Activity Report 2010 - CNRS

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HIGHLIGHT : NANOMAGNETISM AND SPINTRONICS CONTACTS jan.vogel@grenoble.cnrs.fr gilles.gaudin@cea.fr laurent.vila@cea.fr FURTHER READING V.Uhlir et al., Physical Review B, Rapid Communications, 83, 020406 (2011) C. Burrowes et al., Nature Physics, 6, 17 (2010). T.A. Moore et al., Applied Physics Letters, 93, 252604 (2008). I.M. Miron et al., Nature Materials, 9, 230 (2010). 3 DOMAIN WALL DYNAMICS IN NANOSTRIPES Magnetic domain walls in nanostripes have been proposed to be the basic element of a new type of fast and cheap magnetic storage medium. The displacement of the domain walls in these nanostripes is induced by current pulses, through the spin-transfer-torque (STT) effect. Several Grenoble laboratories (Institut Néel, INAC/SPINTEC & NM & LEMMA) work together to obtain groundbreaking results in this very competitive field of research, through the fabrication of innovating materials and the development and the use of advanced tools for characterization and modelling. In 2009, the RTRA project MIDWEST, yields funds to the 4 laboratories to develop and share complementary magnetic imaging techniques allowing a detailed metrology of domain wall dynamics in magnetic nanostructures. Current induced domain wall motion (CIDM) has mainly been studied in nanostripes of soft magnetic permalloy (Ni 80 Fe 20 ) with in-plane magnetization. In these stripes, relatively high domain wall velocities were obtained (> 100 m/s), but the current densities needed were relatively high (> 10 12 A/m 2 ). These high current densities are a drawback both for the power consumption and for the associated Joule heating of the stripes. In collaboration with a team from Unité Mixte de Physique CNRS/Thales, the group of the Institut Néel has used timeresolved Photo Emission Electron Microscope (PEEM) magnetic imaging to investigate CIDM in Co/Cu/NiFe trilayers. Domain wall velocity up to 600 m/s was observed in this system, for current densities that were a factor 2-3 smaller than required for single permalloy layers. Images taken during the application of the current pulses revealed that the NiFe magnetization, which is parallel to the axis of the stripes when no current is applied, tilts in the direction transverse to the stripes during the current pulses. It was shown that this is due to the Oersted magnetic field generated by the current itself, which is relatively large for these trilayer systems where most of the current flows in the Cu and Co layers. The effect of the Oersted field on the magnetization of the stripes and the domain wall may be at the origin of the high efficiency of CIDM in this system [Uhlir2011]. Perpendicular magnetized materials are very attractive for applications since, due to the simpler and narrower domain walls, CIDM is predicted to be much more efficient and the induced displacements reproducible. The group of INAC/NM, together with colleagues from Spintec and IEF Orsay, has studied domain wall motion in FePt alloys and Co/Ni multilayers with perpendicular anisotropy. By studying the probability of depinning a domain wall from a natural or artificial defect, as a function of applied field and current density, important information was provided on the STT efficiency, the socalled non-adiabatic torque. Contrary to what was expected, the results showed that this non-adiabatic torque is relatively insensitive to the domain wall width [Burrowes2008]. Another system with perpendicular magnetic anisotropy, consisting of Pt/Co/AlO x trilayers, was developed by Spintec, and CIDM was studied in collaboration with Institut Néel. It shows a very high CIDM efficiency and, in contrast to almost all other perpendicular systems, long distance current-induced motion of domain walls, essential for most of the applications, can be observed in this system, as shown in Fig. 1. Fig. 1: Differential Kerr images of currentinduced domain wall motion in 500nm wide Pt/Co(0.6nm)/AlOx stripes, for two different current densities (1x10 12 and 1.5x10 12 A/m 2 ) These images were obtained with Kerr microscopy, using the difference between images taken before and after the application of current pulses. For the lower current densities, the domain wall velocity increases with current density following a so-called creep law [Moore2008], while at higher current densities a linear increase is observed, with maximum velocities above 400 m/s. The Rashba effect due to the structural inversion asymmetry is thought to be at the origin of both the high CIDM efficiency and the high velocities [Miron2010].
