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

Reproducible field-free spin-orbit torque (SOT)-drivenperpendicularmagnetization switching is essential for the development of ultralow-powerspintronic devices. However, the perpendicular magnetization of bulk-symmetricmonolayer ferromagnetic films cannot be switched directly by the current-inducedSOT. An additional auxiliary in-plane magnetic field is required torealize deterministic switching, which is unfavorable for device applications.Here, we break the bulk-center symmetry of CoTb alloy films by introducinga vertical composition gradient of heavy metal Pt, and we thus achievethe field-free SOT switching. The experimental results also show thatthe vertical composition gradient of Pt in the magnetic layer inducesan out-of-plane effective field, and the magnitude of this effectivefield is proportional to the Pt vertical composition gradient. Thedata also revealed the presence of the multilevel resistance state,which is consistent with the requirements of artificially intelligentneuro-synapses. This field-free multilevel resistive state providesa way for the design of resistive elements of neural network structures.