C. T. Rettner

Phase-change random access memory: a scalable technology

Nonvolatile RAM using resistance contrast in phase-change materials [or phase-change RAM (PCRAM)] is a promising technology for future storage-class memory. However, such a technology can succeed only if it can scale smaller in size, given the increasingly tiny memory cells that are projected for future technology nodes (i.e., generations). We first discuss the critical aspects that may affect the scaling of PCRAM, including materials properties, power consumption during programming and read operations, thermal cross-talk between memory cells, and failure mechanisms. We then discuss experiments that directly address the scaling properties of the phase-change materials themselves, including studies of phase transitions in both nanoparticles and ultrathin films as a function of particle size and film thickness. This work in materials directly motivated the successful creation of a series of prototype PCRAM devices, which have been fabricated and tested at phase-change material cross-sections with extremely small dimensions as low as 3 nm × 20 nm. These device measurements provide a clear demonstration of the excellent scaling potential offered by this technology, and they are also consistent with the scaling behavior predicted by extensive device simulations. Finally, we discuss issues of device integration and cell design, manufacturability, and reliability.

Current-Controlled Magnetic Domain-Wall Nanowire Shift Register

The controlled motion of a series of domain walls along magnetic nanowires using spin-polarized current pulses is the essential ingredient of the proposed magnetic racetrack memory, a new class of potential non-volatile storage-class memories. Using permalloy nanowires, we achieved the successive creation, motion, and detection of domain walls by using sequences of properly timed, nanosecond-long, spin-polarized current pulses. The cycle time for the writing and shifting of the domain walls was a few tens of nanoseconds. Our results illustrate the basic concept of a magnetic shift register that relies on the phenomenon of spin-momentum transfer to move series of closely spaced domain walls.

Oscillatory dependence of current-driven magnetic domain wall motion on current pulse length

Magnetic domain walls, in which the magnetization direction varies continuously from one direction to another, have long been objects of considerable interest. New concepts for devices based on such domain walls are made possible by the direct manipulation of the walls using spin-polarized electrical current through the phenomenon of spin momentum transfer. Most experiments to date have considered the current-driven motion of domain walls under quasi-static conditions, whereas for technological applications, the walls must be moved on much shorter timescales. Here we show that the motion of domain walls under nanosecond-long current pulses is surprisingly sensitive to the pulse length. In particular, we find that the probability of dislodging a domain wall, confined to a pinning site in a permalloy nanowire, oscillates with the length of the current pulse, with a period of just a few nanoseconds. Using an analytical model and micromagnetic simulations, we show that this behaviour is connected to a current-induced oscillatory motion of the domain wall. The period is determined by the walls mass and the slope of the confining potential. When the current is turned off during phases of the domain wall motion when it has enough momentum, the domain wall is driven out of the confining potential in the opposite direction to the flow of spin angular momentum. This dynamic amplification effect could be exploited in magnetic nanodevices based on domain wall motion.

Nanoscale Nuclear Magnetic Resonance with a Nitrogen-Vacancy Spin Sensor

Nanoscale NMR with Diamond Defects Although nuclear magnetic resonance (NMR) methods can be used for spatial imaging, the low sensitivity of detectors limits the minimum sample size. Two reports now describe the use of near-surface nitrogen-vacancy (NV) defects in diamond for detecting nanotesla magnetic fields from very small volumes of material (see the Perspective by Hemmer). The spin of the defect can be detected by changes in its fluorescence, which allows proton NMR of organic samples only a few nanometers thick on the diamond surface. Mamin et al. (p. 557) used a combination of electron spin echoes and pulsed NMR manipulation of the proton spins to detect the very weak fields. Staudacher et al. (p. 561) measured statistical polarization of a population of about 104 spins near the NV center with a dynamical decoupling method. The optical response of the spin of a near-surface atomic defect in diamond can be used to sense proton magnetic fields. [Also see Perspective by Hemmer] Extension of nuclear magnetic resonance (NMR) to nanoscale samples has been a longstanding challenge because of the insensitivity of conventional detection methods. We demonstrated the use of an individual, near-surface nitrogen-vacancy (NV) center in diamond as a sensor to detect proton NMR in an organic sample located external to the diamond. Using a combination of electron spin echoes and proton spin manipulation, we showed that the NV center senses the nanotesla field fluctuations from the protons, enabling both time-domain and spectroscopic NMR measurements on the nanometer scale.

