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Noise-Enhanced Synchronization of Stochastic Magnetic Oscillators

N. Locatelli, A. Mizrahi, A. Accioly, R. Matsumoto, A. Fukushima, H. Kubota, S. Yuasa, V. Cros, L. G. Pereira, D. Querlioz, J.-V. Kim, and J. Grollier
Phys. Rev. Applied 2, 034009 – Published 16 September 2014
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  • INTRODUCTION
  • MAGNETIC TUNNEL JUNCTION AS A STOCHASTIC…
  • EXPERIMENTAL STUDY
  • CONCLUSION
  • ACKNOWLEDGEMENTS
    • References

    Abstract

    We present an experimental study of phase locking in a stochastic magnetic oscillator. The system comprises a magnetic tunnel junction with a superparamagnetic free layer, whose magnetization dynamics is driven with spin torques through an external periodic driving current. We show that synchronization of this stochastic oscillator to the input current is possible for current densities below 3×106  A/cm2, and occurs for input frequencies lower than the natural mean frequency of the stochastic oscillator. We show that such injection locking is robust and leads to a drastic reduction in the phase diffusion of the stochastic oscillator, despite the presence of a frequency mismatch between the oscillator and the excitation.

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    • Received 16 May 2014

    DOI:https://doi.org/10.1103/PhysRevApplied.2.034009

    © 2014 American Physical Society

    Authors & Affiliations

    N. Locatelli1,2, A. Mizrahi1,2, A. Accioly1,2,3, R. Matsumoto4, A. Fukushima4, H. Kubota4, S. Yuasa4, V. Cros1, L. G. Pereira3, D. Querlioz2, J.-V. Kim2, and J. Grollier1,*

    • 1Unité Mixte de Physique CNRS/Thales, 1 avenue A. Fresnel, Campus de l’Ecole Polytechnique, 91767 Palaiseau, France, and Université Paris-Sud, 91405 Orsay, France
    • 2Institut d’Electronique Fondamentale, UMR CNRS 8622, Université Paris-Sud, 91405 Orsay, France
    • 3Instituto de Física, Universidade Federal do Rio Grande do Sul, Porto Alegre 91501-970, Brazil
    • 4National Institute of Advanced Industrial Science and Technology (AIST) 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan

    • *julie.grollier@thalesgroup.com

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    Issue

    Vol. 2, Iss. 3 — September 2014

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    Images

    • Figure 1
      Figure 1

      (a) Schematic of the magnetic tunnel junction stack. (b) Schematic of double-well energy landscape for the free-layer magnetization, that can hop between the P and AP states with the assistance of thermal fluctuations and spin torque. (c) Sample of the telegraphic temporal resistance evolution generated by the MTJ, measured at room temperature under a Idc=100  μA current and an applied field of −30  Oe which combined with the stray field from the SAF favors the P state. (d) Associated dwell-time distributions for P and AP states, fitted by an exponential envelope corresponding to a Poissonian distribution. (e) Resistance versus dc current curve obtained at zero effective field, i.e., with an applied field of −37  Oe compensating the stray field of the synthetic antiferromagnet.

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    • Figure 2
      Figure 2

      MTJ response to different current amplitudes Iac=250, 200, 150, and 100  μA. (a) Matching time between the resistance response and the excitation signal. (b) Average oscillation frequency of the resistance response ⟨F⟩ versus excitation frequency Fac. The dashed line corresponds to a match between response and excitation frequencies.

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    • Figure 3
      Figure 3

      SP MTJ response to square-wave current excitations with 250  μA amplitude and frequencies Fac=450  Hz (left), Fac=100  Hz (center), Fac=7.8  Hz (right). Sample of the temporal evolution of (a) the current through the MTJ, (b) its resistance response, and (c) the piecewise linear reconstructed phase for the current (φe, black) and for the resistance (φs, red). (d) Associated dwell-time distributions for both high (AP) and low (P) resistance states.

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    • Figure 4
      Figure 4

      (a) Effective diffusion constant Deff as a function of the input excitation frequency Fac for a current amplitude Iac=250  μA. Ten time traces of the stochastic oscillator phase for input frequencies of (b) 210 Hz and (c) 700 Hz, indicated by the red stars in (a). The linear variation corresponding to the average frequency is subtracted for clarity.

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