Vortex state formation and stability in single and double layer nanorings and nanodisks

Quasi-static magnetic properties of microscopic Ni80Fe20 disks, rings, and double layered disks and rings were studied via longitudinal magneto-optical Kerr effect (MOKE) and magnetic force microscopy (MFM), concomitant with micromagnetic simulations to elucidate the systems' vortex properties and remagnetization behavior. The features were fabricated lithographically, with diameters between 0.6 and 2 micrometers, and thicknesses of 50 and 100 nm. Key results are: (i) Dual-vortex to single-vortex transitions were observed at nucleation in 100 nm thick disks. (ii) Vortex nucleation and annihilation fields increase with single layer disk thickness whereas they decrease with thickness in single layer ring features. (iii) Double-layer disks separated by a Ru spacer indicate interactions of vortex cores at nucleation and avoidance of vortex core movements, whereas double-layer rings show successive switching in- and out- of vortex states with strong interactions only at vortex-to-onion transition.