Field-Free SOT-Switching Based on a Vertical Composition Gradient of Ferrimagnetic Alloys

Abstract

Reproducible field-free spin–orbit torque (SOT)-driven perpendicular magnetization switching is essential for the development of ultralow-power spintronic devices. However, the perpendicular magnetization of bulk-symmetric monolayer ferromagnetic films cannot be switched directly by the current-induced SOT. An additional auxiliary in-plane magnetic field is required to realize deterministic switching, which is unfavorable for device applications. Here, we break the bulk-center symmetry of CoTb alloy films by introducing a vertical composition gradient of heavy metal Pt, and we thus achieve the field-free SOT switching. The experimental results also show that the vertical composition gradient of Pt in the magnetic layer induces an out-of-plane effective field, and the magnitude of this effective field is proportional to the Pt vertical composition gradient. The data also revealed the presence of the multilevel resistance state, which is consistent with the requirements of artificially intelligent neuro-synapses. This field-free multilevel resistive state provides a way for the design of resistive elements of neural network structures.

Neuromorphic Computing with Emerging Antiferromagnetic Ordering in Spin–Orbit Torque Devices

“…In sample NiO-15 nm, LTD and LTP correspond to consecutive negative and positive current pulses, respectively. The mechanism underlying this phenomenon is likely the nucleation and growth of domains within the Hall bar device, as reported previously. , Note that in NiO-based devices, the gradual response of R Hall versus pulse number (both increasing and decreasing) indicates synaptic behavior that can be applied to NC. ,,− …”

“…In addition to the switching induced by current-driven SOT, we observed an analog change in the magnetization of the Co layer when the devices were exposed to a pulsed electric current, as shown in Figure . This trait could potentially be used as a synapse ,,− ,,− for information transmission in high-efficiency NC applications. Figure illustrates the neuromorphic characteristics of NiO-based SOT devices under an applied pulse current.…”

“… 28 , 56 Note that in NiO-based devices, the gradual response of R Hall versus pulse number (both increasing and decreasing) indicates synaptic behavior that can be applied to NC. 11 , 34 , 52 − 55 …”

“…These results are in line with previous findings. 11 , 55 The recognition accuracy of the proposed devices was also assessed using the Fashion MNIST data set. 58 As shown in Figure 5 c, we implemented an ANN utilizing 300 R Hall states as synaptic weights under the same sigmoidal neuron.…”