DETECTING SINGLE NANOPARTICLE MAGNETIZATION REVERSAL WITH A NANOTUBE Nanospintronics benefits from advances in quantum transport and molecular electronics. Combining these concepts provides new devices highly sensitive to the local electromagnetic environment such as carbon nanotube quantum dots. Moreover molecular objects offer great versatility in their functionnalization with magnetic systems such as nanoparticles or molecular magnets, offering new routes towards nanoscale spin detection. Magneto-Coulomb effect in nanotube quantum dots filled with magnetic nanoparticles “Fil de l’eau” PhD student 2007: Subhadeep DATTA Carbon nanotubes at low temperature behave as quantum dots (QD) for which charging processes become quantized, giving rise to Coulomb blockade. Any small change in the electrostatic environment (tuned by the gate electrode, see Fig. 1, top) can induce a shift of the energy of the QD, leading to conductivity variation. A carbon nanotube can therefore be a very accurate electrometer. If a magnetic system is electronically coupled to a nanotube, its spin state can influence sequential tunneling through the nanotube (socalled magneto-Coulomb effect). The context in which the magneto- Coulomb effect (MCE) was first observed was that of single-electron transistors connected with two ferromagnetic leads. This effect was characterized by an enhanced magnetoresistance (MR) of the island in the Coulomb blockade regime. This feature originated from the Zeeman energy of the ferromagnetic contacts, inducing a shift between the majority and minority spin energy bands. It resulted in a modification of the island chemical potential, which is equivalent to effective electrostatic gating. The MCE thus enabled the single electron transistor to be driven by the magnetic field. In our case, the hollow center of a double wall carbon nanotube is filled with magnetic nanoparticles such as iron (collaboration with Institut Carnot CIRIMAT, University of Toulouse). We observe unprecedented high MR reaching up to 53% at 40 mK with a hysteretic behaviour and sharp jumps at specific magnetic fields corresponding to the magnetization reversal of the encapsulated particle (see Fig. 1, bottom). Moreover these features are strongly gate dependent and reflect directly the features from Coulomb blockade in the QD. Indeed, the spin flip of the iron island at non-zero magnetic field causes a sharp change in the nanoparticle chemical potential due to the Zeeman energy, which is acting on the nanotube like an effective offset charge. Such coupling allows the detection of a single reversal event, with high accuracy considering its strong influence on the magnetoresistance. This effect is thus a new gate dependent MR, which differs from that described in previous reports in which the MCE originated from the contact between nanostructures and ferromagnetic leads. Here the MCE is induced by the local coupling of a nanotube quantum dot with low-dimensional magnets. This coupling allows differentiating the sensor from the probed magnetic object. It opens up new possibilities for exploiting the versatility of nanotube QD functionnalization with different nanomagnets, using double wall nanotube (DWNTs) filled with various materials, functionalized on their surface with molecular magnets. Fig. 1: Top: transistor based on a double wall carbon nanotube, the inner tube of which is filled with magnetic iron nanomagnet; Bottom: strong resistance variations R and strong MR hysteresis observed for different applied gate voltages Vg. Quantum transport and magnetization reversal are directly correlated and appear as color changes. CONTACTS 4 HIGHLIGHT : NANOMAGNETISM AND SPINTRONICS laetitia.marty@grenoble.cnrs.fr wolfgang.wernsdorfer@grenoble.cnrs.fr FURTHER READING L. Bogani et al., Nature Mater. 7, p179, (2008)
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Date of publication: May 2011 We wo
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Nanosciences Foundation Activity Re
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Part I: ACHIEVEMENTS & PROSPECTS Th
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THE NANOSCIENCES FOUNDATION AT THE
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OUTSTANDING BENEFITS CATALYZED BY T
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The laureates of the 2010 Chairs of
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‘Mobilité d’Excellence’: a n
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INTEGRATION ON SITE BETWEEN BASIC R
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OVERVIEW Fig.3: LANEF framework. Ba
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The Nanosciences Foundation budget
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The evolution of the funding progra
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OVERVIEW Fig. 6: Total number of vi
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ACKNOWLEDGEMENTS One couldn’t fin
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Part II: SCIENTIFIC REPORT 1 - QUAN
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1 - QUANTUM NANOELECTRONICS Moore
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Core/shell nanowires: conductance C
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2 - NANOMAGNETISM AND SPINTRONICS B
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than 10 8 V/m and to compare this t
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sites. The calculations indicate th
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3 - NANOPHOTONICS The optical prope
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Cavity-feeding : decoherence as a r
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4 - MOLECULAR ELECTRONICS Molecular
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5 - NANOMATERIALS, NANOASSEMBLY AND
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Phase-change memory RTRA Project 20
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6 - NANO- CHARACTERISATION AND NANO
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when compared, for example, to the
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7 - NANO APPROACHES TO LIFE SCIENCE
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Innovative biochips to detect and s
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Implantable brain computer interfac
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8 - NANOMODELING, THEORY & SIMULATI
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Growth, patterning, defects & struc
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9 - TECHNOLOGICAL FACILITIES Create
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CRG: The French Cooperative Researc
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The Numerical Simulation Facility T
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10 - EDUCATION AND SCIENTIFIC ANIMA
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Q-NET: a new European Initial Train
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Thesis Prize Award Ceremonies 2009
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2010 Speaker From Date Title Resear
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Workshop “Super Resolution Optica
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Workshop “Electronic Noise and Re
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Workshop « New trends in Electrica
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List of the Quantum Nanoelectronics
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Hélène LE SUEUR Laboratoire de Ph
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Quantum Nanoelectronics Seminars in
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Invitations to talk at seminars: "
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- Anton ANDREEV (University of Wash
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Cellulose Hybrid block copolymers /
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PART 3 S. J. C. Yates, J. J. A. Bas
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D. A. Stewart, I. Savic, and N. Min
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DISPOGRAPH / RTRA project Post-doct
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2009 Call for Proposals MUSCADE Cha
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Piotr Laczkowski, Danny Hykel, Serg
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Part IV: SUPPLEMENTS Appendix 1: th
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Page 178 and 179: Roland HERINO (past Director) Jean-
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