Weak ferromagnetism in low-dimensional cuprates Ba2Cu3O4Cl2 and Ba3Cu2O4Cl2

Ba2Cu3O4Cl2 is a two-dimensional quantum antiferromagnet with two Cu sublat tices showing a small spontaneous magnetization M-0. A group theoretical an alysis shows that M-0 may be due to (i) induced magnetization in the Cu II sublattice or (ii) spin canting of the Cu I moments, i.e., weak ferromagnet ism (WFM). In case (i) the interaction is of pseudodipolar type. In case (i i) the Cu I moments are subject to some kind of anisotropy. The main reason for the occurrence of M-0, compared to similar high-T-c cuprates related s tructures, is that the center between two Cu I moments is not an inversion center. Ba3Cu2O4Cl2 behaves like a classical antiferromagnet with a spin-fl op transition. For this compound our analysis shows that, in the case where the magnetic and the crystallographic unit cells are identical, the Cu mom ents of equivalent sites should be parallel and the antiferromagnetic order is related to opposite directions of the moments at different sites. Weak antiferromagnetism but no WFM is permitted, in agreement with the experimen tal results. (C) 2000 American Institute of Physics. [S0021-8979(00)17908-3 ].