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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - EuSuper (Superconducting Magnetic RAM for Next Generation of Supercomputers)

Teaser

Summary

The continuous search for computing-power and data-storage is quickly approaching the physical limits of conventional silicon-based electronics. To overcome this limit different approaches beyond conventional CMOS technology are nowadays under investigation. On one hand, quantum computers, based on non-classical superposition of logic units (bit), offer bright perspectives. On the other hand, energy-efficient superconducting circuits based on Josephson junctions have already demonstrated a computational speed 500 times larger than conventional CMOS-based ones. The complete implementation of a supercomputer based on this technology is nowadays limited by the lack of memories operating at cryogenic temperatures.
In this context, EuSuper aims at developing a new generation of nano-sized superconducting non-volatile magnetic memories, with improved efficiency and enhanced functionalities, by exploiting a wise hybridization between ferromagnetic insulators (FI) and conventional superconducting (S) metals.
The peculiar behaviour of FI/S systems is determined by interfacial quantum phenomena arising at the boundary between the ferromagnetic and superconducting materials. Within a distance from the interface of the order of the superconducting coherence length, the exchange interaction of the FI induces a spin split of the density of states into the S, as per an effective Zeeman splitting generated by an external magnetic field of up to few Tesla (magnetic proximity effect). Due to the key role played by the FI/S interfaces, the overall quality of the growing technique is crucial for observing and manipulating magnetic proximity effects.
The results of this project can pave the way for innovative superconducting spintronic applications, i.e. in classical large-scale supercomputing, suitable in all fields of science (solid state physics, meteorology, finance, medicine, biology, etc.) where increasing speed of calculation and storage are exponentially increasing on demand. Also, from a fundamental physics point of view, the results obtained within my project will help in clarifying the interplay between ferromagnetism and superconductivity in FI/S mesoscopic junctions and spin-valve. From the technological side, I will employ the small size and scalability of these systems to develop an innovative class of superconducting memories ready for the market of incoming next-generation cryogenics supercomputers.
EuSuper is a synergic Marie Sklodowska Curie Action that combines the capabilities and expertise of two of the most outstanding and globally recognized research institutions: Massachusetts Institute of Technology (USA) and Consiglio Nazionale delle Ricerche (Italy).

Work performed

EuSuper is a powerful playground in which state-of-the-art FI/S physics merges with the experienced researcher proven background in ferromagnetic/superconductor physics and expertise in state-of-the-art nanofabrication, aiming at the investigation and exploitation of novel physics in EuS/Al and GdN/NbN heterostructures.
The research objectives (ROs) of the project are the following: i) Growth and characterization of state-of-the-art FI/S-based thin-film heterostructures, (RO1); ii) Miniaturization of FI/S-based hybrid heterostructures, (RO2). iii) Nanoscale engineering of novel FI/S-based ground-breaking superconducting MRAM prototype (Fig. 1), (RO3).
During the first year at MIT, as an experienced researcher, I have successfully achieved the first two objectives (RO1,2) by focusing all his efforts on the growth and characterization of EuS/Al and GdN/NbN bilayers and more complex stacks (exchange-coupled Josephson junctions). I have been deeply trained and I have learned the most advanced evaporation and sputtering growth techniques that allowed me to reproduce early works and study new and fascinating properties of these FI/S heterostructures. Furthermore, I have been able to develop two specific reliable top-down nanofabrication approaches, especially designed for FI/S multilayers: dry and/or wet etching, after the and material deposition and polymer mask patterning, by Electron Beam Lithography (EBL). These achievements are at the basis of the miniaturization of FI/S mesoscopic heterojunctions (e.g., EuS/Al, GdN/NbN, EuS/Al/Al2O3/Al/EuS, GdN/NbN/AlN/NbN/GdN, etc.), starting from the simplest well-established configurations towards more complex structures by varying the layer thicknesses, device geometry and growth conditions.
All the above-mentioned accomplishments pave the way towards the miniaturization of the FI/S-based superconducting spin-valves, which will ultimately lead to the realization of the first prototype of a reliable nanometric superconducting non-volatile RAM cell, that will be developed during the return phase, at NEST-CNR.
Besides the experimental work, I have given informal/internal seminars and mini-courses (for students and postdocs) on EBL processes and techniques, which on the one hand have allowed me to transfer my knowledge to the outgoing host (MIT), and on the other hand have enabled the host group and me to improve/engineer/optimize the integration of their FI/S structure growth recipes to the nanofabrication processes.

Final results

The proposed research objectives of the overall project will go beyond the state-of-the-art of the literature and technology since they will provide a strategic step forward towards the development of the novel physics emerging from the precise control of the nanoscale interaction between the ferromagnetism and superconductivity. The original research view is to revolutionize standard approaches in superconducting spintronics by designing ad-hoc experimental techniques and nanofabrication processes and, thus opening new opportunities for developing ground-breaking nanodevice concepts that constitute the essential ingredients for future applications in high-performance superconducting computing technology as a real alternative to existing power-hungry computers based on conventional silicon semiconductor technology. Although the project is only at its half, EuSuper has produced results that can be widely considered beyond the state-of-the-art. In particular, the achievements reached in EuS/Al miniaturization, preserving the pristine properties of the materials, may be considered at the basis of novel device concepts for fundamental research and applications within ad over the project development (i.e. field-effect superconducting magnetoelectric devices). EuSuper has already exploited its own potential even in terms of side results. A significant discovery may revolutionize the technology based on superconducting NbN nanocircuits. In fact, we have demonstrated a large superconducting critical current enhancement (~ 30%) and hysteretic infinite electro-resistance in NbN micro and nanobridges induced by electrostatic field. This first-generation of superconducting field effect transistor (SuFET) is of fundamental interest and become highly attractive for potential all-metallic superconducting cryogenic-nanoelectronics. Further, we observed similar results in proximity coupled NbN/ferromagnetic-insulator nano-bridges, which essentially could lead to an additional novel superconducting spintronic device: a triplet paired SuFETs. The abovementioned results, make the final goal of EuSuper more intriguing and appealing: the superconducting MRAM may become more versatile, tunable and performant than hypothesized in the project proposal.