Spin transfer torque switching of Co/Pd multilayers and Gilbert damping of Co-based multilayers

The critical current density (J(c0)) and thermal stability factor Delta for the spin transfer torque switching of [Pt/Co](6)/Cu/[Co/Pd](3) nanopillars with various Co/Pd layer compositions were investigated. Moreover, the Gilbert damping constants a of Co/Pd and Co/Pt multilayers (MLs) with various layer compositions were also studied to understand the variations in the J(c0) and Delta of the nanopillars. The J(c0) and Delta of Co/Pd MLs were found to be almost independent of the pillar diameters, and the effective reversal size contributing to Delta was found to be much smaller than the physical pillar diameter. From the spin transfer torque (STT) switching of Co/Pd MLs with various layer compositions, J(c0) was found to gradually increase with increasing thickness ratio of Pd and Co layers, t(Pd)/t(Co), up to t(Pd)/t(Co) = 2, and further increase in t(Pd)/t(Co) resulted in the decrease in J(c0). On the other hand, Delta was roughly independent of the thickness ratio. The Gilbert damping constants a of Co/Pd and Co/Pt MLs increased with increasing thickness ratio of noble metal and Co layers, t(NM)/t(Co). The large damping constant of the Co/Pt ML compared with that of the Co/Pd ML means that Co/Pt and Co/Pd MLs are appropriate for use as reference and memory layers, respectively. The increase in a with t(Pd)/t(Co) is considered to be responsible for the increase in J(c0) up to t(Pd)/t(Co) = 2. (C) 2018 The Japan Society of Applied Physics