Weak ferromagnetism of quasi-one-dimensional S = 1 2 antiferromagnet BaCu 2 Ge 2 O 7

Weak ferromagnetism of quasi-one-dimensional $S=\frac{1}{2}$ antiferromagnet ${\mathrm{BaCu}}_{2}{\mathrm{Ge}}_{2}{\mathrm{O}}_{7}$ is studied by the magnetization measurement. The spontaneous magnetization appears along the b axis below 8.8 K. The local symmetry between the intrachain nearest-neighbor spins allows the presence of the Dzyaloshinskii-Moriya interaction, and the only possible spatial configuration of the weak ferromagnetic moment per spin definitely determines the sign of the interchain interaction. A weak a-axis magnetic field can change the direction of the magnetization to the a-axis direction, which shows that the spin chain forms a weakly coupled weak-ferromagnetic chain system.