Memristive switching by bulk spin–orbit torque in symmetry-broken ferromagnetic films

Bulk spin–orbit torque (SOT) driven memristive switching is demonstrated in perpendicularly magnetized CoPt alloy films by introducing a composition gradient to break the inversion symmetry in the out-of-plane direction. An analog-like magnetization switching consisting of multiple intermediate states can be robustly formed by applying current pulses with different amplitudes or repetition number. The programmable magnetization manipulation is also presented in a continuous manner to simulate the weight update of biology synapses by means of ramped pulses. Furthermore, controllable switching probability dependent on pulse frequency or repetition number is manifested to emulate the integrate-and-fire function of a biological neuron. With the capability to reproduce both functionalities of synapses and neurons in commonly used CoPt films, it will be a promising candidate to advance the SOT-based neuromorphic hardware.