Annealing effect and under/capping layer study on Co/Ni multilayer thin films for domain wall motion

摘要

Co/Ni multilayer structure with Perpendicular Magnetic Anisotropy (PMA) is considered to be one of the most promising film structures for current-driven domain wall motion. In this work, the field annealing effect on Co/Ni multilayer films with different underlayers is studied. The annealing temperature ranges from 250 ℃ to 375 ℃. The effect of Pt capping layer is also investigated. It was found that the annealing process influences magnetic properties of Co/Ni multilayers with different underlayers differently. For Co/Ni multilayers with Ni underlayer, no PMA is observed in the as-deposited state, but they become perpendicular after annealing, and the effective perpendicular magnetic anisotropy (K~(eff)) increases linearly with annealing temperature. The origin of the K~(eff) increase is discussed. For Co/Ni multilayers with Pt underlayers, large PMA is observed for as-deposited films; however, K~(eff) decreases after annealing. The effects of interfacial lattice mismatch, roughness, and impurities to surface anisotropy are discussed. It was also found that with Pt underlayer, the presence of a thin Pt capping layer is crucial for Co/Ni multilayers to cancel the in-plane Rashba field and stabilize the current-driven domain wall motion.