  In the past several years, many important innovations in nanotechnology were
made. Today it becomes possible to make nanosize magnetic particles, and
development of high storage-density magnetic device is desired. In such a
magnetic particle system, dipole interaction plays the main role. In this note,
we consider stable states of dipolar systems at low temperature: Some systems
show ``antiferromagnetic structure'', and others show magnetic domain
structure, depending on lattice shapes.
