W. Van Roy

imec Belgium, Louvain, Flanders, Belgium

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Publications (125)341.14 Total impact

  • Advanced Optical Materials. 11/2014;
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    ABSTRACT: Spin-dependent quantum transport experiments on InSb and InAs heterostructures and Bi thin films are discussed, focusing on mesoscopic geometries where spin-orbit interaction and quantum coherence determine the properties. The narrow-bandgap semiconductors InSb and InAs, and the semimetal Bi have substantial spin-orbit interaction. The experiments use antilocalization to study spin-orbit interaction and spin coherence lengths in nanolithographic wires fabricated on the materials. In the three systems the spin coherence lengths increase with decreasing wire widths if other parameters stay constant, of technological importance for spin-based devices. The experiments also indicate that Bi has surface states with Rashba-like spin-orbit interaction. A quasi-one-dimensional model of antilocalization, as fitted to the data, is explained and its consequences for quantum coherence in mesoscopic structures is explored. A united understanding of the experiments is presented relying on the duality between the Aharonov-Bohm and the Aharonov-Casher phases, the latter resulting from spin-orbit interaction. The duality strengthens the analogy between phenomena under magnetic fields and under spin-orbit interaction.
    SPIE NanoScience + Engineering; 08/2014
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    ABSTRACT: We propose a write scheme for perpendicular spin-transfer torque magnetoresistive random-access memory that significantly reduces the required tunnel current density and write energy. A sub-nanosecond in-plane polarized spin current pulse is generated using the spin-Hall effect, disturbing the stable magnetic state. Subsequent switching using out-of-plane polarized spin current becomes highly efficient. Through evaluation of the Landau-Lifshitz-Gilbert equation, we quantitatively assess the viability of this write scheme for a wide range of system parameters. A typical example shows an eight-fold reduction in tunnel current density, corresponding to a fifty-fold reduction in write energy, while maintaining a 1 ns write time.
    Applied Physics Letters 12/2013; 104(1). · 3.52 Impact Factor
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    ABSTRACT: Magnetic vortices in thin films are in-plane spiral spin configurations with a core in which the magnetization twists out of the film plane. Vortices result from the competition between atomic-scale exchange forces and long-range dipolar interactions. They are often the ground state of magnetic dots, and have applications in medicine, microwave generation and information storage. The compact nature of the vortex core, which is 10-20 nm wide, makes it a suitable probe of magnetism at the nanoscale. However, thus far the positioning of a vortex has been possible only in confined structures, which prevents its transport over large distances. Here we show that vortices can be propagated in an unconstrained system that comprises electrical nanocontacts (NCs). The NCs are used as tunable vortex attractors in a manner that resembles the propelling of space craft with gravitational slingshots. By passing current from the NCs to a ferromagnetic film, circulating magnetic fields are generated, which nucleate the vortex and create a potential well for it. The current becomes spin polarized in the film, and thereby drives the vortex into gyration through spin-transfer torques. The vortex can be guided from one NC to another by tuning attractive strengths of the NCs. We anticipate that NC networks may be used as multiterminal sources of vortices and spin waves (as well as heat, spin and charge flows) to sense the fundamental interactions between physical objects and fluxes of the next-generation spintronic devices.
    Nature Nanotechnology 12/2013; · 31.17 Impact Factor
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    ABSTRACT: Measurements of low-temperature magnetotransport in lithographic wires of submicron widths fabricated from high-mobility AlGaSb/InAs/AlGaSb two-dimensional electron system heterostructures are presented. The dependence of the spin and phase coherence lengths on wire width and diffusion constant is investigated by analyzing the conductance in low applied magnetic fields with antilocalization models. Predominantly diffusive boundary scattering is deduced from the magnitude and wire width dependence of the conductance. Diffusive boundary scattering leads to a diffusion constant decreasing with wire width and hence allows the dependence of spin coherence on wire width and diffusion constant to be investigated concurrently. The spin coherence lengths are experimentally found to be proportional to the ratio of the diffusion constant to wire width. The phase coherence lengths follow Nyquist decoherence for low-dimensional wires.
    Physical Review B 11/2013; 88(20). · 3.66 Impact Factor
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    ABSTRACT: A classical 3-port optical circulator is demonstrated on the silicon-on-insulator (SOI) platform. A garnet die with a magneto-optical cerium-doped yttrium iron garnet (Ce:YIG) layer is bonded on top of a Mach-Zehnder interferometer circuit using a thin adhesive bonding layer. The power transmission between different ports is characterized in the presence of an external magnetic field, transversal to the light propagation direction. An isolation of 22 dB is measured at a wavelength of 1562 nm.
    Optics Letters 03/2013; 38(6):965-7. · 3.39 Impact Factor
