  In this work, we propose a `1-transistor 1-nanotrack' (1T1N) synapse based on
movement of magnetic skyrmions using spin polarised current pulses. The
proposed synaptic bit-cell has 4 terminals and fully decoupled spike
transmission- and programming- paths. With careful tuning of programming
parameters we ensure multi-level non-volatile conductance evolution in the
proposed skyrmionic synapse. Through micromagnetic simulations, we studied in
detail the impact of programming conditions (current density, pulse width) on
synaptic performance parameters such as number of conductance levels and energy
per transition. The programming parameters chosen used all further analysis
gave rise to a synapse with 7 distinct conductance states and 1.2 fJ per
conductance state transition event. Exploiting bidirectional conductance
modulation the 1T1N synapse is able to undergo long-term potentiation (LTP) &
depression (LTD) according to a simplified variant of biological spike timing
dependent plasticity (STDP) rule. We present subthreshold CMOS spike generator
circuit which when coupled with well known subthreshold integrator circuit,
produces custom pre and post-neuronal spike shapes, responsible for
implementing unsupervised learning with the proposed 1T1N synaptic bit-cell and
consuming ~ 0.25 pJ/event. A spiking neural network (SNN) incorporating the
characteristics of the 1T1N synapse was simulated for two seperate
applications: pattern extraction from noisy video streams and MNIST
classification.
