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We experimentally demonstrate the operation of a Josephson magnetic random access memory unit cell, built with a Ni80Fe20/Cu/Ni pseudo spin-valve Josephson junction with Nb electrodes and an integrated readout SQUID in a fully planarized Nb fabrication process. We show that the parallel and anti-parallel memory states of the spin-valve can be mapped onto a junction equilibrium phase of either zero or π by appropriate choice of the ferromagnet thicknesses, and that the magnetic Josephson junction can be written to either a zero-junction or π-junction state by application of write fields of approximately 5 mT. This work represents a first step towards a scalable, dense, and power-efficient cryogenic memory for superconducting high-performance digital computing.

We have measured spin-triplet supercurrent in Josephson junctions of the form S/F'/F/F'/S, where S is superconducting Nb, F' is a thin Ni layer with in-plane magnetization, and F is a Ni/[Co/Ni]n multilayer with out-of-plane magnetization. The supercurrent in these junctions decays very slowly with F-layer thickness, and is much larger than in similar junctions not containing the two F' layers. Those two features are the characteristic signatures of spin-triplet supercurrent, which is maximized by the orthogonality of the magnetizations in the F and F' layers. Magnetic measurements confirm the out-of-plane anisotropy of the Co/Ni multilayers. These samples have their critical current optimized in the as-prepared state, which will be useful for future applications.

Josephson junctions containing ferromagnetic layers are of considerable interest for the development of practical cryogenic memory and superconducting qubits. Such junctions exhibit a ground-state phase shift of π for certain ranges of ferromagnetic layer thickness. We present studies of Nb based micron-scale ellipticallyshaped Josephson junctions containing ferromagnetic barriers of Ni 81 Fe 19 or Ni 65 Fe 15 Co 20 . By applying an external magnetic field, the critical current of the junctions are found to follow characteristic Fraunhofer patterns, and display sharp switching behavior suggestive of single-domain magnets. The high quality of the Fraunhofer patterns enables us to extract the maximum value of the critical current even when the peak is shifted significantly outside the range of the data due to the magnetic moment of the ferromagnetic layer. The maximum value of the critical current oscillates as a function of the ferromagnetic barrier thickness, indicating transitions in the phase difference across the junction between values of zero and π. We compare the data to previous work and to models of the 0-π transitions based on existing theories.

The phase state of a spin-triplet Josephson junction containing three ferromagnetic layers is magnetically controlled.

Josephson junctions containing ferromagnetic materials are being considered for applications in cryogenic random access memory. The road to such applications requires thorough characterization of junction properties, including critical current and ground-state phase shift, as a function of the thickness of a single ferromagnetic layer. We carried out such a study for elliptically-shaped submicron Josephson junctions containing a Ni0.73Fe0.21Mo0.06 alloy similar to commercial Supermalloy. From the field dependence of the critical current, we conclude that the ferromagnets in our junctions are primarily single-domain. These measurements also produce pertinent information about the switching properties of the nanomagnet. We observe a 0–π transition occurring at a NiFeMo thickness of 2.25 ± 0.10 nm, while the critical current decays exponentially with a characteristic length scale of 0.48 ± 0.04 nm.

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