J.-M. George

Université Paris-Sud 11, Orsay, Île-de-France, France

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Publications (86)239.54 Total impact

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    ABSTRACT: We consider the excitation of large amplitude gyrotropic vortex core precession in a Permalloy nanodisk by spin-orbit torques originating from the surface of a topological insulator. We demonstrate analytically and by micromagnetic modelling that the in-plane microwave current along the surface of the topological insulator can efficiently excite vortex core rotation. We consider the dependence of this excitation on the frequency and magnitude of the microwave current. These findings opens the possibility to excite gyrotropic vortex motion with the current densities far lower than by any other means.
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    ABSTRACT: Facing the ever-growing demand for data storage will most probably require a new paradigm. Magnetic skyrmions are anticipated to solve this issue as they are arguably the smallest spin textures in magnetic thin films in nature. We designed cobalt-based multilayered thin films where the cobalt layer is sandwiched between two heavy metals providing additive interfacial Dzyaloshinskii-Moriya interactions, which reach about 2 mJ/m2 in the case of the Ir|Co|Pt multilayers. Using a magnetization-sensitive scanning x-ray transmission microscopy technique, we imaged magnetic bubble-like domains in these multilayers. The study of their behavior in magnetic field allows us to conclude that they are actually magnetic skyrmions stabilized by the Dzyaloshinskii-Moriya interaction. This discovery of stable skyrmions at room temperature in a technologically relevant material opens the way for device applications in a near future.
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    ABSTRACT: It was recently realized that the spin Hall effect (SHE) can be very useful in the area of spintronics, due to its ability to generate pure spin current from charge current, without the use of any magnetic materials or magnetic field. The maximum conversion factor is given by the spin Hall angle íµí¼ƒ SH , which can take rather important values (above 10% in absolute value was reported for β-Ta and β-W). This phenomenon is usually observed in materials with large spin-orbit coupling, either intrinsic (Pt, Ta, W, etc.) or induced by heavy impurities (Cu doped with Bi or Ir). To investigate this property, several groups studied the reciprocal effect, the so-called inverse spin Hall effect (ISHE), converting a pure "pumped" spin current into a charge current (measured by voltage detection in an "open circuit"). We focus here on the 5d Pt material. Values published nowadays for íµí¼ƒ SH in Pt are scattered over one order of magnitude, with a clear correlation between the spin diffusion length ℓ sf and the íµí¼ƒ SH , both quantities being related to the spin-orbit strength or its inverse. We performed measurements of spin pumping in a cavity and measured the resulting ISHE voltage. We propose a model including spin-current discontinuity or spin memory loss at the interfaces that may reconcile all the different observations. In particular, we demonstrate consistent values of spin diffusion length (ℓ sf = 3.4 ± 0.4 nm) and spin Hall angle (íµí¼ƒ SH = 0.056 ± 0.010) for Pt in different metallic multilayer systems.
    SPIE, Spintronics VII; 08/2014
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    ABSTRACT: Binary information encoded within the spin of carriers can be transferred into corresponding right- or left-handed circularly polarized photons emitted from an active semiconductor medium via carrier-photon angular momentum conversion. In order to attain maximized spin injection at out-of-plane magnetic remanence, a number of material systems have been explored as possible solid-state spin injectors. However, the circular polarization (PC) of emitted light was still limited at 3–4% at remanence. Here, we demonstrate a sizable electroluminescence circular polarization from a III-V-based spin light-emitting diode at zero magnetic field with a perpendicular spin injector consisting of an ultrathin CoFeB ferromagnetic layer (1.2 nm) grown on a MgO tunnel barrier (2.5 nm). The maximum value of PC measured at zero field is as large as 20% at 25 K and still 8% at 300 K. These types of ultrathin perpendicular spin injectors are of great interest (i) to realize the electrical switching of the magnetization of the injector layer owing to the advanced spin-transfer torque properties of the CoFeB layer and (ii) to be directly embedded in optical cavities for spin lasers due to their very low optical absorption loss.
    Physical Review B 08/2014; 90(8):085310. DOI:10.1103/PhysRevB.90.085310 · 3.66 Impact Factor
