  Oscillating fields can make domain patterns change into various types of
structures. Numerical simulations show that concentric-ring domain patterns
centered at a strong defect are observed under a rapidly oscillating field in
some cases. The concentric-ring pattern appears near the threshold of
spatially-uniform patterns in high-frequency cases. The threshold is
theoretically estimated and the theoretical threshold is in good agreement with
numerical one in a high-frequency region. The theoretical analysis gives also
good estimations of several characteristics of domain patterns for
high-frequencies.
