The last ten years have seen rapid growth in both superconducting classical and quantum logic devices. The most recent single flux quantum based logic systems have demonstrated clock-speeds of 10s of GHz [1], and switching energies as low as 0.1 aJ [2], both surpassing CMOS based platforms by orders of magnitude. Regarding superconducting quantum computing, companies such as Google and IBM are continuing to increase the logical qubit count, this year to 433 qubits, with the aggressive scaling so far meeting projections [3]. However, for the promised advances of these technologies to be realised a viable, dense, cryogenic memory must be developed [4].
Here we present our work forming such a prototype memory device based on the tunable magnetic properties of the rare-earth nitride series of ferromagnetic semiconductors [5]. The combination and competition of spin and un-quenched orbital angular momentum on the tri-valent lanthanide ions allow the tuning of various magnetic properties, to a much greater degree than in transition-metal based ferromagnets. Using combinations of (Gdx,Sm1-x)N we demonstrate independent control of the net-magnetisation and coercive field [6,7]. We show the incorporation of these layers into tri-layer structures and the formation micron-scale switchable magnetic dots. The fringe field of these dots is determined by the orientation of the relative orientation of the ferromagnetic layers. The dots can affect the superconducting state of a nearby Josephson junction, which is used for electrical readout of the memory state of the device.
[1] O. A. Mukhanov, “Energy-Efficient Single Flux Quantum Technology,” IEEE Transactions on Applied Superconductivity, vol. 21, 2011.
[2] Igor I. Soloviev, Nikolay V. Klenov, Sergey V. Bakurskiy, Mikhail Yu. Kupriyanov, Alexander L. Gudkov and Anatoli S. Sidorenko, “Beyond Moore’s technologies: operation principles of a superconductor alternative,” Beilstein Journal of Nanotechnology, vol. 8, 2017.
[3] International Roadmap For Devices And Systems™ 2022 Edition, Cryogenic Electronics And Quantum Information Processing, IEEE, 2022.
[4] Alam, S., Hossain, M.S., Srinivasa, S.R. et al. Cryogenic memory technologies. Nat Electron 6, 185–198, 2023.
[5] F. Natali, B.J. Ruck, N.O.V. Plank, H.J. Trodahl, S. Granville, C. Meyer, W.R.L. Lambrecht, Rare-earth mononitrides, Progress in Materials Science, Volume 58, Issue 8, 2013.
[6] J. D. Miller, H. J. Trodahl, M. Al Khalfioui, S. Vézian, B. J. Ruck; Unlocking perpendicular magnetic anisotropy with Gd substitution in SmN. Appl. Phys. Lett. 27 February 2023.
[7] J. D. Miller, J. F. McNulty, B. J. Ruck, M. Al Khalfioui, S. Vézian, M. Suzuki, H. Osawa, N. Kawamura, and H. J. Trodahl, Phys. Rev. B 106, 174432, 2022.