Neuromorphic computing attempts to emulate the remarkable efficiency of the human brain in vision, perception and cognition related tasks. Nanoscale devices that offer a direct mapping to the underlying neural computations have emerged as a promising candidate for such neuromorphic architectures. In this paper, a Magnetic Tunneling Junction (MTJ) has been proposed to perform the thresholding operation of a biological neuron. A crossbar array consisting of programmable resistive synapses generates an excitatory / inhibitory charge current input to the neuron. The magnetization of the free layer of the MTJ is manipulated by Spin-Transfer Torque generated by the net synaptic current. Algorithm, device and circuit co-simulation framework suggest the possibility of ∼ 1.63 - 1.79x power savings in comparison to a 45nm digital CMOS implementation.