Enhancement of Zero-Field Skyrmion Density in [Pt/Co/Fe/Ir]2 Multilayers by FORC

Magnetic skyrmions are novel topological spin textures on the nanoscale, and significant efforts have been taken to improve their zero-field density at room temperature (RT). In this work, we reported an approach of improving zero-field skyrmion density in [Pt/Co/Fe/Ir]2 multilayers at RT by using the first-order reversal curve (FORC) technique. Firstly, we investigated the nucleation and annihilation mechanism of magnetic skyrmions using polar magneto-optical Kerr effect measurement. Secondly, the FORC technique was used to obtain information on the irreversible or reversible behaviors in the magnetization switching process. It was found from FORC diagram that the magnetization reversal mechanism can be characterized into three stages: (1) reversible labyrinth stripe domains expanding or shrinking stage; (2) irreversible stripe domains fracturing stage; and (3) irreversible skyrmion annihilation stage. At the end, we demonstrated that the zero-field skyrmion density can be highly improved by choosing reversal field from the irreversible stages. Our results have established the FORC measurement as a valuable tool for investigating magnetic multilayers of high skyrmion densities.