Ferro-antiferromagnetic delta-chain system studied by high field magnetization measurements

High field magnetization processes were measured for the single crystal of an S = 1/2 ferro-antiferromagnetic delta-chain system [Cu(bpy)(H ₂O)][Cu(bpy)(mal)-(H ₂O)](ClO ₄) ₂. At low temperatures, the observed magnetization curve shows a rapid increase to half of the saturation moment below about 1 T and then followed by a gradual increase to fully polarized state below about 7 T. Concerning this characteristic magnetization process, we discuss the ground state of the system qualitatively based on a non-interacting two sublattice model, where a delta-chain is divided into apical free spins and remaining basal spins regarded as quasi one dimensional antiferromagnetic chain. Essential effect of ferromagnetic interaction, which connects an apical spin and basal spins, is pointed out from the comparison with theoretical curve for the non-interacting two sublattice model as well as with the numerically calculated curve. Presence of the Dzyaloshinskii-Moriya interaction is also suggested from the arguments for the saturation behavior and/or saturation field itself based on the spin wave theory.