Motivated by recent experiments on volborthite single crystals showing a wide 13-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 J2, as well as ferromagnetic J′ and J1. Simulations of the derived spin Hamiltonian show good agreement with the experimental low-field magnetic susceptibility and high-field magnetization data. The 13-plateau phase pertains to polarized magnetic trimers formed by strong J bonds. An effective J→ model shows a tendency towards condensation of magnon bound states preceding the plateau phase.
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