New-Tech Europe Magazine | November 2018

Previously, the imec team validated functioning of spin torque majority gates through micromagnetic simulations. Meanwhile, first devices have been built on 300mm wafers (see figure below), and part of the truth table has been demonstrated. Based on materials and process optimization, it is expected that full device function will be soon demonstrated. In general, spin torque majority gate circuits are compact and could target low-power operation. However, compared to spin-wave based operation, they consume more energy. A major advantage of spin torque majority gates compared to wave-based circuits is the ‘ease’ with which circuits can be designed. Circuits are typically made up of multiple computational stages, and cascading between devices, as mentioned above, has to happen in order to build meaningful logic circuits. In hardware, this means that the output of one set of devices goes to the input of the next stage of devices. Wave-based majority gates rely on the interference of waves, and these waves can easily flow back into the circuit. And this makes the design of ‘cascadable’ circuits very challenging. Spin torque majority gate operation relies on the interaction between domain walls. Domain walls are less likely to be channeled back into the device. Based on modeling, imec has proposed special implementations which reduce this effect even further. Conclusion In this article, we have reviewed the status, challenges and benefits of three types of majority gates: spin- wave majority gates and plasmonic majority gates – both wave-based computational circuits – and spin torque majority gates. With these beyond-CMOS technologies, low-

Fig 6: exploratory device team imec

R&D Engineer, currently working on the fabrication of spin torque majority gate devices; Adrien Vaysset, Researcher, in charge of the micromagnetic modeling of spin torque majority gates; Iuliana Radu, Distinguished Member of Technical Staff, leading the beyond CMOS activities at imec; Christoph Adelmann, Principal Member of Technical Staff, doing research on materials and devices for spintronic logic and interconnects; and Florin Ciubotaru, Senior Researcher, focusing on the development of logic, radio-frequency and sensor devices based on magnetic spin- related phenomena. References [1] ‘Experimental prototype of a spin-wave majority gate’, T. Fischer et al., Applied Physics Letters 110, 152401 (2017) [2] ‘Proposal for nanoscale cascaded plasmonic majority gates for non- Boolean computation’, Dutta S. et al., Scientific Reports 2017, 7, 17866 [3] ‘Electrically driven unidirectional optical nanoantennas’, Gurunarayanan S. et al., Nano Lett. 2017, 17, 7433−7439

power and compact arithmetic circuits can be built, that completely change the way we build logic circuits. Once mature, they are envisioned to perform, in a hybrid architecture, specific functions – depending on their individual strengths. While spin-wave based majority gates promise to be compact and ultralow power, the main asset of the low- power spin torque majority gates is the ease with which multiple-stage circuits can be built. On the longer term, plasmonic majority gates have the potential to be used for applications that require extremely high throughput and speed, and for which energy efficiency is a second consideration. Acknowledgements This work is the result of the collaborative effort of the imec exploratory device team, which includes, from left to right, Surya Gurunarayanan, PhD student, focusing on electrically-driven nanophotonic devices and circuits; Odysseas Zografos, R&D Engineer, working on simulations and benchmarking for spin waves, and on benchmarking for spin torque majority gates; Danny Wan, Senior

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