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), the magnetic fluctuations of which are converted to resistance fluctuations in the presence of a stable fixed layer. Here, we propose and theoretically analyze a MTJ 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.

中文翻译：

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概率钻头的双层自由磁性隧道结

利用自然热噪声的自然随机设备作为用于加速概率计算应用程序的硬件原语最近引起了人们的关注。一种这样的方法是使用低势垒纳米磁体作为磁性隧道结（MTJ）的自由层，在存在稳定的固定层的情况下，其磁性波动会转换为电阻波动。在这里，我们提出并理论上分析没有固定层但有两个自由层的MTJ，这些自由层是圆形磁盘磁体。我们使用一个以实验为基准的模型，该模型考虑了有限温度的磁化动力学，与偏压有关的电荷和自旋极化电流以及自由层之间的偶极耦合。我们获得的波动统计平均值的分析结果与数值模型非常吻合。我们发现，低直径的自由层会波动，以近似独立于偏置的方式使MTJ的电阻随机化。我们展示了如何使用此类MTJ来构建二进制随机神经元（或$p$位）。与需要在特定偏置点上工作以产生随机波动的早期随机MTJ不同，所提出的设计对于宽范围的偏置值可以是随机的，而与自旋传递扭矩钉扎无关。此外，在没有经过仔细优化的稳定固定层的情况下，可以使用当今的磁阻随机存取存储器（MRAM）技术，通过对制造工艺进行最小的改动来制造对称的双层自由层堆栈。此类设备可用作节能计算方案中的硬件加速器，该方案需要可调节随机位的大吞吐量。