Impact microstructure of bottom ferromagnetic layer to spin orbit torque efficiency in FM/NM/FM tri-layer system*

Recently, spin orbit torque (SOT) arising from ferromagnetic metals (FM) has attracted interest. Experimental results found that in the FM/spacer/FM tri-layer structure, the SOT is as efficient as 5d heavy metals with considerable spin Hall effect. Moreover, magnetic spin Hall effect and spin orbit precession, which are magnetization-dependent processes from FM, can generate x and z-polarized spin currents that are forbidden by the symmetry of the spin Hall effect widening the potential application of FM as the spin current source [1].

 

However, the SOT efficiency of FM is scattered from system to system, and there is a lack of a clear strategy for engineering FM tri-layer systems to improve SOT efficiency. To address the issue, we start with CoPt alloy as a model system, whose microstructure can be effectively tuned by underlayer during sputtering deposition. SOT efficiency is characterized by ST-FMR, hysteresis loop shift, and SOT switching in CoPt/spacer/FM structure. We found a strong correlation between microstructure and SOT efficiency, and the mechanism can be explained by the strength of spin orbit coupling which is modulated by microstructure modification. Our finding provides a pathway toward improving SOT efficiency in FM tri-layer structure.