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International audience; The spin Hall effect (SHE) [1] allows for a reciprocal conversion between charge and spin currents using the spin orbit coupling which can be at the core of several promising spintronics devices. The spin orbit interaction is used to produce a transverse flow of spin or charge in response to a longitudinal excitation, these are the direct or inverse SHE. The spin Hall angle (SHA), the ratio of longitudinal and transverse electronic conductivities, is the characterizing parameter of this conversion. So far, large SHA have been reported in transition metals like Pt, Pd, W, Beta-Ta and in a few alloys with large spin orbit coupling impurities: CuIr, CuBi or CuPb [2]. In this presentation we will report on our study of the SHA in AuW alloys [3] which exhibits a non-monotonic relation with the W concentration. In the regime of diluted alloys the behaviour suggests a dominant side-jump contribution to the spin Hall resistivity, the SHA increasing with the W concentration. We will present experiments demonstrating the requirement of new spin-absorption model in lateral spin valves adapted to the case of strong spin absorption for a correct evaluation of the SHA. Altogether with complementary Ferromagnetic Resonance Spin-Pumping studies, it then leads to SHA as large as +15%. At higher W content the SHA sign is reversed, becoming negative as for pure W.

The dynamics of the graphene-catalyst interaction during chemical vapor deposition are investigated using in situ, time- and depth-resolved X-ray photoelectron spectroscopy, and complementary grand canonical Monte Carlo simulations coupled to a tight-binding model. We thereby reveal the interdependency of the distribution of carbon close to the catalyst surface and the strength of the graphene-catalyst interaction. The strong interaction of epitaxial graphene with Ni(111) causes a depletion of dissolved carbon close to the catalyst surface, which prevents additional layer formation leading to a self-limiting graphene growth behavior for low exposure pressures (10(-6)-10(-3) mbar). A further hydrocarbon pressure increase (to ∼10(-1) mbar) leads to weakening of the graphene-Ni(111) interaction accompanied by additional graphene layer formation, mediated by an increased concentration of near-surface dissolved carbon. We show that growth of more weakly adhered, rotated graphene on Ni(111) is linked to an initially higher level of near-surface carbon compared to the case of epitaxial graphene growth. The key implications of these results for graphene growth control and their relevance to carbon nanotube growth are highlighted in the context of existing literature.

Atomic collisions in solids induced by ion beam are often associated with the concept of disorder. In fact, the mobility induced in solids by ion irradiation at appropriate temperatures leads to the production of a wealth of phases which may (or not) be related to the equilibrium phase diagram. Despite, many attempts are made to understand the phase stability and the enhanced mobility of defect under irradiation even at the atomic scale, no clear picture of ion beam mixing exists. The major problem associated with ion beam mixing comes from the fact that it remains quite difficult to accurately measure a concentration over few nanometers. The X Ray Reflectometry (XRR), extensively used in micro electronics, appears to be a useful technique to overcome this difficulty. In this work, we apply the XRR technique to study the nanostructuration of Cr/Si layers induced by 80 keV Kr ion beam at room temperature, a textbook example of ion beam mixing. The analysis of XRR profiles allows computing accurate profiles of Si and Cr concentrations with a resolution equals to 1 nanometer. From these experimental profiles, we point out that the ion beam mixing appears to be a complex process which can not be only described as a diffusion controlled process.

Spinski Hallov efekt (SHE) je pojava kod koje dolazi do akumuliranja/razdvajanja spinova u vodiču, uslijed asimetrije u udarnom presjeku raspršenja nosioca naboja različitog spina. Osim ovog direktnog SHE-a značajan je i tzv. inverzni SHE (ISHE) u kojem spinska struja -- struja u kojoj postoji tok spina ali ne i naboja -- uslijed spinski ovisnog raspršenja generira tok naboja odnosno električnu struju. S obzirom da se pomoću (I)SHE može ostvariti transformacija obične električne struje u spinsku struju i obratno, oba efekta su od vrlo velikog interesa za daljnji razvoj spintronike i uređaja koji se zasnivaju na manipulaciji spina elektrona. U našem radu proučavan je inverzni SHE injektiranjem spinske struje iz NiFe elektroda u CuIr elektrodu i njegova ovisnost o koncentraciji primjesa iridija. Pokazano je da se uslijed asimetričnog raspršenja (skew scattering) na nemagnetskim primjesama iridija generira tok naboja (inverzni SHE). Učinak je proporcionalan koncentraciji iridija i praktički temperaturno neovisan za 4.2K < T < 200K. Efikasnost pretvorbe `spinska struja' – `električna struja' (tzv. spinski Hallov kut) veća je nego u dosadašnjim mjerenjima SHE u čistim metalima, i u skladu je s vrijednostima iz literature od prije tridesetak godina. Dobiveni rezultati pokazuju da je raspršenje elektrona na nemagnetskim primjesama jedan od mogućih načina za ostvarivanje velikog iznosa spinskog Hallovog efekta.

