  Two dimensional magnetic films with perpendicular magnetization spontaneously
form magnetic domain patterns that evolve or undergo symmetry transformations
as a function of temperature. When the system is driven from equilibrium by a
rapid change in temperature, topological pattern defects are the elementary
pattern excitations that affect this evolution. An elastic continuum model is
adapted to describe how a metastable population of topological defects alters
the domain density and the magnetic susceptibility of the "stripe" magnetic
domain pattern. Temporal changes in the susceptibility are interpreted using a
dynamical equation describing the defect population. Recent experiments provide
a quantitative verification of the model, and illustrate the use of the
magnetic susceptibility to follow the population dynamics of topological
defects in this system, and its potential role in investigating a pattern
melting phase transition.
