  Analog electronic non-volatile memories mimicking synaptic operations are
being explored for the implementation of neuromorphic computing systems.
Compound synapses consisting of ensembles of stochastic binary elements are
alternatives to analog memory synapses to achieve multilevel memory operation.
Among existing binary memory technologies, magnetic tunneling junction (MTJ)
based Magnetic Random Access Memory (MRAM) technology has matured to the point
of commercialization. More importantly for this work, stochasticity is natural
to the MTJ switching physics e.g devices referred as p-bits which mimic binary
stochastic neurons. In this article, we experimentally demonstrate a novel
compound synapse that uses stochastic spin-orbit torque (SOT) switching of an
ensemble of nano-magnets that are located on one shared spin Hall effect (SHE)
material channel, i.e. tantalum. By using a properly chosen pulse scheme, we
are able to demonstrate linear potentiation and depression in the synapse, as
required for many neuromorphic architectures. In addition to this experimental
effort, we also performed circuit simulations on an SOT-MRAM based 784*200*10
deep belief network (DBN) consisting of p-bit based neurons and compound
synapses. MNIST pattern recognition was used to evaluate the system
performance, and our findings indicate that a significant reduction in
recognition error rates can be achieved when using our incremental pulse scheme
rather than a non-linear potentiation and depression as obtained when employing
identical pulses.
