Perpendicular-Spin-Transfer-Torque Magnetic-Tunnel-Junction Neuron for Spiking Neural Networks Depending on the Nanoscale Grain Size of the MgO Tunnelling Barrier

Unlike conventional neuromorphic chips fabricated with C-MOSFETs and capacitors, those utilizing p-STT MTJ neuron devices can achieve fast switching (on the order of several tens of nanoseconds) and extremely low...

STDP implementation using multi-state spin-orbit torque synapse

The abundance of data to be processed calls for new computing paradigms, which could accommodate, and directly map artificial neural network (ANN) architectures at the hardware level. Neuromorphic computing has emerged as a potential solution, proposing the implementation of artificial neurons and synapses on physical substrates. Conventionally, neuromorphic platforms are deployed in complementary metal-oxide–semiconductor (CMOS) technology. However, such implementations still cannot compete with the highly energy-efficient performance of the brain. This calls for novel ultra-low-power nano-scale devices with the possibility of upscaling for the implementation of complex networks. In this paper, a multi-state spin-orbit torque (SOT) synapse based on the three-terminal perpendicular-anisotropy magnetic tunnel junction (P-MTJ) is proposed. In this implementation, P-MTJs use common heavy metals (HMs) but with different cross-section areas, thereby creating multiple states that can be harnessed to implement synapses. The proposed multi-state SOT synapse can solve the state-limited issue of spin-based synapses. Moreover, it is shown that the proposed multi-state SOT synapse can be programmed to reproduce the spike-timing-dependent plasticity (STDP) learning algorithm.

Easy-cone state in spin-torque diode under combined action of magnetostatics and perpendicular anisotropy

Spin-torque diodes (STDs) with interfacial perpendicular magnetic anisotropy (IPMA) in the free layer have outstanding microwave signal rectification performances. Large sensitivity values in such systems are usually associated with an easy cone (EC) magnetic state, when the magnetization in the free layer is tilted from the normal to the plane of the film. Here we theoretically investigate the phase diagram of the existence of an EC state in an infinite free layer of the magnetic tunnel junction (MTJ) considering both IPMA (first and second order) and magnetostatic interaction. We show that the increase of the magnetostatic field leads to extension of the EC existence region. Then we consider the effect of finite dimensions in case of two differently spatially oriented elliptic nanopillar MTJ on the obtained phase diagrams. And finally, we consider dynamic properties and rectification of two elliptic STD under microwave current injection. These results clarify magnetostatic interaction influence on IPMA based STD rectification and demonstrate possible approach to extend the parameters area of the EC STD highly effective rectification.

Critical switching current density of magnetic tunnel junction with shape perpendicular magnetic anisotropy through the combination of spin-transfer and spin-orbit torques

Recently, magnetic tunnel junctions (MTJs) with shape perpendicular magnetic anisotropy (S-PMA) have been studied extensively because they ensure high thermal stability at junctions smaller than 20 nm. Furthermore, spin-transfer torque (STT) and spin-orbit torque (SOT) hybrid switching, which guarantees fast magnetization switching and deterministic switching, has recently been achieved in experiments. In this study, the critical switching current density of the MTJ with S-PMA through the interplay of STT and SOT was investigated using theoretical and numerical methods. As the current density inducing SOT ($$J_{\text {SOT}}$$ J SOT ) increases, the critical switching current density inducing STT ($$J_{\text {STT,c}}$$ J STT,c ) decreases. Furthermore, for a given $$J_{\text {SOT}}$$ J SOT , $$J_{\text {STT,c}}$$ J STT,c increases with increasing thickness, whereas $$J_{\text {STT,c}}$$ J STT,c decreases as the diameter increases. Moreover, $$J_{\text {STT,c}}$$ J STT,c in the plane of thickness and spin-orbit field-like torque ($$\beta$$ β ) was investigated for a fixed $$J_{\text {SOT}}$$ J SOT and diameter. Although $$J_{\text {STT,c}}$$ J STT,c decreases with increasing $$\beta$$ β , $$J_{\text {STT,c}}$$ J STT,c slowly increases with increasing thickness and increasing $$\beta$$ β . The power consumption was investigated as a function of thickness and diameter at the critical switching current density. Experimental confirmation of these results using existing experimental techniques is anticipated.