Guillaume Agnus

Strain-controlled magnetic domain wall propagation in hybrid piezoelectric/ferromagnetic structures

The control of magnetic order in nanoscale devices underpins many proposals for integrating spintronics concepts into conventional electronics. A key challenge lies in finding an energy-efficient means of control, as power dissipation remains an important factor limiting future miniaturization of integrated circuits. One promising approach involves magnetoelectric coupling in magnetostrictive/piezoelectric systems, where induced strains can bear directly on the magnetic anisotropy. While such processes have been demonstrated in several multiferroic heterostructures, the incorporation of such complex materials into practical geometries has been lacking. Here we demonstrate the possibility of generating sizeable anisotropy changes, through induced strains driven by applied electric fields, in hybrid piezoelectric/spin-valve nanowires. By combining magneto-optical Kerr effect and magnetoresistance measurements, we show that domain wall propagation fields can be doubled under locally applied strains. These results highlight the prospect of constructing low-power domain wall gates for magnetic logic devices.

Two-chamber AFM: probing membrane proteins separating two aqueous compartments

Biological membranes compartmentalize and define physical borders of cells. They are crowded with membrane proteins that fulfill diverse crucial functions. About one-third of all genes in organisms code for, and the majority of drugs target, membrane proteins. To combine structure and function analysis of membrane proteins, we designed a two-chamber atomic force microscopy (AFM) setup that allows investigation of membranes spanned over nanowells, therefore separating two aqueous chambers. We imaged nonsupported surface layers (S layers) of Corynebacterium glutamicum at sufficient resolution to delineate a 15 Å–wide protein pore. We probed the elastic and yield moduli of nonsupported membranes, giving access to the lateral interaction energy between proteins. We combined AFM and fluorescence microscopy to demonstrate the functionality of proteins in the setup by documenting proton pumping by Halobacterium salinarium purple membranes.

Two‐Terminal Carbon Nanotube Programmable Devices for Adaptive Architectures

2010 WILEY-VCH Verlag Gm Devices based on nanoscale objects with well-defined structures and original electronic properties are of great interest for the development of innovative electronic circuits, in particular if they offer novel functionalities (such as memory or sensing) and are compatible with large-scale self-assembly techniques. Among these devices, two-terminal ones such as memristors are attracting intense interest due to their potential superior capabilities in terms of integration. However, at the nanometer scale, one faces the critical issue of variability among devices, both in terms of device-to-device performances and in terms of precise positioning of individual objects. It is, thus, very unlikely that conventional circuit architectures developed for silicon complementary metal oxide semiconductor (CMOS) devices will be ideally suited for these new devices. Adaptive architectures comprising a programming or a learning step are probably more reasonable candidates, as they are naturally tolerant to variability. Targeting adaptive circuits is a challenging approach, which requires the development of devices that combine several key properties among which a well-controlled memory effect is the most critical. In this context, single-walled carbon nanotubes (SWNTs) are of special relevance, as they combine nanometer-scale size and 1D character with exceptional electronic, mechanical, and chemical properties. In particular, carbon nanotube-based field-effect transistors (CNTFETs), when aggressively scaled, compete favorably with predictions of ultimate silicon-based devices of the same size. However, circuits based on such three-terminal devices do not present sufficient improvement in terms of scaling, performances, and functionality to compensate for the strong issues related to their integration. Indeed, despite important progress in the field, there is still no satisfactory solution for the implementation of a pure CNTFET-based technology that would be useful in terms of function and competitive and compatible with existing technologies to allow their co-integration. Recently, we showed that CNTFETs, coated with a thin film of photoconductive polymer, combine light sensitivity with a strong and well-controlled non-volatile memory effect. In this Communication, we show that such optically gated carbon nanotube transistors (OG-CNTFETs) can be used as two-terminal memory devices, i.e., programmable resistors, which have all the required characteristics to serve as building blocks for adaptive architectures. In particular, the nanotube channel resistivity can be precisely adjusted in a very large range, spanning several orders of magnitudes (without using the gate electrode), and then stored in a non-volatile way. We also establish the capability to handle the programming of multiple devices and demonstrate how this approach addresses the crucial issue of variability among devices. All together, it shows that such mixed nanotube–polymer devices have all the characteristics of artificial synapses that can provide an elegant solution to build neural network types of circuits. The conductivity of an OG-CNTFETat a fixed source/drain bias (VDS) can be controlled independently using either a gate potential (VGS) or illumination at a wavelength corresponding to an absorption peak of the polymer. Such transistor can be built from an individual SWNT, as in our previous study, or from networks of SWNTs. Constructing circuits based on single-nanotube devices presents severe fabrication difficulties in the present state of development of the field, in particular, due to the mixing of metallic and semiconducting nanotubes in available sources and to the need for complex steps to achieve the precise positioning of individual nano-objects. Conversely, the use of dense networks of randomly oriented SWNTs allows the efficient fabrication of multiple and interconnected devices. Moreover, the use of parallel stripes of SWNTs as illustrated in Figure 1a and 2a allows obtaining routinely on/off (ION/IOFF) current ratios of several decades. [8] As we showed in ref. [9], such use of nanotube networks has no impact on the optical-gating mechanism so that the conclusions of the present work can be extended to the single-nanotube-device case. The transistors are fabricated as described in the Experimental section. In the dark, the devices behave as p-type Schottky transistors. Under illumination, the conductivity of the device increases due to the accumulation and trapping of photogenerated electrons at the nanotube/dielectric interface.

