W. Van Roy

Publications (12)22.07 Total impact

• Article: Understanding Nanoscale Temperature Gradients in Magnetic Nanocontacts.

  S Petit-Watelot, R M Otxoa, M Manfrini, W Van Roy, L Lagae, J-V Kim, T Devolder
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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. · 7.37 Impact Factor
• Source

  Available from: Claude Chappert

  Article: Vortex Nucleation Phase in Spin Torque Oscillators Based on Nanocontacts

  T. Devolder, Joo-Von Kim, S. Petit-Watelot, R. Otxoa, C. Chappert, M. Manfrini, W. Van Roy, L. Lagae
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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; · 1.36 Impact Factor
• Article: Nanocontact size dependence of the properties of vortex‐based spin torque oscillators

  R. M. Otxoa, M. Manfrini, T. Devolder, J.-V. Kim, W. van Roy, L. Lagae, C. Chappert
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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. · 1.32 Impact Factor
• Source

  Available from: Claude Chappert

  Article: Frequency shift keying in vortex-based spin torque oscillators

  M. Manfrini, T. Devolder, Joo-Von Kim, P. Crozat, C. Chappert, W. Van Roy, L. Lagae
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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; · 2.17 Impact Factor
• Source

  Available from: Claude Chappert

  Article: Direct experimental measurement of phase-amplitude coupling in spin torque oscillators

  L. Bianchini, S. Cornelissen, Joo-Von Kim, T. Devolder, W. Van Roy, L. Lagae, C. Chappert
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  ABSTRACT: We study spin-torque induced oscillations of MgO magnetic tunnel junctions in the time domain. By using the Hilbert transform on the time traces, we obtain for the first time a direct experimental measure of the coupling between the power and the phase fluctuations. We deduce the power restoration rate and we obtain low values for the coupling strength, which is consistent with the weak frequency dependence on the applied voltage. Comment: 3 pages, 3 figures
  04/2010;
• Article: Free layer versus synthetic ferrimagnet layer auto-oscillations in nanopillars processed from MgO-based magnetic tunnel junctions

  S Cornelissen, L Bianchini, T Devolder, Joo-Von Kim, W Van Roy, L Lagae, C Chappert
  Physical Review B. 04/2010; 81(14):144408.
• Article: Influence of oscillation modes on the line width of rf emissions in MgO based nanopillars

  G Hrkac, A Goncharov, J Dean, T Schrefl, Joo-Von Kim, T Devolder, C Chappert, S Cornelissen, W Van Roy, L Lagae
  Journal of Applied Physics. 01/2010; 108(2):023917.
• Source

  Available from: Claude Chappert

  Article: Vortex nucleation in spin-torque nanocontact oscillators

  T Devolder, Joo-Von Kim, M Manfrini, W Van Roy, L Lagae, C Chappert
  Applied Physics Letters 01/2010; 97(7):072512. · 3.84 Impact Factor
• Source

  Available from: Claude Chappert

  Article: Agility of vortex-based nanocontact spin torque oscillators

  M. Manfrini, T. Devolder, Joo-Von Kim, P. Crozat, N. Zerounian, C. Chappert, W. Van Roy, L. Lagae, G. Hrkac, T. Schrefl
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  ABSTRACT: We study the agility of current-tunable oscillators based on a magnetic vortex orbiting around a point contact in spin-valves. Theory predicts frequency-tuning by currents occurs at constant orbital radius, so an exceptional agility is anticipated. To test this, we have inserted an oscillator in a microwave interferometer to apply abrupt current variations while time resolving its emission. Using frequency shift keying, we show that the oscillator can switch between two stabilized frequencies differing by 25% in less than ten periods. With a wide frequency tunability and a good agility, such oscillators possess desirable figures of merit for modulation-based rf applications. Comment: 3 pages, 3 figures
  10/2009;
• Source

  Available from: Claude Chappert

  Article: Quantized spin wave modes in magnetic tunnel junction nanopillars

  A. Helmer, S. Cornelissen, T. Devolder, J.-V. Kim, W. Van Roy, L. Lagae, C. Chappert
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  ABSTRACT: We present an experimental and theoretical study of the magnetic field dependence of the mode frequency of thermally excited spin waves in rectangular shaped nanopillars of lateral sizes 60x100, 75x150, and 105x190 nm2, patterned from MgO-based magnetic tunnel junctions. The spin wave frequencies were measured using spectrally resolved electrical noise measurements. In all spectra, several independent quantized spin wave modes have been observed and could be identified as eigenexcitations of the free layer and of the synthetic antiferromagnet of the junction. Using a theoretical approach based on the diagonalization of the dynamical matrix of a system of three coupled, spatially confined magnetic layers, we have modeled the spectra for the smallest pillar and have extracted its material parameters. The magnetization and exchange stiffness constant of the CoFeB free layer are thereby found to be substantially reduced compared to the corresponding thin film values. Moreover, we could infer that the pinning of the magnetization at the lateral boundaries must be weak. Finally, the interlayer dipolar coupling between the free layer and the synthetic antiferromagnet causes mode anticrossings with gap openings up to 2 GHz. At low fields and in the larger pillars, there is clear evidence for strong non-uniformities of the layer magnetizations. In particular, at zero field the lowest mode is not the fundamental mode, but a mode most likely localized near the layer edges. Comment: 16 pages, 4 figures, (re)submitted to PRB
  07/2009;
• Article: Effect of patterning on the saturation magnetization in MgO based nanopillars

  S. Cornelissen, L. Bianchini, A. Helmer, T. Devolder, Joo-Von Kim, M. Op de Beeck, W. Van Roy, L. Lagae, C. Chappert
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  ABSTRACT: We have studied the effect of nanopillar patterning on the saturation magnetization of the CoFeB free layer in MgO magnetic tunnel junctions. Before patterning, the free layer magnetization is measured by ferromagnetic resonance and is found to be close to the bulk value, with no detectable interface anisotropy. After patterning, the shape anisotropy and the frequency of the main spin wave mode indicate that the free layer magnetization is substantially reduced. Current dependent measurements indicate that this is not due to Joule heating. Size dependent measurements indicate that the magnetization reduction most likely arises from process damage during the etching step.
  Journal of Applied Physics 05/2009; · 2.17 Impact Factor
• Article: Time-resolved zero field vortex oscillations in point contacts

  T Devolder, Joo-Von Kim, P Crozat, C Chappert, M Manfrini, M Van Kampen, W Van Roy, L Lagae, G Hrkac, T Schrefl
  Applied Physics Letters 01/2009; 95(1):012507. · 3.84 Impact Factor