Double Free-Layer Magnetic Tunnel Junctions for Probabilistic Bits

Abstract

Naturally random devices that exploit ambient thermal noise have recently attracted attention as hardware primitives for accelerating probabilistic computing applications. One such approach is to use a low barrier nanomagnet as the free layer of a magnetic tunnel junction (MTJ) whose magnetic fluctuations are converted to resistance fluctuations in the presence of a stable fixed layer. Here, we propose and theoretically analyze a magnetic tunnel junction with no fixed layers but two free layers that are circularly shaped disk magnets. We use an experimentally benchmarked model that accounts for finite temperature magnetization dynamics, bias-dependent charge and spin-polarized currents as well as the dipolar coupling between the free layers. We obtain analytical results for statistical averages of fluctuations that are in good agreement with the numerical model. We find that the free layers with low diameters fluctuate to randomize the resistance of the MTJ in an approximately bias-independent manner. We show how such MTJs can be used to build a binary stochastic neuron (or a p-bit) in hardware. Unlike earlier stochastic MTJs that need to operate at a specific bias point to produce random fluctuations, the proposed design can be random for a wide range of bias values, independent of spin-transfer-torque pinning. Moreover, in the absence of a carefully optimized stabled fixed layer, the symmetric double-free layer stack can be manufactured using present day Magnetoresistive Random Access Memory (MRAM) technology by minimal changes to the fabrication process. Such devices can be used as hardware accelerators in energy-efficient computing schemes that require a large throughput of tunably random bits.

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• A full-stack view of probabilistic computing with p-bits: devices, architectures and algorithms
• CMOS + stochastic nanomagnets: heterogeneous computers for probabilistic inference and learning
• Experimental demonstration of an integrated on-chip p-bit core utilizing stochastic Magnetic Tunnel Junctions and 2D-MoS2 FETs
• Computational capability for physical reservoir computing using a spin-torque oscillator with two free layers
• Voltage-insensitive stochastic magnetic tunnel junctions with double free layers
• Double-Free-Layer Stochastic Magnetic Tunnel Junctions with Synthetic Antiferromagnets
• Nonvolatile bistable memory and Ising machine based on Micro-Electro-Mechanical Systems
• Connecting physics to systems with modular spin-circuits
• Engineering-Oriented Design of Drift-Resilient MTJ Random Number Generator via Hybrid Control Strategies
• Superparamagnetic and Stochastic-Write Magnetic Tunnel Junctions for High-Speed True Random Number Generation in Advanced Computing
• Stray magnetic fields from elliptical-shaped and stadium-shaped ferromagnets
• Metrics for spin-based computing