Ta-thickness effect on the field-free switching and spin-orbit torque efficiency in ferromagnetically coupled Co/Ta/CoFeB trilayer


 Current induced spin-orbit torque (SOT) switching of magnetization is a promising technology for nonvolatile spintronic memory and logic applications. In this work, we systematically investigated the Ta thickness effect on the magnetic properties, field-free switching and spin-orbit torque efficiency in a ferromagnetically coupled Co/Ta/CoFeB trilayer with perpendicular magnetic anisotropy. It is found that both the anisotropy field and coercivity increase with increasing the Ta thickness from 0.15 to 0.4 nm. With further increasing Ta thickness to 0.5 nm, a two-step switching is observed, indicating that the two magnetic layers are magnetically decoupled. The measurements of pulse-current induced magnetization switching and harmonic Hall voltages show that the critical switching current density increases, while the field-free switching ratio and spin-orbit torque efficiency decrease with increasing Ta thickness. Both the enhanced spin memory loss and reduced interlayer exchange coupling might be responsible for the β
 dl decrease as Ta spacer thickness increases. The studied structure with the incorporation of CoFeB layer is able to realize field-free switching in strong ferromagnetic coupling region, which may contribute to the further development of magnetic tunnel junctions for better memory applications.