Full-Scale Field-Free Spin-Orbit Torque Switching in HoCo Structure with a Vertical Composition Gradient

Current-induced magnetization switching without the assistance of an in-plane magnetic field by using the spin orbit torque (SOT) has attracted lots of attention for the novel energy-efficient and nonvolatile spintronics devices. In this study, a full-scale field-free SOT switching in perpendicularly magnetized Ru/Pt/HoCo/Ru multilayers with a designed vertical composition gradient is reported. Through annealing the multilayers at 150 degrees C for 0-40 min, a structural change from a multilayer to an alloying structure with mixed distribution of Ho and Co is found. Simultaneously, the field-free switching gradually turns from a full-scale to a partial switching with the structural change. The multiple mechanisms brought by fabricating the HoCo layers with vertical composition gradient (including the non-uniform Dzyaloshinskii-Moriya interaction (DMI), the defects, the inevitable lateral gradient, or the exchange bias effect along the thickness direction) lead to the deterministic field-free switching. In addition, a detailed study on the stack of Ru/gradient HoCo/Ru shows that HoCo itself can also produce a certain amount of SOT to realize a partial switching in the absence of the in-plane field. This work provides a route via engineering rare-earth-transition-metal ferrimagnet to manipulate DMI and SOT for fully electrically controlled spintronics devices.