Modeling Study of Domain Wall Motion in Dual Magnetic Layers Driven by Spin Hall Effect

Future high density data storage also desires fast read/write and low power capability . One of the candidates for meeting the demand is current-driven domain wall motion memory. Domain walls can be moved by current via the Spin Hall effect in the presence of Dzyaloshinskii–Moriya interaction (DMI). In such a case, the domain wall motion is propelled by the polarized pure spin current injected into the magnetic layer from the electron flow in an adjacent heavy-metal layer. Much of existing work has been focusing on enhancing the domain wall motion by interlayer interaction, including exchanging coupling and dipolar interaction. Here, we focus on creating fast domain wall motion, required for high speed switching, with increased spin injection efficiency. In particular the magnetic layer are sandwiched by two heavy metal layers, enabling spin injection from both the top and bottom sides of the magnetic layer. In this thesis, we present a micromagnetic modeling investigation on symmetric dual magnetic layers with heavy metals on both sides. Specifically, the domain wall motion behavior of symmetric Pt/Co/Ir/Co/Pt multilayer has been investigated. The study focuses on the effect of interlayer interaction between the two magnetic layers during the current driven domain wall motion.
We first verified that we could adjust the spin current and chirality of the domain wall to control domain wall motion by manipulating the Pt/Co/Ir film stack order. Based on such understanding, the magnetic layer in the dual magnetic layers system with ferromagnetic/antiferromagnetic coupling is further investigated. We discover that the velocities increase while the ferromagnetic exchanging coupling strength decreases.
The inner magnetization of domain walls in different layers will create a certain angle to facilitate the domain wall motion. The velocities saturate when they create a 180-degree angle. On the other hand, the domain wall motion is accelerated once the two magnetic layers are antiferromagnetic coupled together. The exchange coupling interaction creates an extra torque, which increases the velocities of the domain wall. The domain wall also transforms into an “S” shape instead of being linear to reduce the demagnetization field. Hence the domain wall motion is faster at low current density.