Abstract
Magnetic pseudo spin valves (PSVs) with superconducting Nb electrodes, have been leading candidates for an energy-efficient memory solution compatible with cryogenic operation of ultra-low power superconducting logic. Integration of these PSV Josephson junctions in a standard multi-layer Nb process requires growing high-quality thin magnetic films on a thick Nb bottom electrode (i.e. ≥1.5kÅ, to achieve bulk superconducting properties). However, as deposited, 1.5kÅ Nb exhibits a rough surface with a characteristic rice grain morphology, which severely degrades the switching properties of subsequently deposited PSVs. Therefore, in order to achieve coherent switching throughout a PSV, the Nb interface must be modified. Here, we demonstrate that the Nb surface morphology and PSV crystallinity can be altered with the incorporation of separate 50Å Cu or 100Å Al/50Å Cu non-magnetic seed layers, and demonstrate their impact on the magnetic switching of a 15Å Ni80Fe20/50Å Cu/20Å Ni PSV, at both room temperature and at 10 K. Most notably, these results show that the incorporation of an Al seed layer leads to an improved face centered cubic templating through the bulk of the PSV, and ultimately to superior magnetic switching.
Highlights
Superconducting digital technologies, such as reciprocal quantum logic (RQL)[1] and efficient rapid single flux quantum logic[2] offer a significant improvement in energy efficiency over CMOS technology, and are under consideration for ‘beyond-CMOS’ power-efficient, highperformance computing systems[3,4] To this end, a compatible memory solution must be developed that is power efficient, dense, and fast
It is necessary that smooth and crystallographically ordered interfaces are maintained throughout the pseudo spin valves (PSVs) film stack, with nanometer-thick films grown on top of thick Nb electrodes
The peak positions for the Al layer are indexed to an face-centered cubic (FCC) crystal structure and occur at 2θ positions exactly matching body centered cubic (BCC) Nb, indicating epitaxial layer growth
Summary
Superconducting digital technologies, such as reciprocal quantum logic (RQL)[1] and efficient rapid single flux quantum (eRSFQ) logic[2] offer a significant improvement in energy efficiency over CMOS technology, and are under consideration for ‘beyond-CMOS’ power-efficient, highperformance computing systems[3,4] To this end, a compatible memory solution must be developed that is power efficient, dense, and fast. The junction’s equilibrium phase can be toggled between a zero and a π state based on the relative magnetization of the PSV barrier that stores the data, facilitating readout of the bit via the Josephson Effect. Systems comprised of Ni, Cu, Ni80Fe20 and (NixFe1-x)1-yMy (where M = a normal metal dopant) have demonstrated the ability to carry a measureable critical current, the magnetic quality of these systems has not been well characterized.[5,6,7,10,11] Here, we use magnetization and structural data to show that the switching behavior in Ni80Fe20/Cu/Ni/-based PSVs, grown on thick Nb films, can be significantly improved by the incorporation of a normal metal seed-layer
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