Controlling magnetic skyrmion Brownian motion through Focused-Ion-Beam patterning

Complementary metal-oxide-semiconductor (CMOS) technology has dominated the industry thanks to the scaling of device dimensions. Today, miniaturization is approaching its physical limits, encountering high power losses due to quantum effects. In this scenario, the need for new technologies led to the development of new research fields, such as spintronics that exploits the electron spin rather than the electron charge. This thesis focuses on the properties of magnetic textures found in ferromagnetic multilayers, which enable magnetic skyrmions to perform computing. Magnetic skyrmions are topologically protected spin-textures in ultra-thin ferromagnetic layers and can be displaced through currents or magnetic fields and perform as information carriers. It has been observed that skyrmions are also characterized by diffusion. This peculiar motion makes them good candidates for unconventional computing applications. The analysis reports the systematic study of skyrmions’ Brownian motion in W/CoFeB/MgO/Ta magnetic structures irradiated using Gallium ions. Indeed, Focused-Ion-Beam patterning modifies the potential energy landscape of magnetic multilayer film stacks to create magnetic skyrmions. By tailoring the amount of implanted ions, the disorder of the magnetic stack is increased, and this affects the density of stabilized skyrmions. For low ion doses, a hopping-like diffusion is retrieved, skyrmions fluctuate between pinning centers. The latter are generated at defects or atomic steps between the interfaces of the magnetic layers. Gallium implantation introduces intermixing at those interfaces, preventing pinning. Consequently, at higher ion doses, the diffusivity of the system increases and skyrmions pass from confined motion to free diffusion. This demonstrates the possibility of tuning Brownian motion with ion irradiation. Then the application of a current to skyrmions in ion irradiated states is analyzed, resulting in their directed motion. But skyrmions don’t lose their fluctuating behavior; as a matter of fact, diffusion continues to distort their velocity. Therefore, instead of concentrating on methods to eliminate this kind of noisy motion, this research moved on to find a way to exploit diffusion. By analogy with biological systems, diffusion can be harnessed if it is biased by a control mechanism. The idea is to create a gradient of ion doses, and skyrmions are expected to move from regions with low ion doses towards regions of high ion doses. The gradient is realized with FIB according to the observed behavior of Brownian motion with respect to the different ion doses. While creating the gradient, it appeared clear that it is not enough to direct diffusion. Thus, the effect of geometrical confinement is tested on skyrmions, with the idea of reducing the space for diffusion and forcing skyrmions along a direction. Once the geometrical aspect ratio with limited edge repulsion for skyrmions is found, the combination of geometrical confinement and gradients is checked on skyrmions. Rectangular strips of gradients are patterned, and even though directed motion is not yet achieved, in this system, skyrmions acquire more diffusivity. Their diffusion becomes more relevant and thanks to its tunability with ion irradiation it can be exploited for probabilistic computing purposes, as in a stochastic reshuffler, or for neuromorphic devices, as artificial neurons.