Unraveling spin-orbit torque induced multistate magnetization switching in Co/Gd ferrimagnetic multilayers for physically unclonable functions

Abstract

Ferrimagnetic materials driven by spin-orbit torque (SOT) exhibit the distinctive characteristic of multistate magnetization switching and enable versatile applications. However, the underlying mechanism governing multistate magnetization switching in ferrimagnetic materials remains unelucidated. Here, by studying the SOT induced magnetization switching in Co/Gd ferrimagnetic multilayers with perpendicular magnetic anisotropy (PMA), we demonstrate that the multistate magnetization switching behavior is not only observed in [Co/Gd]n, but also remains in [Co/Gd]L/CoFeB stacks and in size-shrinking dot devices. Under the stimulus of numerous pulsed SOT currents, the anomalous Hall resistance of our devices is found to change successively and to finally saturate at a specific value, depending on the SOT current density. This behavior suggests a substantial pinning effect that prevents the domain wall from expansion, as verified by Magneto-optical Kerr experiments. Because the pinning effect is intrinsic and specific to each Hall-bar device, we further realize analogue physically unclonable functions (PUFs) in a 10×10 Hall-bar array, and we generate multiple PUFs by using SOT pulse current of different amplitudes. Our work unravels the underlying mechanism of SOT-driven multistate switching in ferrimagnets and shed insights into materials engineering to realize spintronic analogue PUFs.