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    ABSTRACT: We have determined the temperature profile in magnetic nanocontacts under applied current densities typical of spin-torque oscillators (∼10^{8} A/cm^{2}). The study combines experimental measurements of the electrical and magnetic properties of the nanocontacts and full three-dimensional simulations of the heat and current flow in these systems. It is found that the quadratic current-induced increase of the resistance due to Joule heating is independent of the applied temperature from 6 to 300 K. In terms of magnetization dynamics, the measured current-induced vortex nucleation, a thermally activated process, is found to be consistent with local temperatures increases of between 147 and 225 K. Simulations reproduce the experimental findings and show that significant thermal gradients exist out to 450 nm from the nanocontact.
    Physical Review Letters 12/2012; 109(26):267205. · 7.73 Impact Factor
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    ABSTRACT: We demonstrate an optical isolator integrated with a silicon-on-insulator waveguide platform realized by the adhesive bonding of a Ce:YIG/SGGG die on top of a Mach-Zehnder interferometer (MZI). The design is based on the different nonreciprocal phase shifts (NRPS) experienced by both arms of the interferometer, which have different waveguide widths. Simulation of NRPS versus silicon waveguide width is shown for a few different benzocyclobutene bonding layer thicknesses for a particular silicon waveguide thickness and Ce:YIG/SGGG stack. Variations of NRPS as a function of MZI arm lengths with bonded stack on top of MZI are measured. Optical isolation of 11 dB is experimentally obtained for a device with a footprint of 1.5 mm × 4 μm.
    IEEE Photonics Technology Letters 09/2012; 24(18):1653-1656. · 2.18 Impact Factor
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    ABSTRACT: We determine the temperature profile in magnetic nanocontacts submitted to the very large current densities that are commonly used for spin-torque oscillator behavior. Experimentally, the quadratic current-induced increase of the resistance through Joule heating is independent of the applied temperature from 6 K to 300 K. The modeling of the experimental rate of the current-induced nucleation of a vortex under the nanocontact, assuming a thermally-activated process, is consistent with a local temperature increase between 150 K and 220 K. Simulations of heat generation and diffusion for the actual tridimensional geometry were conducted. They indicate a temperature-independent efficiency of the heat sinking from the electrodes, combined with a localized heating source arising from a nanocontact resistance that is also essentially temperature-independent. For practical currents, we conclude that the local increase of temperature is typically 160 K and it extends 450 nm about the nanocontact. Our findings imply that taking into account the current-induced heating at the nanoscale is essential for the understanding of magnetization dynamics in nanocontact systems.
    06/2012;
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    ABSTRACT: A waveguide optical isolator realized by adhesive bonding of a garnet die, containing a Ce:YIG magneto-optic layer, on a silicon-on-insulator waveguide circuit is demonstrated. The die was bonded on top of an asymmetric Mach-Zehnder interferometer using a 100nm thick DVS-BCB adhesive bonding layer. A static magnetic field applied perpendicular to the light propagation direction results in a non-reciprocal phase shift for the fundamental quasi-TM mode in the hybrid waveguide geometry. A maximum optical isolation of 25 dB is obtained.
    Optics Express 01/2012; 20(2):1839-48. · 3.53 Impact Factor
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    ABSTRACT: Localized surface plasmon resonances possess very interesting properties for a wide variety of sensing applications. In many of the existing applications only the intensity of the reflected or transmitted signals is taken into account, while the phase information is ignored. At the center frequency of a (localized) surface plasmon resonance, the electron cloud makes the transition between in- and out-of-phase oscillation with respect to the incident wave. Here we show that this information can experimentally be extracted by performing phase-sensitive measurements, which result in linewidths that are almost one order of magnitude smaller than those for intensity based measurements. As this phase transition is an intrinsic property of a plasmon resonance, this opens up many possibilities for boosting the figure of merit (FOM) of refractive index sensing by taking into account the phase of the plasmon resonance. We experimentally investigated this for two model systems: randomly distributed gold nanodisks and gold nanorings on top of a continuous gold layer and a dielectric spacer and observed FOM values up to 8.3 and 16.5 for the respective nanoparticles.
    Nano Letters 01/2012; 12(3). · 13.03 Impact Factor
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    ABSTRACT: In diffusive two-dimensional electron systems (2DESs) with linear spin-orbit interaction (SOI), quasi-one-dimensional (Q1D) confinement in narrow wires of width W is theoretically predicted to result in an enhancement of the spin relaxation length LS, such that LS~1/W. We present our experimental data of the dependence of LS on W for three different systems: 1) ballistic InSb wires with specular boundary scattering and strong Dresselhaus SOI, 2) ballistic InAs wires with diffusive boundary scattering and Rashba SOI, and 3) diffusive bismuth wires with a large density of states at the surface. For all three systems, information on the spin relaxation is gathered from the weak-antilocalization effect (WAL), a magnetotransport measurement sensitive to both the spin-orbit coherence length and the phase coherence length Lφ. We find a dependence of LS on W, where LS increases with decreasing W in all three systems. However, the theory is valid for Q1D diffusive wires with linear SOI, which does not fit the profile of all our systems. Thus, we conclude that the increase in LS in narrow wires is a more universal phenomenon that can be extended to systems outside the theory, which impacts various applications.
