WEAK FERROMAGNETISM IN THE LOW-TEMPERATURE TETRAGONAL PHASE OF THE CUPRATES

The criterion for the existence of weak ferromagnetism in the low-temp erature distorted phases of La2CuO4-type compounds is derived on the b asis of a general superexchange Hamiltonian. It is found that a ferrom agnetic canting out of the CuO2 planes is excluded in the pure low-tem perature tetragonal phase. The anisotropic superexchange interactions between the copper spins are obtained from a Hubbard model which inclu des all copper 3d and oxygen 2p orbitals, and all on-site Coulomb matr ix elements. The results apply when the oxygen octahedra surrounding e ach copper have tetragonal symmetry. Explicit expressions for the net ferromagnetic moment and the in-plane and the out-of-plane gaps in the spin-wave spectrum are derived and estimated in terms of the microsco pic parameters of the cuprates.