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    ABSTRACT: An efficient electrical spin injection into an InGaAs/GaAs quantum well light emitting diode is demonstrated thanks to a CoFeB/MgO spin injector. The textured MgO tunnel barrier is fabricated by two different techniques: sputtering and molecular beam epitaxy. The maximal spin injection efficiency is comparable for both methods. Additionally, the effect of annealing is also investigated for the two types of samples. Both samples show the same trend: an increase of the electroluminescence circular polarization (Pc) with the increase of annealing temperature, followed by a saturation of Pc beyond 350 °C annealing. Since the increase of Pc starts well below the crystallization temperature of the full CoFeB bulk layer, this trend could be mainly due to an improvement of chemical structure at the top CoFeB/MgO interface. This study reveals that the control of CoFeB/MgO interface is essential for an optimal spin injection into semiconductor.
    Applied Physics Letters 07/2014; 105(1):012404-012404-5. DOI:10.1063/1.4887347 · 3.52 Impact Factor
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    ABSTRACT: We demonstrate a large electrical spin injection into GaAs at zero magnetic field thanks to an ultrathin perpendicularly magnetized CoFeB contact of a few atomic planes (1.2 nm). The spin-polarization of electrons injected into GaAs was examined by the circular polarization of electroluminescence from a Spin Light Emitting Diode with embedded InGaAs/GaAs quantum wells. The electroluminescence polarization as a function of the magnetic field closely traces the out-of-plane magnetization of the CoFeB/MgO injector. A circular polarization degree of the emitted light as large as 20% at 25 K is achieved at zero magnetic field. Moreover the electroluminescence circular polarization is still about 8% at room temperature.
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    ABSTRACT: We report an experimental study of a gold-tungsten alloy (7% at. W concentration in Au host) displaying remarkable properties for spintronics applications using both magneto-transport in lateral spin valve devices and spin-pumping with inverse spin Hall effect experiments. A very large spin Hall angle of about 10% is consistently found using both techniques with the reliable spin diffusion length of 2 nm estimated by the spin sink experiments in the lateral spin valves. With its chemical stability, high resistivity and small induced damping, this AuW alloy may find applications in the nearest future.
    Applied Physics Letters 03/2014; 104(14). DOI:10.1063/1.4870835 · 3.52 Impact Factor
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    ABSTRACT: Compact rolled-up nanomembranes of high quality (111)-oriented Co/Pt multilayers with perpendicular magnetic anisotropy are realized by combining strain engineering with top-down preparation methods. Magnetic force microscopy analyses were performed in the demagnetized state and under magnetic fields applied on rolled-up Co/Pt tubes. Observed magnetic domains are composed of radially polarized stripeshaped domains. The stripes running along the rolled-up tubes axis are observed for magnetic fields applied along this direction, whereas a salmon-like pattern is observed for magnetic fields applied along the tube diameter. Present results indicate potential for applications as magnetic encoders and servo motors.
    RSC Advances 01/2014; 4(16):8410. DOI:10.1039/c3ra46340k · 3.71 Impact Factor
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    ABSTRACT: We fabricated and characterized an optically pumped (100)-oriented InGaAs/GaAsP multiple quantum well Vertical External Cavity Surface Emitting Laser (VECSEL). The structure is designed to allow the integration of a Metal-Tunnel-Junction ferromagnetic spin-injector for future electrical injection. We report here the control at room temperature of the electromagnetic field polarization using optical spin injection in the active medium of the VECSEL. The switching between two highly circular polarization states had been demonstrated using an M-shaped extended cavity in multi-modes lasing. This result witnesses an efficient spin-injection in the active medium of the LASER.
    Applied Physics Letters 12/2013; 103(25):252402-252402-4. DOI:10.1063/1.4850676 · 3.52 Impact Factor
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    ABSTRACT: We have measured the inverse spin Hall effect (ISHE) in \textit{n}-Ge at room temperature. The spin current in germanium was generated by spin pumping from a CoFeB/MgO magnetic tunnel junction in order to prevent the impedance mismatch issue. A clear electromotive force was measured in Ge at the ferromagnetic resonance of CoFeB. The same study was then carried out on several test samples, in particular we have investigated the influence of the MgO tunnel barrier and sample annealing on the ISHE signal. First, the reference CoFeB/MgO bilayer grown on SiO$_{2}$ exhibits a clear electromotive force due to anisotropic magnetoresistance and anomalous Hall effect which is dominated by an asymmetric contribution with respect to the resonance field. We also found that the MgO tunnel barrier is essential to observe ISHE in Ge and that sample annealing systematically lead to an increase of the signal. We propose a theoretical model based on the presence of localized states at the interface between the MgO tunnel barrier and Ge to account for these observations. Finally, all of our results are fully consistent with the observation of ISHE in heavily doped $n$-Ge and we could estimate the spin Hall angle at room temperature to be $\approx$0.001.
