Weak ferromagnetism in Ba2Cu3−xCoxO4Cl2 (0⩽x⩽0.1) and Ba2−ySryCu3O4Cl2 (0⩽y⩽2)

Magnetization measurements were carried out for the oxychlorides Ba2Cu3−xCoxO4Cl2 (0⩽x⩽0.1) and Ba2−ySryCu3O4Cl2 (0⩽y⩽2) with lamellar structure. The weak ferromagnetic moment observed in Ba2Cu3O4Cl2 increased drastically by substitution of a small amount of Co for Cu, in contrast with the case of Ni substitution where the moment disappears sharply by a small substitution of Ni. In the case of Ba2−ySryCu3O4Cl2, although the substitution was made not for the magnetic Cu ion but for the non-magnetic Ba ion, the weak ferromagnetic moment decreased gradually by substitution and at the same time the Néel temperature increased. We suggest that these extreme changes of the weak ferromagnetism in the Ba2Cu3O4Cl2 system are caused by the conflicting magnetic interactions in two Cu sublattices.