  Magnetic domain patterns under an oscillating field is studied theoretically
by using a simple Ising-like model. We propose two ways to investigate the
effects of the oscillating field. The first one leads to a model in which
rapidly oscillating terms are averaged out, and the model can explain the
existence of the maximum amplitude of the field for the appearance of patterns.
The second one leads to a model that includes the delay of the response to the
field, and the model suggests the existence of a traveling pattern which moves
very slowly compared with the time scale of the driving field.
