  We generalize the nonlinear sigma model treatment of quantum spin chains to
cases including ferromagnetic bonds. When these bonds are strong enough, the
classical ground state is no longer the standard Neel order and we present an
extension of the known formalism to deal with this situation. We study the
alternating ferromagnetic-antiferromagnetic spin chain introduced by Hida. The
smooth crossover between decoupled dimers and the Haldane phase is
semi-quantitatively reproduced. We study also a spin ladder with diagonal
exchange couplings that interpolates between the gapped phase of the two-leg
spin ladder and the Haldane phase. Here again we show that there is good
agreement between DMRG data and our analytical results.
