Magnetic remanent states and magnetization reversal in patterned trilayer nanodots

The magnetic properties of a trilayer system involving two ferromagnetic disks separated by a nonmagnetic spacer differ from those of isolated dots due to the magnetostatic and interlayer exchange coupling across the spacer. The reversal process for this system has been investigated experimentally, analytically and numerically for permalloy (Ni₈₀Fe₂₀) dots with thicknesses of up to 400.3em0exnm and radii of 0.25--1.250.3em0exμm separated by a Cu spacer up to 450.3em0exnm thick. Hysteresis measurements and photoemission electron microscopy images of the remanent states show evidence of the vortex remanent state and reversal, respectively. Single and multidomain states, however, can be stabilized through flux closure between layers resulting in a reduction of the phase space occupied by the vortex state. Micromagnetic simulations indicate that the disks will each support oppositely directed vortices at remanence that reverse through coordinated nucleation, displacement and annihilation of vortices in the two layers. Experimental and numerical magnetic susceptibilities are in agreement with analytical calculations based on the rigid vortex model.