  We conducted a thorough experimental and numerical study of the micromagnetic
properties of Permalloy (Ni$_{80}$Fe$_{20}$) microdisks exhibiting target
domain structures at remanence. Vortex configurations are quite common in such
microdisks and correspond to an in-plane flux closure configuration of
cylindrical symmetry with an out-of-plane magnetized core. In contrast, target
domain configuration are observed in thicker microdisks and are characterized
by a vortex configuration of the in-plane component of the magnetization
superposed to an out-of-plane component of magnetization which oscillates as a
function of the distance to the microdisk center resulting in the formation of
concentric domains. The ratio of the out-of-plane oscillatory component of the
magnetization to the in-plane vortex one increases with the thickness of the
microdisk. Hysteresis loops were measured under in-plane and out-of-plane
field. The results at remanence and under magnetic field could be interpreted
by micromagnetic simulations in which the microdisks were described as an
assembly of partially coupled columns representing the granular nanostructure
of the films from which the microdisks were patterned. Quite original
magnetization processes take place in these microdisks exhibiting target domain
remanent configuration. These include in particular entire flipping of the
domain configuration and annihilation/creation of ring domains.
