Abstract
We investigate the magnetism of the CuO2 chains in the spin-chain compounds (Ca,Sr)14Cu24O41 by means of LDA+U electronic structure calculations. The study is based on an effective model of two coupled CuO2 chains, which captures the structural prerequisites of the exchange interaction. We determine the total energy of various spin configurations on coupled CuO2 chains. In particular, we find that the ground state of Ca-rich spin-chain compounds is given by an antiferromagnetic intrachain spin order with $4k_{{\rm F}}$-periodic charge order, complemented by likewise antiferromagnetic interchain ordering.