Over the duration of this project, the foundations of a fully integrated photonic-spintronic memory technology have been laid out:
The first area is the development of magnetic materials that can be optically switched. Work has converged on using a material stack based on an optically-switchable Tb/Co multilayer, and a conventional ferromagnetic electrode FeCoB layer. We have shown that this layer can be switched by single picosecond optical pulses, which is a key achievement for the proposed technology. The energy density needed for switching is comparable to best-in-class results of similar technologies. The technology has been evaluated including necessary thermal anneal steps, which is required for further integration.
The second area is the fabrication of the memory elements. A process flow was first established and validated. Secondly, indium tin oxide was selected for the top contact, to allow for optical access, and a process to fabricate magnetic tunnel junction (MTJ) elements with optically transparent top contacts was established. MTJ elements were realized that could be switched optically, by a single short pulse, and read out electrically, by a change in resistance, thereby confirming usefulness as a memory element. The process is available for third parties.
The third area is the development of the photonic distribution layer. An energy-efficient switch network for the optical pulses was developed, based on optimized silicon pin modulators. Gated operation, to reduce the energy consumption, was experimentally achieved, and it was calculated that such networks could work at 100 fJ-per-bit levels. Elements to couple the optical pulse from the distribution layer into the memory element were designed and realized, with close to diffraction-limited focal spots. Active control of the polarization was achieved, for polarization based switching, and solutions for deterministic toggle-based switching were designed and implemented in the circuits. Initial work on reducing the energy required for switching, using plasmonic concentrators, was started. Switches are available for third parties.
The fourth area is the development of design tools. A material simulation software tool has been further optimized for spintronic applications. The tool has been applied to calculating, o.a. the temperature dependence of the tunnel magnetoresistance. A simulation flow, including this tool, and other, open-access, tools, was established, to include magnetization dynamics. These results can then be exported to circuit-level electronic-design-automation tools. This latter work resulted in a compact model, that was shared with the community for open use.
These results converged into our Demonstrator effort, where a chip with an MTJ array was coupled to a photonic switching network. Due to limitations in the power handling capacity of the silicon photonic chip, the optical power that could be delivered was not enough to achieve switching in a fully integrated configuration. The operation of the individual chips was validated, though, and the path towards full integration was outlined.
