  We investigate the interaction between spherical magnetic nanoparticles which
present either a single domain or a vortex structure. First the magnetic
structure of a uniaxial soft sphere is revisited, and then the interaction
energy is calculated from a micromagnetic simulation. In the vortex regime the
orientation of the vortex relative to the easy axis depends on both the
particle size and the anisotropy constant. We show that the leading term of the
interaction is the dipolar interaction energy between the magnetic moments. For
particles presenting a vortex structure, we show that the polarization due to
the dipolar field must be included. The parameters entering in the dipolar
interaction are deduced from the magnetic behavior of the isolated particle.
