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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.
Zeng et al. (Mon,) studied this question.