Magnetic behavior of the low-dimensional compounds Ba2Cu3O4Cl2 and Ba3Cu2O4Cl2

Orthorhombic Ba3Cu2O4Cl2 contains folded Cu2O4 chains along the crystallographic a direction with two different Cu sites. In single crystals of this compound the magnetization measured for a field applied parallel to the a axis shows a spin-flop transition at a threshold field of 2.6 T for temperatures below T-N approximate to 20 K. Above T-N a Curie-Weiss behavior is found with a paramagnetic moment of 2.0 mu(B) per Cu atom. Parallel to the b or c axis the magnetization increases linearly with the field strength, and no metamagnetic transition was detected for these directions. It is concluded that Ba3Cu2O4Cl2 has localized magnetic moments which, at low temperatures, order antiferromagnetically parallel to the a axis. The bulklike magnetic behavior of this compound is probably caused by a strong coupling between the Cu2O4 chains. Tetragonal Ba2Cu3O4Cl2 is built up of Cu3O4 planes, also with two types of copper atoms (Cu-A, Cu-B) Its magnetization increases nearly linearly with the field. Below T-NA = 337 K single-crystalline Ba2Cu3O4Cl2 shows a spontaneous magnetization and ferromagnetic hysteresis for fields applied parallel to the tetragonal [100] or [110] directions. For T < T-NB approximate to 33 K, the coercive field is more than one order of magnitude larger compared to that measured in the temperature range T-NB < T < T-NA. The spontaneous magnetization observed in T-NB < T < T-NA may be attributed to weak ferromagnetism of the Dzyaloshinsky-Moriya type. The behavior of Ba2Cu3O4Cl2 at lower temperatures, T less than or equal to T-NB, is not yet understood. (C) 1998 American Institute of Physics. [S0021-8979(98)25211-X].