Stability of homogeneous magnetic phases in a generalized t-J model

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 ope rators. At mean-field order we find, in agreement with other authors, that the inclusion of further-neighbor hopping and Coulomb repulsion makes the c ompressibility positive, thereby stabilizing at this level the spiral and N eel 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 bri efly considered the effects of the correlated hopping term that is obtained in the reduction from the Hubbard to the t-J model.