Nanotube devices based crossbar architecture: toward neuromorphic computing

Nanoscale devices such as carbon nanotube and nanowires based transistors, memristors and molecular devices are expected to play an important role in the development of new computing architectures. While their size represents a decisive advantage in terms of integration density, it also raises the critical question of how to efficiently address large numbers of densely integrated nanodevices without the need for complex multi-layer interconnection topologies similar to those used in CMOS technology. Two-terminal programmable devices in crossbar geometry seem particularly attractive, but suffer from severe addressing difficulties due to cross-talk, which implies complex programming procedures. Three-terminal devices can be easily addressed individually, but with limited gain in terms of interconnect integration. We show how optically gated carbon nanotube devices enable efficient individual addressing when arranged in a crossbar geometry with shared gate electrodes. This topology is particularly well suited for parallel programming or learning in the context of neuromorphic computing architectures.

Low depinning fields in Ta-CoFeB-MgO ultrathin films with perpendicular magnetic anisotropy

We have studied the domain-wall dynamics in Ta-CoFeB-MgO ultra-thin films with perpendicular magnetic anisotropy for various Co and Fe concentrations in both the amorphous and crystalline states. We observe three motion regimes with increasing magnetic field, which are consistent with a low fields creep, transitory depinning, and high fields Walker wall motion. The depinning fields are found to be as low as 2 mT, which is significantly lower than the values typically observed in 3d ferromagnetic metal films with perpendicular magnetic anisotropy. This work highlights a path toward advanced spintronics devices based on weak random pinning in perpendicular CoFeB films.

Interface electronic structure in a metal/ferroelectric heterostructure under applied bias

The effective barrier height between an electrode and a ferroelectric (FE) depends on both macroscopic electrical properties and microscopic chemical and electronic structure. The behavior of a prototypical electrode/FE/electrode structure, Pt/BaTiO

Design and Modeling of a Neuro-Inspired Learning Circuit Using Nanotube-Based Memory Devices

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Ferroelectric Pb(Zr,Ti)O3 epitaxial layers on GaAs

/Nb-doped SrTiO

Performance analysis of MgO-based perpendicularly magnetized tunnel junctions

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Memristive and neuromorphic behavior in a Li x CoO 2 nanobattery

, under in-situ bias voltage is investigated using x-ray photoelectron spectroscopy. The full band alignment is measured and is supported by transport measurements. Barrier heights depend on interface chemistry and on the FE polarization. A differential response of the core levels to applied bias as a function of the polarization state is observed, consistent with Callen charge variations near the interface.