Magnetic Behavior of Volborthite Cu3V2O7(OH)2•2H2O Determined by Coupled Trimers Rather than Frustrated Chains

Motivated by recent experiments on volborthite single crystals showing a wide 1/3-magnetization plateau, we perform microscopic modeling by means of density functional theory (DFT) with the single-crystal structural data as a starting point. Using DFT + U, we find four leading magnetic exchanges: antiferromagnetic J and J(2), as well as ferromagnetic J' and J(1). Simulations of the derived spin Hamiltonian show good agreement with the experimental low-field magnetic susceptibility and high-field magnetization data. The 1/3-plateau phase pertains to polarized magnetic trimers formed by strong J bonds. An effective J. 8 model shows a tendency towards condensation of magnon bound states preceding the plateau phase.