We have fabricated air-stable n-type, ambipolar carbon nanotube field effect transistors (CNFETs), and used them in nanoscale memory cells. N-type transistors are achieved by annealing of nanotubes in hydrogen gas and contacting them by cobalt electrodes. Scanning gate microscopy reveals that the bulk response of these devices is similar to gold-contacted p-CNFETs, confirming that Schottky barrier formation at the contact interface determines accessibility of electron and hole transport regimes. The transfer characteristics and Coulomb Blockade (CB) spectroscopy in ambipolar devices show strongly enhanced gate coupling, most likely due to reduction of defect density at the silicon/silicon-dioxide interface during hydrogen anneal. The CB data in the "on"-state indicates that these CNFETs are nearly ballistic conductors at high electrostatic doping. Due to their nanoscale capacitance, CNFETs are extremely sensitive to presence of individual charge around the channel. We demonstrate that this property can be harnessed to construct data storage elements that operate at the few-electron level.

We measure the microwave signals produced by spin-torque-driven magnetization dynamics excited by direct currents in patterned magnetic multilayer devices at room temperature as a function of the angle of a magnetic field applied in the sample plane. We find strong variations in the frequency linewidth of the signals, with a decrease by more than a factor of 20 as the field is rotated from the magnetic easy axis to the in-plane hard axis. Based on micromagnetic simulations, we identify these variations as due to a transition from spatially incoherent to coherent precession.

The successful operation of spin-based data storage devices depends on thermally stable magnetic bits. At the same time, the data-processing speeds required by today's technology necessitate ultrafast switching in storage devices. Achieving both thermal stability and fast switching requires controlling the effective damping in magnetic nanoparticles. By carrying out a surface chemical analysis, we show that through exposure to ambient oxygen during processing, a nanomagnet can develop an antiferromagnetic sidewall oxide layer that has detrimental effects, which include a reduction in the thermal stability at room temperature and anomalously high magnetic damping at low temperatures. The in situ deposition of a thin Al metal layer, oxidized to completion in air, greatly reduces or eliminates these problems. This implies that the effective damping and the thermal stability of a nanomagnet can be tuned, leading to a variety of potential applications in spintronic devices such as spin-torque oscillators and patterned media.

The torque generated by the transfer of spin angular momentum from a spin-polarized current to a nanoscale ferromagnet can switch the orientation of the nanomagnet much more efficiently than a current-generated magnetic field, and is therefore in development for use in next-generation magnetic random access memory (MRAM). Up to now, only DC currents and square-wave current pulses have been investigated in spin-torque switching experiments. Here we present measurements showing that spin transfer from a microwave-frequency pulse can produce a resonant excitation of a nanomagnet and lead to improved switching characteristics in combination with a square current pulse. With the assistance of a microwave-frequency pulse, the switching time is reduced and achieves a narrower distribution than when driven by a square current pulse alone, and this can permit significant reductions in the integrated power required for switching.Resonantly excited switching may also enable alternative, more compact MRAM circuit architectures. 2A spin-polarized current can exert a spin-transfer torque 1,2 on a nanomagnet strong enough to reverse its magnetization without an applied magnetic field. 3,4 This provides a promising mechanism for writing information in the next generation of magnetic random access memory (MRAM). Previous studies have indicated that the spintorque switching proceeds via a process of magnetic precession with increasing precession amplitude. [5][6][7][8] This suggests that using a high-frequency (RF) current to drive precession on resonance 9,10 might improve the switching characteristics. 11 Here we report low temperature proof-of-principle experiments, using a combination of RF and square-wave current pulses, which show that resonantly-excited precession can enhance the switching speed and the reproducibility of switching times for spin-torque-driven nanoscale magnetic devices, and also significantly reduce the integrated power required for switching.The initial experiments on spin transfer switching studied primarily multilayered nanopillar devices (Fig. 1a inset) in which the magnetic moment directions were approximately collinear for the "free" magnetic layer that undergoes switching and the "pinned" magnetic layer that polarizes the current. [12][13][14] Since the spin torque vanishes at zero offset angle, spin-transfer switching in such devices requires that the moments have an initial deviation from completely parallel or anti-parallel alignment that was usually initiated by thermal fluctuations. The switching process was therefore stochastic, with a broadened distribution of switching times. An alternative strategy is to start with a nonzero equilibrium angle between the two moments by biasing one of the two layers away from the easy axis defined by shape anisotropy. 6,[15][16][17] However, in this case using a square-wave current pulse (with a single sign of current) to drive switching may not 3 provide optimum efficiency. Consider the initial stage of the switching process in a thin film nanopillar devic...

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