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

Abstract

The critical current density (Jc0) and thermal stability factor Δ 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 α of Co/Pd and Co/Pt multilayers (MLs) with various layer compositions were also studied to understand the variations in the Jc0 and Δ of the nanopillars. The Jc0 and Δ of Co/Pd MLs were found to be almost independent of the pillar diameters, and the effective reversal size contributing to Δ 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, Jc0 was found to gradually increase with increasing thickness ratio of Pd and Co layers, tPd/tCo, up to tPd/tCo = 2, and further increase in tPd/tCo resulted in the decrease in Jc0. On the other hand, Δ was roughly independent of the thickness ratio. The Gilbert damping constants α of Co/Pd and Co/Pt MLs increased with increasing thickness ratio of noble metal and Co layers, tNM/tCo. 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 α with tPd/tCo is considered to be responsible for the increase in Jc0 up to tPd/tCo = 2.