Abstract
For the isotropic Heisenberg Hamiltonian with ferromagnetic nearest- and antiferromagnetic next-nearest-neighbor interactions: H=J1Σ(nn)SnSn+δ1+J2Σ(nnn)SnSn+δ2, J1<0, J2>0, necessary and sufficient conditions for a ferromagnetic ground state are obtained for all Bravais lattices with periodic boundaries and arbitrary spin s. For some lattices, e.g., the cubic lattices in two and three dimensions, these conditions are spin dependent. For the Ising and classical Heisenberg model, the threshold values for |J1|J2 are calculated and compared with the quantum case. The results are also consistent with most of the experimental information.