Helimagnetism and weak ferromagnetism in edge-shared chain cuprates

The present understanding of a novel growing class of chain cuprates with intriguing magnetic properties is reviewed. Among them, several undoped edge-shared CuO2 chain compounds show at low temperature a clear tendency to helicoidal magnetical ordering with acute pitch angles and sometimes also to weak ferromagnetism. Our analysis is based on the isotropic 1D frustrated J(1)-J(2) Heisenberg model with ferromagnetic (FM) 1st neighbor and antiferromagnetic 2nd neighbor exchange. The achieved assignment is supported by microscopic calculations of the electronic and magnetic structure. We consider Na(Li)Cu2O2, LiVCuO4 as the best studied helimagnets, Li2ZrCuO4 and other systems close to a FM quantum critical point, as well as Li2CuO2 with FM inchain ordering. The interplay of frustrated inchain couplings, anisotropy and interchain exchange is discussed. (C) 2007 Elsevier B.V. All rights reserved.