Ferromagnetic Moment and Spin Rotation Transitions in Tetragonal Antiferromagnetic ${\mathrm{Sr}}_2{\mathrm{Cu}}_3{O}_4{\mathrm{Cl}}_2$

${\mathrm{Sr}}_2{\mathrm{Cu}}_3{\mathrm{O}}_4{\mathrm{Cl}}_2$ is a variant of the lamellar copper oxides, containing an extra ${\mathrm{Cu}}^{2+}$ ion in the center of every second plaquette of the square ${\mathrm{CuO}}_2$ lattice. The two types of Cu form interpenetrating Heisenberg antiferromagnets, which order at 380 and 40 K. Magnetization measurements 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 coupling constants, relevant to other lamellar, chain, and ladder copper oxides.