The spin Hall effect (SHE) is one of the important phenomena in spintronics since it enables to create spin currents in non-magnetic materials without using a ferromagnet. Recently-reported large SHEs in Pt and Au have triggered the search for a larger SHE in metallic devices. The mechanism of the large SHEs, however, have not been figured out yet ; whether it is intrinsic or extrinsic, if it is extrinsic whether it stems from the skew scattering mechanism or the side jump one. In this work, we focus on the extrinsic SHE. By adding Ir impurity into pure Cu, we have studied the impurity concentration dependence of the SHE in CuIr. We have used the lateral spin valve structure which consists of two Py wires and a CuIr middle wire.This device allows to estimate precisely how much spin currents are injected into CuIr. By flowing electric current from Py to one of the Cu electrodes, pure spin currents are injected into the CuIr middle wire and we are able to measure the inverse SHE. No SHE has been observed when there is no Ir impurity. With increasing the Ir impurity, however, the SH resistivity increases linearly with the resistivity of CuIr. This linear dependence strongly indicates that the SHE in CuIr is due to the skew scattering mechanism. From the slope of the SHE resistivity vs CuIr resistivity curve, it turns out that the SH angle in this system is 1.5%, which is consistent with the previous anomalous Hall effect measurement in CuMnIr.

Due to the large diversity of their physical properties and the potentiality to modulate them, the role of oxides in electronics has continuously increased over the past few years. The field of spintronics is no exception. This functional character is further increased by the possibility to obtain radically new properties at their interfaces. To illustrate our work in this field, we will present the results on all manganite tunnel junctions in which ferromagnetism is induced in the barrier by proximity effect resulting in a related exchange splitting of the conduction band and the ensuing spin filtering phenomena. Also, the experiments on heterostructures combining ferroelectric tunnel barriers of BaTiO3 and ferromagnetic electrodes (Fe or Co) will be discussed. This kind of heterostructures allows to generate, within a single device, not only a tunnel magnetoresistance (TMR) phenomena related to the relative orientation of the magnetization of the electrodes and a tunnel electroresistance (TER) induced by the ferroelectric polarisation of the barrier. They also give rise to an interfacial magnetoelectric coupling that results in an unusual modulation of the spin polarisation at the interface by the ferroelectricity resulting in a TEMR (Tunnel Electro MagnetoResistance) effect in these artificial multiferroic heterostructures. Finally, we will show the recent results on the conduction at the interface between two insulating materials SrTiO3 and LaAlO3 and the experiments we have performed in order to determine the extension of this electron gas.

International audience

We present experimental results which show that doping Cu with (nonmagnetic) 6d impurities can induce a very large Spin Hall Effect. The Spin Hall conductivity can exceed 10^7 Ohm^(-1)m^(-1), to be compared to 10^4-10^6 Ohm^(-1)m^(-1) in Pt or Au (Kimura et al, PRL 2007 ; Seki et al, Nature Materials ; 2008), 10^3 Ohm^(-1)m^(-1) in Al (Valenzuela and Tinkham, Nature 2006) or 1 Ohm^(-1)m^(-1) in InGaAs (Kato et al, Science 2004). Resonant skew scattering on spin-orbit split impurity states explains the large value of the SHE induced by nonmagnetic impurities like Ir, Ta, Lu or Sb. It also explains that the different signs of the SHE we observe at the beginning (Lu) and end (Ir) of the 5d series are related to the position of the resonant 5d states with respect to the Fermi level. We will present a theoretical model of resonant skew scattering accounting for our results and discuss the advantage of resonant scattering on other mechanisms. An extension to resonant side-jump scattering will be also presented.

