Magnetic behavior of the low-dimensional compounds Ba2Cu3O4Cl2 and Ba3Cu2O4Cl2

Orthorhombic Ba3Cu2O4Cl2 contains folded Cu2O4 chains along the crystallogr aphic a direction with two different Cu sites. In single crystals of this c ompound the magnetization measured for a field applied parallel to the a ax is shows a spin-flop transition at a threshold field of 2.6 T for temperatu res below T-N approximate to 20 K. Above T-N a Curie-Weiss behavior is foun d 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 n o metamagnetic transition was detected for these directions. It is conclude d that Ba3Cu2O4Cl2 has localized magnetic moments which, at low temperature s, order antiferromagnetically parallel to the a axis. The bulklike magneti c 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 ne arly linearly with the field. Below T-NA = 337 K single-crystalline Ba2Cu3O 4Cl2 shows a spontaneous magnetization and ferromagnetic hysteresis for fie lds 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 magni tude 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 attri buted 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)252 11-X].