  Motivated by a recent magnetization reversal experiment on a TbFeCo thin
film, we study a topological excitation in the anisotropic nonlinear sigma
model together with the Zeeman and magnetic dipole-dipole interactions.
Dipole-dipole interactions turn a ferromagnet into a frustrated spin system,
which allows a nontrivial spin texture such as a giant skyrmion. We derive an
analytic formula for the skyrmion radius. The radius is controllable by the
external magnetic field. It is intriguing that a skyrmion may have already been
observed as a magnetic domain. A salient feature is that a single skyrmion can
be created or destroyed experimentally. An analysis is made also on skyrmions
in chiral magnets.
