Superparamagnets for Stochastic Computing INVITED

Abstract

Interacting nanomagnets with low thermal stability have been proposed for probabilistic computing, which has potential uses in low power sensing and logic, as well as in encryption and decryption [1]. Superparamagnetic tunnel junctions (SP-MTJs) are an ideal type of nanomagnet because their time-averaged magnetization of the free layer can be programmed with a voltage or current (Figure 1). We have previously used SP-MTJs for random number generation (RNG), and analog multiplication [2]. While voltage-controlled magnetic anisotropy (VCMA) has advantages for RNG due to ultra-low power consumption, spin transfer torque (STT) or spin orbit torque (SOT) is superior for logic devices due to the wider tuning range. For small sizes, magnetic tunnel junctions can be superparamagnetic near zero bias voltage, and the average retention time can be as short as a few hundred nanoseconds, so GHz speeds are feasible [3]. Pairs of superparamagnetic tunnel junctions fluctuate independently when isolated (Figure 2, top), but when coupled, the output of one is the NOT of the other (Figure 2, bottom). For AND and NAND gates the coupling between three SP-MTJs is optimized by Boltzmann machine simulations to statistically favor the set of microstates consistent with the truth table. Experimental results show that fast operation can be achieved with a hybrid SP-MTJ – CMOS circuit and without the need for a separate microprocessor. Prospects for optimizing the speed and performance of these devices will be discussed, and benchmarked with non-magnetic stochastic computing technologies.