Weak ferromagnetism in low-dimensional cuprates Ba2Cu3O4Cl2 and Ba3Cu2O4Cl2

Abstract

Ba 2 Cu 3 O 4 Cl 2 is a two-dimensional quantum antiferromagnet with two Cu sublattices showing a small spontaneous magnetization M0. A group theoretical analysis shows that M0 may be due to (i) induced magnetization in the Cu II sublattice or (ii) spin canting of the Cu I moments, i.e., weak ferromagnetism (WFM). In case (i) the interaction is of pseudodipolar type. In case (ii) the Cu I moments are subject to some kind of anisotropy. The main reason for the occurrence of M0, compared to similar high-Tc cuprates related structures, is that the center between two Cu I moments is not an inversion center. Ba3Cu2O4Cl2 behaves like a classical antiferromagnet with a spin-flop transition. For this compound our analysis shows that, in the case where the magnetic and the crystallographic unit cells are identical, the Cu moments 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 experimental results.