We use Monte Carlo and quaternion molecular dynamics
simulations to study the self-assembly of intriguing 
structures which form in colloidal suspensions of small
magnetite particles. We show that the only stable isomers
with few particles, a ring and a chain, can be 
efficiently interconverted using a magnetizable tip. We 
propose to use the oscillating dipole field of the tip 
to locally anneal the aggregates to either a ring in zero
field or a chain in nonzero applied field.