Magnetic vortex state and multi-domain pattern in electrodeposited hemispherical nanogranular nickel films

• Magnetic states of electrodeposited nickel in isolated spherical and agglomerated nanogranules, and a continuous film.
• Preferential magnetization reversal mechanism in isolated granules is vortex state.
• Micromagnetic simulations confirm the three-dimensional vortex.
• Transition between the vortex state and multi-domain magnetic pattern causes a significant decrease in the coercive force.
• Continuous nickel films electrodeposited on silicon substrate exhibit AMR whose magnitude increases with the film thickness.

Magnetic states of nickel nanogranular films were studied in two distinct structures of individual and agglomerated granules electrodeposited on n-type Si(1 1 1) surface from a modified Watts bath at a low pH of 2. Magnetic force microscopy and micromagnetic simulations revealed three-dimensional out-of-plane magnetic vortex states in stand-alone hemispherical granules and their arrays, and multi-domain patterns in large agglomerates and integrated films. Once the granules coalesce into small chains or clusters, the coercivity values increased due to the reduction of inter-granular spacing and strengthening of the magnetostatic interaction. Further growth leads to the formation of a continuous granulated film which strongly affected the coercivity and remanence. This was characterized by the domain wall nucleation and propagation leading to a stripe domain pattern. Magnetoresistance measurements as a function of external magnetic field are indicative of anisotropic magnetoresistance (AMR) for the continuous films electrodeposited on Si substrate.