformation and crack damage, causing a dramatic electrical signal decrease. Two self-healing regimes in the nanoindented ZnO nanobelts are revealed. The physical mechanism for the observed phenomena is analyzed in terms of the nanoindentation-induced dislocations, the short-range atomic diffusion in nanostructures, and the local heating of the dislocation zone in the electrical measurement.

Coherent control of nanomagnet dynamics via ultrafast spin torque pulses

We demonstrate reliable manipulation of the magnetization dynamics of a precessing nanomagnet by precisely controlling the spin transfer torque on the subnanosecond time scale. Using a simple pulse shaping scheme consisting of two ultrafast spin torque pulses with variable amplitudes and delay, we demonstrate coherent control over the precessional orbits and the ability to tune the switching probability of a nanomagnet at room temperature and 77 K. Our measurements suggest that appropriately shaped spin transfer can be used to efficiently manipulate the orientation of a free layer nanomagnet, thus providing an alternative for spin torque driven spintronic devices.

Electron Beam Irradiation Stiffens Zinc Tin Oxide Nanowires

We report a remarkable phenomenon that electron beam irradiation (EBI) significantly enhances the Youngs modulus of zinc tin oxide (ZTO) nanowires (NWs), up to a 40% increase compared with the pristine NWs. In situ uniaxial buckling tests on individual NWs were conducted using a nanomanipulator inside a scanning electron microscope. We propose that EBI results in substantial atomic bond contraction in ZTO NWs, accounting for the observed mechanically stiffening. This argument is supported by our experimental results that EBI also reduces the electrical conductivity of ZTO NWs.

Growth direction modulation and diameter-dependent mobility in InN nanowires

Diameter-dependent electrical properties of InN nanowires (NWs) grown by chemical vapor deposition have been investigated. The NWs exhibited interesting properties of coplanar deflection at specific angles, either spontaneously, or when induced by other NWs or lithographically patterned barriers. InN NW-based back-gated field effect transistors (FETs) showed excellent gate control and drain current saturation behaviors. Both NW conductance and carrier mobility calculated from the FET characteristics were found to increase regularly with a decrease in NW diameter. The observed mobility and conductivity variations have been modeled by considering NW surface and core conduction paths.

Electron coherence at low temperatures: The role of magnetic impurities

We review recent experimental progress on the saturation problem in metallic quantum wires. In particular, we address the influence of magnetic impurities on the electron phase coherence time. We also present new measurements of the phase coherence time in ultra-clean gold and silver wires and analyse the saturation of tf in these samples, cognizant of the role of magnetic scattering. For the cleanest samples, Kondo temperatures below 1 mK and extremely small magnetic-impurity concentration levels of less than 0.08 ppm have to be assumed to attribute the observed saturation to the presence of magnetic impurities.

Temperature dependent carrier mobility in graphene: Effect of Pd nanoparticle functionalization and hydrogenation

The two dimensional nature of graphene, with charge carriers confined within one atomic layer thickness, causes its electrical, optical, and sensing properties to be strongly influenced by the surrounding media and functionalization layers. In this study, the effect of catalytically active Pd nanoparticle (NP) functionalization and subsequent hydrogenation on the hall mobility and carrier density of chemical vapor deposition synthesized graphene has been investigated as a function of temperature. Prior to functionalization, the mobility decreased monotonically as the temperature was reduced from 298 to 10u2009K, indicating coulomb scattering as the dominant scattering mechanism as expected for bilayer graphene. Similar decreasing trend with temperature was also observed after 2u2009nm Pd deposition, however, hydrogenation of the Pd NP led to significant enhancement in mobility from ∼2250 to 3840u2009cm2/V s at room temperature, which further monotonically increased to 5280u2009cm2/V s at 10u2009K. We attribute this contrasti...

Enhanced spin-dependent shot noise in magnetic tunnel barriers

We report the observation of enhanced spin-dependent shot noise in magnetic tunnel barriers, suggesting transport through localized states within the barrier. This is supported by the existence of negative magnetoresistance and structure in the differential conductance curves. A simple model of tunneling through two interacting localized states with spin-dependent tunneling rates is used to explain our observations.

Low frequency noise characteristics of submicron magnetic tunnel junctions

We report that low frequency (up to 200 kHz) noise spectra of magnetic tunnel junctions with areas 10cm at 10 Kelvin deviate significantly from the typical 1/f behavior found in large area junctions at room temperature. In most cases, a Lorentzian-like shape with characteristic time between 0.1 and 10 ms is observed, which indicates only a small number of fluctuators contribute to the measured noise. By investigating the dependence of noise on both the magnitude and orientation of an applied magnetic field, we find that magnetization fluctuations in both free and reference layers are the main sources of noise in these devices. At small fields, where the noise from the free layer is dominant, a linear relation between the measured noise and angular magnetoresistance susceptibility can be established.We report that low frequency (up to 200 kHz) noise spectra of magnetic tunnel junctions with areas ~10^{-10}cm^2

Effect of resistance feedback on spin torque-induced switching of nanomagnets

at 10 Kelvin deviate significantly from the typical 1/f behavior found in large area junctions at room temperature. In most cases, a Lorentzian-like shape with characteristic time between 0.1 and 10 ms is observed, which indicates only a small number of fluctuators contribute to the measured noise. By investigating the dependence of noise on both the magnitude and orientation of an applied magnetic field, we find that magnetization fluctuations in both free and reference layers are the main sources of noise in these devices. At small fields, where the noise from the free layer is dominant, a linear relation between the measured noise and angular magnetoresistance susceptibility can be established.

Comment on "anomalous conductance distribution in quasi-one- dimensional gold wires: Possible violation of the one-parameter scaling hypothesis"

Abstract In large magnetoresistance devices spin torque-induced changes in resistance can produce GHz current and voltage oscillations which can affect magnetization reversal. In addition, capacitive shunting in large resistance devices can further reduce the current, adversely affecting spin torque switching. Here, we simultaneously solve the Landau–Lifshitz–Gilbert equation with spin torque and the transmission line telegraphers equations to study the effects of resistance feedback and capacitance on magnetization reversal of both spin valves and magnetic tunnel junctions. While for spin valves parallel (P) to anti-parallel (AP) switching is adversely affected by the resistance feedback due to saturation of the spin torque, in low resistance magnetic tunnel junctions P–AP switching is enhanced. We study the effect of resistance feedback on the switching time of magnetic tunnel junctions, and show that magnetization switching is only affected by capacitive shunting in the pF range.