Executive Summary : | In recent years, low-dimensional topological spin textures found in magnetic materials, such as magnetic skyrmions/antiskyrmions, possess a great technological advantage for their application in the field of spintronics. In general, these topological spin textures are found to exist in single crystals of non-centrosymmetric magnetic materials and multilayer thin films. One of the main mechanisms to stabilize the skyrmions is the Dzyaloshinskii–Moriya (DM) interaction, which arises when the inversion symmetry of the crystal lattice is broken. Skyrmions have been experimentally observed in different bulk materials including the B20 compounds and thin films with interfacial DMI. The main characteristics of skyrmions include, i) stable particle-like states with nanometer scale dimension and can move with a low depinning current density (as small as ∼10⁶ A/m²), and (ii) nontrivial spin structures with high mobility. Their special topologically protected spin configurations makes them promising candidates for future high-density and low-power skyrmion-based memory/logic devices. One of the most promising and expected skyrmion-based memory devices is the racetrack memory (RM). In RM, the magnetic skyrmions must be written, deleted, moved and read in a narrow track using external stimuli such as applied magnetic or electric field, spin-polarized current injection and light pulses. The dynamic behavior and stability of the created skyrmions in the race track is studied by using Lorentz transmission electron microscopy (LTEM) and magnetic force microscopy (MFM) techniques.
Occasionally, skyrmions can also be nucleated at naturally occurring defects in the material. Though these defects are uncontrollable and random in location as well as character, still it offers a slight control over the formation of skyrmions. Nevertheless, they are not reliable nucleation method for real time devices. Recently, it is found that skyrmions can be nucleated systematically at nanoscale defect sites formed in a controlled manner using the ion irradiation approach in magnetic multilayer thin films. The perpendicular magnetic anisotropy (PMA), DM interactions and the coercivity of the asymmetric magnetic multilayer can be tailored by irradiation fluence. This is due to the formation of disorders at the interfaces. Also, the local irradiation has been used to create and confines the skyrmion tracks. However, the driving and annihilation processes are still the key issues in skyrmion-based RM devices. Therefore, more details of the motion dynamics, particularly in the presence of artificial defects the motion of skyrmionic bits has to be systematically studied. |