The future developments of new spintronic devices require high mobility 2DEG to inject and manipulate spins. Recently, such high mobility electron gas has been reported in the LaAlO_3/SrTiO_3 system and attributed to charge transfer at the polar interface. Such oxide based 2DEG is promising since it could be combined with other oxide materials taking advantage of their attractive versatility and multifunctional character. In this charge transfer mechanism the carrier concentration is very high, about 10^21 cm^(-3) and presents a moderate mobility compared to doped SrTiO3 single crystal. An alternative way to obtain high mobility electron gas while controlling the carrier concentration and its extension is by ion etching. We will present analysis of the distribution of defects and mobile carriers generated by means of preferential ion-beam etching of SrTiO_3 single crystals by combining positron annihilation spectroscopy (PAS), conductive-tip atomic force microscopy (CT-AFM) and low temperature magnetotransport. We demonstrate that vacancies generated at the surface by room-temperature ion irradiation diffuse on the micron scale and that consequently, carriers are distributed on the same scale.

La recherche portant sur les semiconducteurs magnétiques est d’ une grande importance pour l’ électronique de spin. Récemment des expériences ont aussi suggéré l’ existence de ferromagnétisme dans La_0.5Sr_0.5Ti_O_(3-delta) dopé avec du cobalt. Contrairement a d’ autres systemes dilués, celui-ci présente l’ originalité d’ avoir comme matrice hôte un métal fortement corrélé et non semiconducteurs, ce qui est susceptible de donner lieu a des mécanismes d’ échange de nature différente. Nous avons suivi cette approche en dopant SrTiO_3, préalablement dopé avec du lanthane, avec l.5% de cobalt (Co-LSTO). Les échantillons ont été fabriqués par ablation laser pulsé avec un laser Nd:YAG. Dans le but de comprendre l’ origine des propriétés magnétiques de Co-LSTO, nous avons procédé a une large étude structurale, ainsi qu’ a la mesure d’ une polarisation en spin pour des jonctions tunnels avec des électrodes de Co-LSTO et nous avons trouvé que les conditions de croissance, comme la pression partielle d’ oxyg&egrave ; ; ne, ont une influence importante sur les propriétés magnétiques, révélant le rôle des défauts (lacunes d’ oxygene, p.ex.) Pour mesurer la polarisation en spin du matériau, des jonctions tunnels &agrave ; ; base de Co-LSTO ont été fabriquées par lithographie optique. La mesure de la magnétorésistance tunnel TMR pour des jonction Co-LSTO/LAO/Co donne des valeurs de TMR de l’ ordre de 20% ce qui nous permettent de déduire une polarisation en spin pour Co-LSTO de l’ ordre de 80%. Les expériences de spectroscopie d’ absorption de rayon X et de dichro&iuml ; ; sme circulaire magnétique nous ont permis de sonder les états électroniques des atomes de cobalt et de titane, important pour comprendre les origines microscopique des propriétés magnétiques nos films de Co-LSTO. En particulier, les signatures spectrales du XAS et de l’ XMCD indiquent que le cobalt est dans un état ionique, coexistant dans certains cas avec du Co métallique. Au vu de l’ ensemble de ces résultats, nous tenterons de dégager des conclusions quant a la nature du ferromagnétisme observé dans les couches minces de Co-LSTO.

We have shown, using magnetotransport measurements in high magnetic fields (SdH oscillations and Hall effect), that LAO/STO is not a 2D system, but 3D system. High mobility in this system is due to the doping of STO substrate by oxygen vacancies, and this happens during growth, but only in low oxygen pressure. In higher oxygen pressures, LAO/STO structure is no more of high mobility type.

Nous avons examiné les propriétés de magnétotransport de couches minces de LAO sur des substrats de STO(001) terminés TiO_2 déposées dans des pressions dans une large range de pressions (10^-6 -10^-3 mbar). On a observé que seulement les échantillons déposés aux pressions d' oxygene < 10^-5 mbar possedent une haute mobilité, tandis que les échantillons déposés aux pressions supérieures ont des résistances progressivement croissantes. Ce résultat est indicatif du fait que le déficit d' oxygene joue un rôle essentiel dans la conductivité de ces systemes. Pour analyser la dimensionnalité des échantillons avec la haute mobilité (>10^4 cm&sup2 ; /Vs) on a mesuré la magnétorésistance (MR) avec le champ magnétique appliqué dans le plan de l’ échantillon ou parallele au courant dans le plan de l' échantillon. Les deux MR montrent des oscillations de Shubnikov - de Haas (SdH) pour des températures T < 4 K et des champs B > 6 T. Ces oscillations disparaissent progressivement pour T > 1.5 K, comme attendu. L' observation d' oscillations SdH avec la meme fréquence dans les deux configurations exclut absolument la possibilité de confinement des porteurs a l' interface. Les mesures d' effet SdH et d' effet Hall ont permis une mesure plus précise de la taille de l' épaisseur du systeme conducteur: quelques centaines de microns. Cette épaisseur est en accord avec des calculs basés sur les coefficients de diffusion des lacunes d'oxygene déterminés expérimentalement dans des monocristaux de STO. En fait, différents systemes basés sur STO montrent la meme dépendance en température de la mobilité et la résistance que celles des structures LAO/STO déposées &agrave ; basse pression. Ça c'est une forte évidence de l'origine commune de la haute mobilité: le dopage par des lacunes d'oxygene dans le STO, avec des concentrations entre 10^17 et 10^19 cm^-3. Ces résultats ouvrent des perspectives intéressantes pour l'électronique d'oxydes, car ils pourraient etre utilisés pour fabriquer des heterostructures intégrant des oxydes multifonctionnels avec du STO ayant haute mobilité pour des dispositifs électroniques basés entierement sur des oxydes.

