Magnetostatic and exchange coupling in the magnetization reversal of trilayer nanodots

We present an experimental investigation of the magnetization reversal process in Ni80Fe20(10 nm)/Cu/Co(10 nm) sub-micrometric circular discs for two different thicknesses of the Cu spacer (1 and 10 nm). Magnetic hysteresis loops were measured by the longitudinal magneto-optical Kerr effect and by resonant scattering of polarized soft x-ray. The results for the 10 nm thick Cu interlayer show a complex magnetization reversal process determined by the interplay between the interlayer dipolar interaction and the different reversal nucleation fields in the two layers. It is worth noting that, during the reversal process, the magnetization of the two layers remains in a nearly single domain state due to the dipolar coupling. These findings are confirmed by three-dimensional micromagnetic simulations. In contrast, when the Cu spacer is 1 nm thick both measurements and simulations show that the reversal is accomplished via the formation of a vortex state in both discs due to the presence of a ferromagnetic exchange coupling that competes with the dipolar interaction.