Competition between vortex and "s"-like states in laterally confined magnetic trilayers

We report on the magnetization reversal and micromagnetic configurations of Co(10 nm)/Pd(0.8 nm)/Co(10 nm) and Co (20 nm) nanodisks studied as a function of the disk diameter. Using magneto-optical Kerr effect (MOKE) we show that the magnetic fields of vortex nucleation and annihilation decrease while the nanodisk diameter increases. We have discovered that in an array of trilayer nanodisks with diameters D = 200 nm, direct exchange through pinholes and interlayer indirect ferromagnetic exchange coupling promote the formation of a non-uniform "S"-like state with opposed magnetic moments in adjacent Co layers. In larger trilayers nanodisks (D = 400 - 800 nm) the vortex state, like in single layer nanodisk, is formed.