  The topological nature of magnetic-vortex state gives rise to peculiar
magnetization reversal observed in magnetic microdisks. Interestingly,
magnetostatic and exchange energies which drive this reversal can be
effectively controlled in artificial ferrimagnet heterostructures composed of
rare-earth and transition metals. 25x[Py(t)/Gd(t)] (t=1 or 2 nm) superlattices
demonstrate a pronounced change of the magnetization and exchange stiffness in
a 10-300 K temperature range as well as very small magnetic anisotropy. Due to
these properties, the magnetization of cylindrical microdisks composed of these
artificial ferrimagnets can be transformed from the vortex to
uniformly-magnetized states in a permanent magnetic field by changing the
temperature. We explored the behavior of magnetization in 1.5-micrometer
25x[Py(t)/Gd(t)] (t=1 or 2 nm) disks at different temperatures and magnetic
fields and observed that due to the energy barrier separating vortex and
uniformly-magnetized states, the vortex nucleation and annihilation occur at
different temperatures. This causes the temperature dependences of the Py/Gd
disks magnetization to demonstrate unique hysteretic behavior in a narrow
temperature range. It was discovered that for the 25x[Py(2 nm)/Gd(2 nm)]
microdisks the vortex can be metastable at a certain temperature range.
