  The magnetization ground states (MGSs) for a nanosized Co hollow sphere, with
the outer radius, R < 50 nm, have been studied numerically by micromagnetic
simulation using object oriented micromagnetic framework (OOMMF). In addition
to the originally known single domain and vortex-curling states, a three
dimensional "onion" state with a corresponding analytical expression is
proposed and confirmed as one of the ground states. Two phase diagrams, one for
a single crystalline and the other for a polycrystalline nanosphere, are
obtained for the three MGSs. The result reveals that the magnetic anisotropy
has a significant effect on the phase line in the diagrams. The finite
temperature effect and the blocking properties of the nanosphere for the
magnetization reversal are discussed.
