  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.