LaAlO_3/SrTiO_3 (LAO/STO) structures showing high mobility conduction have recently raised great expectations as they might represent a major step towards the conception of all-oxide electronics devices. The low-temperature high mobility was found for LAO films deposited on STO substrates at oxygen pressures below 10^-4 mbar. Generally, this metallic behavior has been ascribed to charge transfer effects at the interface between the insulators LAO and STO. For the development of oxide-based electronic devices a full understanding of the dimensionality and origin of this conducting electronic system is crucial. We have investigated the magnetotransport properties of LAO films epitaxially grown on STO(001) substrates in a wide range of deposition pressures. We show that metallic behavior and high mobility are only obtained when the films are deposited at low pressure (

The future development of micro- and nanoelectronic devices based on oxide structures requires novel materials with high carrier mobility. In this regard, SrTiO_3 (STO) is being intensively investigated due to the dramatic sensitivity of its electronic properties to extrinsic impurity doping. Stoichiometric STO is a band insulator, but an insulator-to-metal transition is induced when STO is doped with extrinsic impurities such as La, Nb or oxygen vacancies with concentrations above 10^16 cm^ -3. In these conditions, the sheet resistance can be as low as 0.01 ohm sq and the electronic mobility can exceed 10^4 cm^2/Vs at low temperature. Vacuum annealing at high temperatures has been the traditional method to dope STO with oxygen vacancies, but recently we have demonstrated that this doping can also be achieved at a much enhanced rate by thin film deposition of oxides on STO substrates at low oxygen pressure and high temperatures. We will address both theoretically and experimentally the mechanisms of oxygen vacancy formation and diffusion in STO to provide a quantitative approach to control the electronic properties of STO. Finally, we will also present results on the generation of carriers in STO, with the objective of achieving two-dimensional conduction. For this purpose, high-magnetic field transport characterization at low temperature is a powerful tool to get insight on the electronic properties and the dimensionality of the high-mobility gas created in STO by the above-mentioned methods.

We present results on current-induced domain wall motion in Co/Cu/CoFeB and Co/Cu/NiFe trilayered stripes. We find that the threshold current densities in our CoFeB spin valve stripes are around 10^6 A/cm^2 at zero magnetic field i.e. about one order of magnitude smaller than in our NiFe spin valve stripes (that is about two orders of magnitude smaller than in single NiFe stripes). The motion of the domain wall can be reversible or irreversible depending on the experimental conditions. In the irreversible regime, we observe switching of the spin valve by irreversible domain wall displacement in the free layer. The domain wall displacement is field-assisted when the applied field acts in the same direction as the spin transfer torque. The switching current density decreases as the applied field increases. This feature can have interesting applications in spintronics devices such as magnetoresistive random access memories. When the applied field torque is opposed to the spin transfer one, and above a threshold field, we observe a reversible displacement of the domain wall (corresponding to a peak in the differential resistance measurements). This can be ascribed to the onset of domain wall fluctuations, which is confirmed by micromagnetic simulations, taking into account the non adiabaticity in the spin transfer model. To understand the origin of the reversible domain wall behavior, we have performed time dependent resistance measurements that show evidences of domain wall fluctuations dominated by telegraph noise. The domain wall fluctuations occur between two well-defined states, corresponding to domain wall pinning centers distant of 400 nm. The dwell times are measured in the two states as a function of both the magnetic field and the current. The main features of the fluctuations are probably linked to the energy form of the pinning potentials.