  The interplay between magnetic and superconducting states on a square lattice
is studied using the extended Hubbard model, which takes into account the
attraction of electrons located at nearest neighbor sites. Ferro-, antiferro-,
and spiral magnetic states with all possible sets of spiral wave vectors, as
well as singlet superconducting states with s-wave and d-wave pairing order
parameters, are considered. Formation of a state with the intermediate
s+id-symmetry and phase separation between different phases are allowed. The
results of the Hartree-Fock and slave boson approximations are compared in
order to study the role of correlation effects. Both macroscopic phase
separation and microscopic coexistence of the superconducting and magnetic
phases are found under certain model parameters.
