We propose a four-terminal domain wall-magnetic tunnel junction (DW-MTJ) neuron that enables the first-ever purely spintronic multilayer perceptron with unsupervised learning. The leaky integrate-and-fire neuron has a ferromagnetic DW track coupled to a binary MTJ by an electrically insulated layer. Current through the DW track performs integration by moving the DW. Leaking occurs by moving the DW in the opposite direction of integration due to either dipolar magnetic field, anisotropy gradient, or shape variation. When the DW passes underneath the MTJ, it fires by switching between the resistive and conductive states.
In a crossbar perceptron, the DW track of each neuron is connected to the analog three-terminal DW-MTJ synapses and the MTJ terminals cascade multiple layers. Finally, an unsupervised learning algorithm results from the feedback between the neuron MTJ and the analog synapses, providing best results of 98.11% accuracy on the Wisconsin breast cancer clustering task.
Writing magnetic random-access memory (MRAM) by ultrafast and energy-efficient spin-orbit torque (SOT) has been impeded by the orthogonality between spin polarization and thermally stable perpendicular magnetic anisotropy (PMA). Previously proposed approaches to break this symmetry increase the fabrication complexity, are highly sensitive to the SOT current duration and magnitude, or increase the switching energy. To overcome these challenges, we exploit the precessional nature of the field-like SOT to propose a toggle PMA SOT-MRAM with simple structure that is controlled by a unidirectional SOT current. The proposed MRAM achieves field-free and energy-efficient switching that is robust to variations in the SOT current magnitude and duration with greater than 50% tolerance demonstrated through micromagnetic simulation. The deformation-free structure provides efficient data read-out and can be leveraged for directional writing through a simple XOR between the stored and incoming bits.
The challenge of developing an efficient artificial neuron is impeded by the use of external CMOS circuits to perform leaking and lateral inhibition. The proposed leaky integrate-and-fire neuron based on the three terminal magnetic tunnel junction (3T-MTJ) performs integration by pushing its domain wall (DW) with spin-transfer or spin-orbit torque. The leaking capability is achieved by pushing the neurons’ DWs in the direction opposite of integration using a stray field from a hard ferromagnet or a non-uniform energy landscape resulting from shape or anisotropy variation. Firing is performed by the MTJ stack. Finally, analog lateral inhibition is achieved by dipolar field repulsive coupling from each neuron. An integrating neuron thus pushes slower neighboring neurons’ DWs in the direction opposite of integration. Applying this lateral inhibition to a ten-neuron output layer within a neuromorphic crossbar structure enables the identification of handwritten digits with 94% accuracy.
Spintronics is widely explored as a replacement of CMOS for next-generation computing systems. In particular, magnetic skyrmions efficiently carry information due to their topological stability, non-volatility, low-current motion, and small size. Here we propose a skyrmion logic system that includes a direct cascading mechanism, enabling the implementation of large-scale skyrmion computing systems. This system leverages the rich physics of magnetic skyrmions, including the spin-Hall effect, skyrmion-Hall effect, skyrmion-skyrmion repulsion, repulsion between skyrmions and the track boundaries, and electrical current-control of notch depinning. A force generated by the spin-Hall effect moves skyrmions within ferromagnetic tracks due to repulsion from the track boundaries, and the interplay between the skyrmion-Hall effect and skyrmion-skyrmion repulsion enable logical operations at track junctions. Direct cascading is achieved by using the output skyrmions of one logic gate as inputs to other logic gates. To ensure correct functionality and enable large-scale systems, a global clock provides periodic current pulses in order to the synchronize skyrmion motion past notches in the track. In this conservative logic system, skyrmions are never destroyed during logical operations, thus eliminating the need to generate skyrmions, reducing energy consumption, and simplifying experimental demonstration.
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