Exchange integrals and magnetization distribution in BaCu2X2O7 (X=Ge,Si)

Estimating the intrachain and interchain exchange constants in BaCu2X2O7 (X=Ge,Si) by means of density-functional calculations within the local spin-density approximation (LSDA) we find the Ge compound to be a more ideal realization of a one-dimensional spin chain with Dzyaloshinskii-Moriya interaction than its Si counterpart. Both compounds have a comparable magnitude of interchain couplings in the range of 5-10 K, but the nearest neighbor intrachain exchange of the Ge compound is nearly twice as large as for the Si one. Using the LSDA+U method we predict the detailed magnetization density distribution and especially remarkable magnetic moments at the oxygen sites.