Josephson junctions act as a natural spiking neuron-like device for neuromorphic computing. By leveraging the advances recently demonstrated in digital single flux quantum (SFQ) circuits and using recently demonstrated magnetic Josephson junction (MJJ) synaptic circuits, there is potential to make rapid progress in SFQ-based neuromorphic computing. Here we demonstrate the basic functionality of a synaptic circuit design that takes advantage of the adjustable critical current demonstrated in MJJs and implement a synaptic weighting element. The devices were fabricated with a restively shunted Nb/AlO x-Al/Nb process that did not include MJJs. Instead, the MJJ functionality was tested by making multiple circuits and varying the critical current, but not the external shunt resistance, of the oxide Josephson junction that represents the MJJ. Experimental measurements and simulations of the fabricated circuits are in good agreement.

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We report on the strong coupling between a metallic ferromagnetic Fe75Co25 thin film patterned element and a range of superconducting Nb half-wavelength co-planar waveguide (CPW) resonators. By varying the volume of the ferromagnet we demonstrate that the coupling rate scales linearly with the square root of the number of spins and achieve a coupling rate over 700 MHz, approaching the ultrastrong coupling regime. Experiments varying the center conductor width while maintaining constant magnetic volume verify that decreasing the center conductor width increases coupling and cooperativity. Our results show that the frequency dependence of the coupling rate is linear with the fundamental and higher order odd harmonics of the CPW, but with differing efficiencies. The results show promise for scaling planar superconducting resonator/magnetic hybrid systems to smaller dimensions.

We have measured the synaptic properties of a magnetic Josephson junction (MJJ) consisting of a barrier of amorphous Ge containing size-selected Fe nanoclusters between two Nb electrodes. We show that the critical current of the device varies with the magnetic order of the clusters and that the magnetic order can be tuned in a quasi-analog way with short electrical pulses of 16 pJ applied through the device. This is the first demonstration of critical current tuning via magnetic order in a nano-clustered MJJ in a material system other than SiMn. This result eliminates the need for a postdeposition annealing step for synaptic MJJs in neuromorphic applications and opens new options to optimize materials, which has the potential to improve the performance of the MJJs and provide a better understanding of the physics of the device.

A nanoclustered magnetic Josephson junction (nMJJ) is a hybrid magnetic-superconducting device that can be used as an artificial synapse in neuromorphic applications. In this paper, we review the nMJJ from the device level to the circuit level. We describe the properties of individual devices and show how they can be integrated into a neuromorphic circuit. We discuss the current limitations related to the study of the nMJJ, what can be done to improve the device and better understand the underlying physics, and where the community can focus its efforts to develop magnetic Josephson junctions for neuromorphic applications.

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