  A theory of collective states in a magnetically quantized two-dimensional
electron gas (2DEG) with half-filled Landau level (quantized Hall ferromagnet)
in the presence of magnetic 3d impurities is developed. The spectrum of bound
and delocalized spin-excitons as well as the renormalization of Zeeman
splitting of the impurity 3d levels due to the indirect exchange interaction
with the 2DEG are studied for the specific case of n-type GaAs doped with Mn
where the Lande` g-factors of impurity and 2DEG have opposite signs. If the
sign of the 2DEG g-factor is changed due to external influences, then impurity
related transitions to new ground state phases, presenting various spin-flip
and skyrmion-like textures, are possible. Conditions for existence of these
phases are discussed. PACS: 73.43.Lp, 73.21.Fg, 72.15.Rn
