  The two-dimensional (2D) t-t'-Hubbard model is studied within the slave-boson
(SB) theory. At half-filling, a paramagnetic to antiferromagnetic phase
transition of first order at a finite critical interaction strength U_c(t'/t)
is found. The dependences on U/t and t'/t of the sublattice magnetization and
of the local magnetic moment are calculated. Our results reasonably agree with
recent (Projector) Quantum Monte Carlo data. The SB ground-state phase diagram
reveals a t'-induced electron-hole asymmetry, and, depending on the ratio t'/t,
the antiferromagnetic or ferromagnetic phases are stable down to U=0 at a
critical hole doping.
