Stabilized biskyrmion states in annealed CoFeB bilayer with different interfaces

This study investigates the stability of skyrmions and biskyrmions in perpendicular magnetic tunneling junctions with a thick CoFeB/Ta/CoFeB free layer. The samples showed a magnetoresistance of ~ 41% when annealed at 230 {\deg}C. Magnetic force microscopy revealed the existence of skyrmions and biskyrmions at room temperature in the as-deposited state and under an external magnetic field. Annealing at 330 {\deg}C enhanced interfacial Dzyaloshinskii-Moriya interaction (DMI) and crystallinity, enabling the spontaneous coexistence of these topological structures. Micromagnetic simulations using MuMax3 explored the interplay between DMI strength, sign, and skyrmion chirality. Skyrmions exhibited repulsive interactions, while biskyrmions displayed attractive interactions due to the difference in helicities. The study highlights the influence of multilayer structure and varying Ta layer thicknesses on the DMI charility, which modulates the formation of complex spin textures. These results provide an understanding of skyrmion and biskyrmion dynamics and their potential for spintronic applications, including racetrack memory and data storage technologies.