    Physical review. B, Condensed matter 05/2013; 88(6). DOI:10.1103/PhysRevB.88.064403 · 3.66 Impact Factor
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    ABSTRACT: We have studied the perpendicular magnetic anisotropy of Co/Pt multilayers and the electron spin injection efficiency by optical spectroscopy from a [Co(0.6 nm)/Pt(1 nm)]4/Fe(0.3 nm)/MgO perpendicular tunnel spin injector grown on AlGaAs/GaAs semiconductor light-emitting diodes. We observe a 2.5% circular polarization at low temperature close to the magnetic remanence when the 0.3 nm Fe film of the ferromagnetic injector is sufficiently thin to maintain the magnetization out of plane. The acquired squared magnetization cycle is explained by the remaining interlayer exchange coupling existing between Fe and the (Co/Pt) multilayer through Pt or possible perpendicular magnetic anisotropy at the MgO/Fe interface. The corresponding spin polarization of the current is then estimated as 7%, measured by photoluminescence techniques, after the necessary up-renormalization, taking into account the electron spin-flip rate in the quantum well. In contrast, no circular polarization is observed when the thin Fe layer is removed and despite the rather high magnetic polarizability of the 5d9 electronic open shell of Pt at the interface with MgO. This emphasizes the reduced size of tunneling branching of wave functions at the interface, of the order of the atomic plane unit.
    Physical review. B, Condensed matter 11/2012; 86(20). DOI:10.1103/PhysRevB.86.205314 · 3.66 Impact Factor
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    ABSTRACT: We present experiments on a series of permalloy/gold lateral spin valves which show that their magnetoresistance (MR) is enhanced by about 100% in device geometries confining laterally the spin accumulation. We present the interpretation of this enhancement in the frame of the theory of diffusive spin transport in lateral nanostructures and we show that the MR enhancement can be generally related to the enhancement of both spin accumulation and current spin polarization.
    Physical Review B 06/2012; 85(22):220404. DOI:10.1103/PhysRevB.85.220404 · 3.66 Impact Factor
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    ABSTRACT: In this letter, we first show electrical spin injection in the germanium conduction band at room temperature and modulate the spin signal by applying a gate voltage to the channel. The corresponding signal modulation agrees well with the predictions of spin diffusion models. Then by setting a temperature gradient between germanium and the ferromagnet, we create a thermal spin accumulation in germanium without any tunnel charge current. We show that temperature gradients yield larger spin accumulations than pure electrical spin injection but, due to competing microscopic effects, the thermal spin accumulation in germanium remains surprisingly almost unchanged under the application of a gate voltage to the channel.
    Applied Physics Letters 04/2012; 101(2). DOI:10.1063/1.4733620 · 3.52 Impact Factor
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    ABSTRACT: Electrical spin injection into semiconductors paves the way for exploring new phenomena in the area of spin physics and new generations of spintronic devices. However the exact role of interface states in spin injection mechanism from a magnetic tunnel junction into a semiconductor is still under debate. In this letter, we demonstrate a clear transition from spin accumulation into interface states to spin injection in the conduction band of $n$-Ge. We observe spin signal amplification at low temperature due to spin accumulation into interface states followed by a clear transition towards spin injection in the conduction band from 200 K up to room temperature. In this regime, the spin signal is reduced down to a value compatible with spin diffusion model. More interestingly, we demonstrate in this regime a significant modulation of the spin signal by spin pumping generated by ferromagnetic resonance and also by applying a back-gate voltage which are clear manifestations of spin current and accumulation in the germanium conduction band.
    Physical Review Letters 03/2012; 109(10). DOI:10.1103/PhysRevLett.109.106603 · 7.73 Impact Factor
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    ABSTRACT: Using an advanced tight-binding approach, we estimate the anisotropy of the tunnel transmission associated with the rotation of the 5/2 spin of a single Mn atom forming an acceptor state in GaAs and located near an AlGaAs tunnel barrier. Significant anisotropies in both in-plane and out-of-plane geometries are found, resulting from the combination of the large spin-orbit coupling associated with the p-d exchange interaction, cubic anisotropy of heavy-hole dispersion and the low C2v symmetry of the chemical bonds.
    Applied Physics Letters 01/2012; 100(6). DOI:10.1063/1.3683525 · 3.52 Impact Factor
