Research ArticleAPPLIED SCIENCES AND ENGINEERING

Ultralow power artificial synapses using nanotextured magnetic Josephson junctions

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Science Advances  26 Jan 2018:
Vol. 4, no. 1, e1701329
DOI: 10.1126/sciadv.1701329

Figures

  • Fig. 1 Spin-dependent superconducting transport.

    (A) Schematic of the JJ synapse in the magnetically disordered state (left) and the magnetically ordered state (right). (B) Data (blue circles) and fit to the resistively shunted junction (RSJ) model (red line) of the voltage-current characteristic taken at 4 K on a 10-μm-diameter JJ synapse in the disordered state. The data are well fit with an RSJ shown in red. Lower inset: Simulated voltage spike train above Ic; the average of the spike train results in the measured quasi-static voltage measured. Upper inset: Schematic showing the magnetically disordered state of the junction for this measurement. (C) Data (blue circles) and fit to the RSJ model (red line) of the voltage versus current characteristic taken at 4 K on the same 10-μm-diameter JJ synapse in the ordered state. Inset: Schematic showing the magnetically ordered state of the junction for this measurement. (D) Fraunhofer diffraction of the same JJ synapse in varying magnetic ordered states. The most ordered state is in red, disorder is increased for black and magenta, and most disordered data are in blue. Black data are offset by 0.3 mA, magenta data are offset by 0.6 mA, and blue data are offset by 0.9 mA. Data are shown as squares with fits to the Airy function shown as lines.

  • Fig. 2 Device operation and scaling.

    (A) Critical current measured in a zero field after the application of electrical ordering pulses (ordering pulses were applied in a 20-mT magnetic field); the line serves as a visual guide. (B) Electric pulse energy required to magnetically order the JJ synapse in a 20-mT applied magnetic field (red squares) and SFQ pulse energy (blue circles) versus JJ synapse cross-sectional area.

  • Fig. 3 JJ synapse SPICE simulation.

    (A) Circuit diagram used in the simulation, where Ic of JJ1 = JJ2 = JJ3 = 200 μA, L1 = L3 = 3 pH (picohenry), L2 = 5 pH, and R1 = 100 milliohm. (B) Peak current through the coupling inductor L2 versus Ic of the MJJ. Ic values within the blue box are those measured in a single MJJ. (C) Circuit operation with low magnetic order Ic = 100 μA (low synaptic weight) showing the phase modulation of the presynaptic (input) JJ (blue), the JJ synapse (black), the postsynaptic (output) JJ (red), and the output voltage (red right axis). (D) Circuit operation with high magnetic order Ic = 50 μA (high synaptic weight) showing the phase modulation of the presynaptic (input) JJ (blue), the JJ synapse (black), the postsynaptic (output) JJ (red), and the output voltage (red left axis).

  • Fig. 4 SPICE simulation of stochasticity.

    (A) Measured critical current of a 10-μm JJ synapse in the partially ordered magnetic state as a function of temperature (black circles); lines serve as visual guides. Simulated stochasticity of a 100-nm JJ synapse as a function of temperature (blue squares). (B) Block diagram of the circuit model. (C) Voltage at output (black) and magnetic order (blue) as a function of time simulated at 2 K. (D) Voltage at output (black) and magnetic order (blue) as a function of time simulated at 3 K.

  • Fig. 5 Magnetic nanocluster properties.

    (A) Magnetic moment versus temperature under zero field–cooled (ZFC) (black) and field-cooled (red) conditions. The peak in the zero field–cooled curve gives a measure of the energy required to reorient the magnetic clusters. (B) Magnetic moment versus applied magnetic field at 70 K; data are in blue circles, and Langevin fit to the data are shown as a red line. The Langevin fit gives an estimated magnetic cluster density and moment of 2 × 104 Mn clusters/μm2 and 2000 μB, respectively.

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