The core-shell Al 2 O 3 @Co particles were prepared by the ball-milling of the composite Al 2 O 3 -Co particles synthesized by
electroless reduction. During the ball-milling process, both, the core-shell particle size and the ratio of hcp/fcc phases changed.
To perform a phase analysis of the cobalt shell we used an approach based on the magnetic measurements. The use of Al 2 O 3 -Co
composite particles as the precursors for the ball milling significantly shortens the time needed for a change in the phase
composition of cobalt.