  We study the stability of homogeneous magnetic phases in a generalized t-J
model including a same-sublattice hopping t' and nearest-neighbor repulsion V
by means of the slave fermion-Schwinger boson representation of spin operators.
At mean-field order we find, in agreement with other authors, that the
inclusion of further-neighbor hopping and Coulomb repulsion makes the
compressibility positive, thereby stabilizing at this level the spiral and Neel
orders against phase separation. However, the consideration of Gaussian
fluctuation of order parameters around these mean-field solutions produces
unstable modes in the dynamical matrix for all relevant parameter values,
leaving only reduced stability regions for the Neel phase. We have computed the
one-loop corrections to the energy in these regions, and have also briefly
considered the effects of the correlated hopping term that is obtained in the
reduction from the Hubbard to the t-J model.
