Current driven motion of magnetic domains and skyrmions: a study on ultrathin CoFeB films

The improvement of applications concerning CMOS technology is becoming a difficult task, due to physical and technology constrains, such as quantum effects or miniaturization below few nanometers. In order to overcome the related increasing of the power dissipation and rising of the costs, beyond-CMOS technologies have been attracted the attention of recent researches. This thesis explores the basics of one promising beyond-CMOS technology: Racetrack Logic. It is a spintronic technology that can be used for efficient data storaging, thanks to important properties like non-volatility and low power operation, allowing at the same time logic-in-memory. The building block of Racetrack Memories is the motion of magnetic textures into a sub-micron ferromagnetic track, thanks to a current directly injected into the track. This work is a study of the current-driven motion of two magnetic texture that can be used to build a Racetrack Memory: domain walls and skyrmions. The former ones are interfaces between two magnetic domains having opposite magnetization, while skyrmions are particle-like spin configurations which are topologically stable. In first place the domain wall motion is analyzed. The first task to address is the nucleation of a domain into the track, which should be efficient and controlled. Once the domain is created it can be moved by injecting a current into the track. The results of the study shows how to have a controlled way to nucleate domains and the evolution of the domain velocity with respect to the current density injected into the wire, supported by micromagnetic simulations. The study ends with a demonstration of a systematically confinement and shifting of a domain between barriers. If a information is encoded into the domain, it can work as a shift register. In second place, a study of skyrmion size with respect to irradiation dose is presented. Skyrmion diameter is a fundamental parameter that depends on material parameters, like exchange energy, magnetic anisotropy, Dzyaloshinskii–Moriya interaction, and magnetic field. These parameters can be locally varied using Focus Ion Beam irradiation. The study is then completed with micromagnetic simulations. Micromagnetic simulations of skyrmion motion is compared with experimentally measured skyrmion velocities, in order to better understand the skyrmion dynamics. Lastly, a method to systematically nucleate skyrmions is presented. It can be done using ANC in which the magnetic properties of the film are locally degraded. The magnetic structures used for this work are based on CoFeB/MgO thin film stacks. CoFeB, together with MgO oxide exhibit perpendicular magnetic anisotropy (PMA) and low damping, making these stacks the most promising materials for spintronic applications. The two thin films are sandwiched between seed and capping layers. For domain wall motion Ta thin film is used as seed and capping layer, while W thin film is used as seed layer for skyrmion hosting structure and Ta ad capping layer. To vary the material parameters the Focus In Beam technology has been used, in which Ga+ ions are shot towards the stack. Micromagnetic simulation results are derived from Mumax3, a GPU-accelerated micromagnetic simulation program.