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|Title:||Investigations on control of domain wall motion using synthetically textured magnetic nanostructures||Authors:||Jin, Tianli||Keywords:||Science::Physics||Issue Date:||2019||Source:||Jin, T. (2019). Investigations on control of domain wall motion using synthetically textured magnetic nanostructures. Doctoral thesis, Nanyang Technological University, Singapore.||Abstract:||Domain wall motion-based devices such as racetrack memory have been proposed as promising candidates for high capacity, non-volatile information storage. In these devices, multiple domain walls can be propagated through nanowires at a speed of several hundred meters per second using the magnetic field, electrical currents and both, thus allowing data to be written, read and processed efficiently. A major challenge towards the commercial realization for domain wall memories is the stochasticity of domain wall motion. Such stochastic behavior limits the reliability and hence maximum data density. To overcome this issue, researchers have proposed and investigated the concept of forming artificial pinning centers at specific positions along the nanowire, such as creating notches to alter energy barriers at specific positions and forming exchange bias. In this thesis, I investigated the use of synthetic magnetic textures to control and pin domains in nanowires. The synthetic magnetic textures can be achieved by using non-magnetic metal diffusion into magnetic nanowires or ions implantation. In NiFe nanowire, the orthogonal Cr crossbars as overlayer on NiFe nanowire were deposited. Then the device was annealed at 400 ℃ for one hour to allow Cr atoms diffusion into NiFe nanowire at the cross positions, which causes the change of the magnetic properties of NiFe in the saturation magnetization (Ms) and the damping constant (α). We performed wide-field magneto-optical Kerr effect (MOKE) and magnetoresistance measurements to evaluate the pinning field about 100 Oe. Subsequently, as a second concept, we studied the boron ions (B+) implantation effect into Co/Pd multilayer-based nanowires. We have observed that B+ implantation can reduce the anisotropy energy of the Co/Pd multilayers and change the easy-axis of magnetization from perpendicular to in-plane direction. In (Co/Pd)10 nanowire, the pinning field, realized by B+ implantation, can be as high as 900 Oe. We also demonstrated the formation of pinning centers by using exchange interaction between films with perpendicular magnetic anisotropy (PMA) and in-plane magnetic anisotropy (IMA) to create locally tilted magnetization of the PMA layer. The thickness of a Pt spacer layer between the PMA layer and the IMA layer was varied to tune the orientation of magnetization. For Pt layer thickness below 2 nm, the magnetization switching behavior of the PMA and IMA layers suggests the presence of exchange interaction between those two layers. I also carried out micromagnetic simulations to further study the tilted angle change by tuning the exchange coupling strength and the depinning current with various tilted angles. The simulation results indicate that the depinning current density decreased with larger tilted magnetization angle and that the tilted angle can be optimized by tuning the exchange coupling strength between PMA and IMA layers. As a summary, a synthetic magnetic texture can be achieved by modifying the magnetic properties of nanowire using various methods, such as thermal diffusion of non-magnetic species and local ion implantation. These magnetically modified regions are supposed to form the pinning sites. In comparison with lithographically fabricated notches along a nanowire, which is non-uniform and increases the local heating at the position of the notches, the methods proposed in this study, can potentially be used in future domain wall memory applications.||URI:||https://hdl.handle.net/10356/85161
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