  We study the fundamental magnetic states of disc-shaped ferromagnetic
particles with a uniaxial anisotropy along the symmetry axis. Besides the
monodomain, a bidomain state is also identified and studied both numerically
and theoretically. This bidomain state consists of two coaxial oppositely
magnetized cylindrically symmetric domains and remains stable even at zero bias
field, unlike magnetic bubbles in ferromagnetic films. For a given disc
thickness we find the critical radius above which the magnetization
configuration falls into the bidomain bubble state. The critical radius depends
strongly on the film thickness especially for ultrathin films. The effect of an
external field is also studied and the bidomain state is found to remain stable
over a range of field strengths.