    12/2011;
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    ABSTRACT: In this paper we demonstrate an optical isolator on a Silicon-on-Insulator waveguide platform realized by the adhesive bonding of Ce:YIG on top of a Mach-Zehnder interferometer. An optical isolation of 25dB is experimentally obtained.
    Group IV Photonics (GFP), 2011 8th IEEE International Conference on; 10/2011
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    ABSTRACT: We study the starting up phase of a current-controlled oscillator based on a magnetic vortex orbiting around a nanocontact in a spin-valve. From the idle state, current pulses down to a few nanoseconds can create the vortex, which is detected through the electrical signature of its steady-state gyration. Two ns are needed to reach the in-current equilibrium. The process can then be described by an Arrhenius law, with an activation energy that is consistent with the Oersted-field-induced separation of a vortex-antivortex pair. Requirements for deterministic nucleation are deduced, with prospects for instant-on oscillator capability.
    IEEE Transactions on Magnetics 07/2011; · 1.21 Impact Factor
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    ABSTRACT: We study the frequency, linewidth, and power of spin torque driven vortex oscillators, based on a nanocontacted spin-valve (SV). The oscillation frequency strongly decreases with the contact size, and increases with the current. The power delivered by the oscillator is not quadratic with the current, in contrast with the behavior expected from the rigid vortex model (RVM). The linewidth is almost independent of the current at low current and does not strongly depend on the nanocontact size. We compare our findings with the outcomes of the RVM.
    physica status solidi (b) 06/2011; 248(7):1615 - 1618. · 1.61 Impact Factor
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    ABSTRACT: Vortex-based spin-torque oscillators can be made from extended spin valves connected to an electrical nanocontact. We study the implementation of frequency shift keying modulation in these oscillators. Upon a square modulation of the current in the 10  MHz range, the vortex frequency follows the current command, with easy identification of the two swapping frequencies in the spectral measurements. The frequency distribution of the output power can be accounted for by convolution transformations of the dc current vortex waveform, and the current modulation. Modeling indicates that the frequency transitions are phase coherent and last less than 25 ns. Complementing the multi-octave tunability and first-class agility, the capability of frequency shift keying modulation is an additional milestone for the implementation of vortex-based oscillators in radio frequency circuits.
    Journal of Applied Physics 05/2011; · 2.19 Impact Factor
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    ABSTRACT: We investigate electron spin- and phase coherence in an array of quasi-ballistic InAs quantum well mesoscopic rings through observation of Aharonov-Bohm h/e oscillations (AB) and Altshuler-Aronov-Spivak h/2e oscillations (AAS). The temperature dependence of the AAS oscillations is characterized through a single effective coherence length, Leff, following the formalism of Doucot and Rammal, from which the phase coherence length, Lphi and the spin coherence length as limited by spin-orbit interaction, LSO, are extracted. AB oscillations are also present, and can be separated from AAS by Fourier transformation. We contrast the AAS method of extracting the coherence lengths with analysis of the AB oscillation amplitudes. Previous studies have examined Lphi from AB signals in single ballistic rings, or by using AAS amplitudes in large networks, or have observed AB and AAS in single rings with spin-orbit interaction. Here the presence of both AB and AAS in an array with spin-orbit interaction allows for study of both Lphi and LSO, and enables direct juxtaposition of different quantum coherence phenomena as means for measuring coherence lengths (DOE DE-FG02-08ER46532).
    03/2011;
  • MRS Online Proceeding Library 01/2011; 384.
  • ChemInform 10/2010; 27(41).

Publication Stats

1k Citations
341.14 Total Impact Points

Institutions

  • 1995–2013
    • imec Belgium
      • Smart Systems and Energy Technology
      Louvain, Flanders, Belgium
    • Radboud University Nijmegen
      Nymegen, Gelderland, Netherlands
  • 2011
    • Université Paris-Sud 11
      • Institut d'Electronique Fondamentale
      Paris, Ile-de-France, France
  • 2009
    • Inha University
      • Department of Physics
      Seoul, Seoul, South Korea
    • Osaka University
      • Department of Materials Engineering Science
      Ōsaka-shi, Osaka-fu, Japan
  • 2007
    • University of Oxford
      • Department of Materials
      Oxford, England, United Kingdom
  • 2006
    • KU Leuven
      • Institute of Nanoscale Physics and Chemistry (INPAC)
      Leuven, VLG, Belgium
  • 2004
    • Technische Universiteit Eindhoven
      • Department of Applied Physics
      Eindhoven, North Brabant, Netherlands
  • 2002
    • Polish Academy of Sciences
      • Institute of Physics
      Warszawa, Masovian Voivodeship, Poland