Nanoscale magnetic resonance imaging

We have combined ultrasensitive magnetic resonance force microscopy (MRFM) with 3D image reconstruction to achieve magnetic resonance imaging (MRI) with resolution <10 nm. The image reconstruction converts measured magnetic force data into a 3D map of nuclear spin density, taking advantage of the unique characteristics of the “resonant slice” that is projected outward from a nanoscale magnetic tip. The basic principles are demonstrated by imaging the 1H spin density within individual tobacco mosaic virus particles sitting on a nanometer-thick layer of adsorbed hydrocarbons. This result, which represents a 100 million-fold improvement in volume resolution over conventional MRI, demonstrates the potential of MRFM as a tool for 3D, elementally selective imaging on the nanometer scale.

Effect of rotation on the translational and vibrational energy dependence of the dissociative adsorption of D2 on Cu(111)

We have investigated the dependence on the rotational and vibrational states of the translational energy of D2(v,J) formed in recombinative desorption from Cu(111). These results provide information about the effect of rotational energy relative to that of vibrational and translational energy on the dissociative chemisorption of D2 on Cu(111). The range of rovibrational states measured includes rotational states J=0–14 for vibrational state v=0, J=0–12 for v=1, and J=0–8 for v=2. D2 molecules were detected in a quantum‐state‐specific manner using three‐photon resonance‐enhanced multiphoton ionization (2+1 REMPI). Kinetic energies of desorbed molecules were obtained by measuring the flight time of D2+ ions in a field‐free region. The mean kinetic energies determined from these measurements depend strongly on the rotational and vibrational states. Analyzing these results using the principle of detailed balance confirms previous observations that vibrational energy is effective, though not as effective as tr...

Quantum‐state‐specific dynamics of the dissociative adsorption and associative desorption of H2 at a Cu(111) surface

We have determined the dependence of the dissociative adsorption probability in the zero coverage limit, S0, for H2 on Cu(111) as a function of translational energy, Ei, and incidence angle, θi, vibrational state, v, and rotational state, J. We have also obtained information on the effect of surface temperature, Ts, on this probability. These results have been obtained by combining the findings of two separate experiments. We have obtained the form of the dependence of S0 on Ei at Ts=925 K for a range of quantum states from desorption experiments via the principle of detailed balance. We have obtained absolute S0 values from direct molecular beam adsorption experiments, which reveal that S0 scales with the so‐called ‘‘normal energy,’’ En=Ei cos2 θi. The desorption experiments provide detailed information for J=0 to 10 of H2(v=0) and for J=0 to 7 of H2(v=1). The beam experiments additionally provide information on the adsorption of H2(v=2), averaged over J. All measurements are consistent with adsorption f...

Effect of incidence kinetic energy and surface coverage on the dissociative chemisorption of oxygen on W(110)

The dissociative chemisorption of oxygen on W(110) has been studied using molecular beam techniques. Chemisorption probabilities have been measured as a function of incidence angle, θi, and kinetic energy, Ei, and of surface coverage and temperature. In addition, angular scattering distributions have been measured for a range of conditions and LEED has been used to examine surface structure. The initial (zero coverage limit) sticking probability is found to depend strongly on the incidence energy, scaling with En=Ei cos2 θi. This probability is ∼10% at En =0.1 eV, rising to essentially unity above En =0.4 eV. At half a monolayer coverage of atomic oxygen, the sticking probability is close to zero up to a threshold of ∼0.25 eV, above which it rises to over 50% by 1.3 eV. In most cases, the sticking probability is found to fall roughly linearly with increasing surface coverage. However, a less‐than‐linear fall‐off is observed for En ≥1 eV and for En ≤0.03 eV, the sticking probability actually rises with inc...

Ultra-Thin Phase-Change Bridge Memory Device Using GeSb

An ultra-thin phase-change bridge (PCB) memory cell, implemented with doped GeSb, is shown with < 100muA RESET current. The device concept provides for simplified scaling to small cross-sectional area (60nm2) through ultra-thin (3nm) films; the doped GeSb phase-change material offers the potential for both fast crystallization and good data retention

Dynamics of the chemisorption of O2 on Pt(111): Dissociation via direct population of a molecularly chemisorbed precursor at high incidence kinetic energy

We have used the thermal desorption spectroscopy of the O/O2+CO→CO2 system to probe the chemical nature of oxygen that remains on a Pt(111) surface following exposure to a supersonic O2 beam under various conditions. We find that for a surface temperature of 90 K, the resulting CO2 formation thermal desorption spectrum is the same for all beam kinetic energies employed up to 1.1 eV at normal incidence, in all cases resembling that assigned to the O2+CO co‐adsorbate system. This spectrum is clearly distinct from the O+CO case, where atomically chemisorbed oxygen is obtained either by thermal dissociation of O2 on the surface or by exposing the 90 K surface to a beam containing O atoms. These results imply that the dissociative chemisorption of O2 on Pt(111) proceeds by way of a molecular precursor even at relatively high incidence kinetic energies, at least as high as 1.1 eV. This interpretation readily accounts for the strong surface temperature dependence associated with dissociation under these conditio...