FERROMAGNETIC MOMENT AND SPIN ROTATION TRANSITIONS IN TETRAGONAL ANTIFERROMAGNETIC SR2CU3O4CL2

Sr(2)Cn(3)O(4)Cl(2) is a variant of the lamellar copper oxides, contai ning an extra CU2+ ion in the center of every second plaquette of the square CuO2 lattice. The two types of Cu form interpenetrating Heisenb erg antiferromagnets, which order at 380 and 40 K. Magnetization measu rements yield a small spontaneous ferromagnetic moment,below 380 K and two spin rotation transitions. The results are explained in detail by a pseudodipolar coupling between the two Cu lattices. A quantitative analysis of the data yields several previously unknown microscopic cou pling constants, relevant to other lamellar, chain, and ladder copper oxides.