Spin-Based Nonvolatile Memories, Unconventional Computing, and Energy Harvesting

Spin-based magnetic random access memory is emerging as a key enabling low-power technologies, which have already spread over markets from embedded memories to the Internet of Things. In addition, spin devices can offer alternative solutions for unconventional computing and energy harvesting. We present an experimental Ising computer based on magnetic tunnel junctions, which successfully solves a 70-city travelling salesman problem (4761-node Ising problem) [1]. We also propose a spintronic artificial neuron based on the heavy metal (HM)/ferromagnet (FM)/antiferromagnet (AFM) [2], which can reset itself due to the exchange bias. Using our proposed neuron, we further implement a restricted Boltzmann machine (RBM) and stochastic integration multilayer perceptron (SI-MLP). By integrating the electrically connected eight spin--torque oscillators (STOs), we demonstrate the battery-free energy-harvesting system by utilizing the wireless RF energy to power electronic devices (such as LEDs) [3,4].

We present our perspective on spin device applications using emerging 2D materials [6]. Previous proposals for the field-free spin--orbit torque (SOT) switching of perpendicular magnetic anisotropy (PMA) require an additional magnetic field. Exploiting the out-of-plane spins could be a solution to this challenge [6]. Here, we experimentally demonstrate the field-free switching of PMA CoFeB at room temperature by utilizing out-of-plane spins from Weyl semimetals, TaIrTe₄ [7] and PtTe₂/WTe₂ [8]. Finally, we discuss magnon-mediated spin torques, which could minimize Joule heating and corresponding energy dissipation [9]. We demonstrate the magnon current-driven switching of PMA at room temperature and field-free operation [10].

[1] J. Si, et al., “Energy-efficient superparamagnetic Ising machine and its application to traveling salesman problems” Nat. Commun. (2024) 15, 3457.
[2] Q. Yang, et al., “Spintronic Integrate-Fire-Reset Neuron with Stochasticity for Neuromorphic Computing” Nano Lett. (2022) 22, 8437.
[3] R. Sharma et al., “Electrically connected spin-torque oscillators array for 2.4 GHz WiFi band transmission and energy harvesting” Nat. Commun. (2021) 12, 2924.
[4] R. Sharma et al., “Nanoscale spin rectifiers for harvesting ambient radiofrequency energy” Nat. Elec. (2024) 7, 653–661.
[5] H. Yang et al., “Two-dimensional Materials Prospects for Non-volatile Spintronic Memories” Nature (2022) 606, 663-673.
[6] Q. Yang, et al., “Field-free spin–orbit torque switching in ferromagnetic trilayers at sub-ns timescales” Nat. Commun. (2024) 15, 1814.
[7] Y. Liu, et al. “Field-free switching of perpendicular magnetization at room temperature using out-of-plane spins from TaIrTe₄” Nat. Electron. (2023) 6, 732-738.
[8] F. Wang, et al. “Field-free switching of perpendicular magnetization by two-dimensional PtTe₂/WTe₂ van der Waals heterostructures with high spin Hall conductivity” Nat. Mater. (2024) 23, 768-774.
[9] Y. Wang, et al. “Magnetization switching by magnon-mediated spin torque through an antiferromagnetic insulator” Science (2019) 366, 1125-1128.
[10] F. Wang, et al. “Deterministic switching of perpendicular magnetization by out-of-plane anti-damping magnon torques” Nat. Nano. (2024) https://doi.org/10.1038/s41565-024-01741-y.