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Correlated fluctuations in spin orbit torque coupled perpendicular nanomagnets


Low-barrier nanomagnets have attracted a great deal of research interest for their use as sources of high-quality true random number generation. More recently, low-barrier nanomagnets with tunable output have been shown to be a natural hardware platform for unconventional computing paradigms such as probabilistic spin logic. Efficient generation and tunability of high-quality random bits is critical for these novel applications. However, current spintronic random number generators are based on superparamagnetic tunnel junctions with tunability obtained through spin transfer torque, which unavoidably leads to challenges in designing concatenated networks using these two terminal devices. The more recent development of utilizing spin orbit torque (SOT) allows for a three-terminal device design, but can only tune in-plane magnetization freely, which is not very energy efficient due to the needs of overcoming a large demagnetization field. In this work, we experimentally demonstrate a stochastic device with perpendicular magnetic anisotropy that is completely tunable by SOT without the aid of any external magnetic field. Our measurements lead us to hypothesize that a tilted anisotropy might be responsible for the observed tunability. We carry out stochastic Landau-Lifshitz-Gilbert simulations to confirm our experimental observation. Finally, we build an electrically coupled network of two such stochastic nanomagnet-based devices and demonstrate that finite correlation or anticorrelation can be established between their output fluctuations by a weak interconnection, despite having a large difference in their natural fluctuation timescale. Simulations based on a dynamical model for autonomous circuits composed of low-barrier nanomagnets show close agreement with the experimental results.

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