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    ABSTRACT: We have studied the influence of ultra-thin interfacial Fe layers on the structural and magnetoresistance properties of CO/Cu Multilayers. Our results show that the giant magnetoresistance arises from spin dependent scattering at the CO/Cu interfaces and in the bulk of Co, the interfacial contribution being predominant. We also demonstrate that the close-packed crystallographic structure of Co and Cu is very sensitive to the insertion of interfacial bec Fe layers: for small thicknesses, Co as well as Cu adopt a metastable bec structure.
    MRS Online Proceeding Library 01/2012; 313. DOI:10.1557/PROC-313-737
  • Physical Review Letters 12/2011; DOI:10.1103/PhysRevLett.107.249901 · 7.73 Impact Factor
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    ABSTRACT: In this letter, we report on successful electrical spin injection and detection in \textit{n}-type germanium-on-insulator (GOI) using a Co/Py/Al$_{2}$O$_{3}$ spin injector and 3-terminal non-local measurements. We observe an enhanced spin accumulation signal of the order of 1 meV consistent with the sequential tunneling process via interface states in the vicinity of the Al$_{2}$O$_{3}$/Ge interface. This spin signal is further observable up to 220 K. Moreover, the presence of a strong \textit{inverted} Hanle effect points at the influence of random fields arising from interface roughness on the injected spins.
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    ABSTRACT: Although the creation of spin polarization in various non-magnetic media via electrical spin injection from a ferromagnetic tunnel contact has been demonstrated, much of the basic behavior is heavily debated. It is reported here for semiconductor/Al2O3/ferromagnet tunnel structures based on Si or GaAs that local magnetostatic fields arising from interface roughness dramatically alter and even dominate the accumulation and dynamics of spins in the semiconductor. Spin precession in the inhomogeneous magnetic fields is shown to reduce the spin accumulation up to tenfold, and causes it to be inhomogeneous and non-collinear with the injector magnetization. The inverted Hanle effect serves as experimental signature. This interaction needs to be taken into account in the analysis of experimental data, particularly in extracting the spin lifetime and its variation with different parameters (temperature, doping concentration). It produces a broadening of the standard Hanle curve and thereby an apparent reduction of the spin lifetime. For heavily doped n-type Si at room temperature it is shown that the spin lifetime is larger than previously determined, and a new lower bound of 0.29 ns is obtained. The results are expected to be general and occur for spins near a magnetic interface not only in semiconductors but also in metals, organic and carbon-based materials including graphene, and in various spintronic device structures.
    Physical review. B, Condensed matter 01/2011; 84. DOI:10.1103/PhysRevB.84.054410 · 3.66 Impact Factor
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    ABSTRACT: We have measured in the same hybrid semiconductor/ferromagnetic (FM) metal structures the photocurrent obtained under polarized optical excitation and the polarized electroluminescence recorded under forward electric bias (spin light-emitting diode operation). The systematic investigations have been performed on devices with different ferromagnetic spin injectors: tunnel barrier of Al2O3 surmounted by a thin Co ferromagnetic layer or MgO tunnel barriers with a CoFeB FM layer. The semiconductor part of the device is composed of an AlGaAs diode with a GaAs/AlGaAs quantum well embedded in the intrinsic region. Though a very efficient electrical spin injection is demonstrated with a measured circular polarization of the electroluminescence up to 30% for an external field of 0.8 T, very weak polarizations of the photocurrent are evidenced whatever the nature of the device is. The maximum photocurrent polarization obtained under continuous resonant circularly polarized excitation of the quantum well excitons is about 3%. This demonstrates that the investigated devices do not act as an efficient spin filter for the electrons flowing from the semiconductor part toward the ferromagnetic part of these structures though these layers are very efficient spin aligners for electrical spin injection. We interpret the weak measured polarization of the photocurrent in the percent range as a consequence of the Zeeman splitting of the quantum well excitons which yields different absorption coefficients for the polarized excitation laser with different helicities. This leads to different intensities of photocurrent collected for the two different circularly polarized excitations. This interpretation is confirmed by an experiment exhibiting the same results for photocurrent measured on a device with a nonferromagnetic electrode.
    Physical review. B, Condensed matter 11/2010; 82(19). DOI:10.1103/PhysRevB.82.195317 · 3.66 Impact Factor

Publication Stats

3k Citations
239.54 Total Impact Points

Institutions

  • 1992–2015
    • Université Paris-Sud 11
      • Laboratoire de Physique des Solides
      Orsay, Île-de-France, France
  • 1997–2013
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
  • 1994–2002
    • Catholic University of Louvain
      • Institute of Condensed Matter and Nanosciences
      Walloon Region, Belgium
  • 1993
    • Laboratory of Plasma Physics
      Paliseau, Île-de-France, France