Formation of Magnetic Nanoparticles by Annealing Continuous CoFeB/Cu Bilayer Thin Films

This paper reports a novel yet simple maskless method for fabricating monodisperse submicrometer magnetic islands by annealing continuous CoFeB/Cu bilayer films. Annealing drives the diffusion of Co and Fe into the Cu underlayer, causing them to aggregate into submicrometer islands at the Cu grain boundaries. This was established by in situ Auger electron spectroscopy (AES), along with scanning electron microscopy and high-resolution atomic and magnetic force microscopy, to determine the surface composition at various stages of preparation. The composition of the magnetic clusters was established by energy dispersive X-ray spectroscopy. Our results indicate that Co and Fe are coalesced, with their relative ratio remaining similar to the starting composition of the sputtering target, while boron is dissociated from the magnetic species. Also, the areal density of the islands was found to be strongly dependent on the Cu thickness and annealing temperature, while the size of magnetic particles was only weakly dependent. This feature was utilized to control the distribution of monosized magnetic islands.