Decorated Shastry-Sutherland lattice in the spin- 1/2 magnet CdCu2(BO3)2

We report the microscopic magnetic model for the spin-1/2 Heisenberg system CdCu 2(BO 3) 2, one of the few quantum magnets showing the 1/2-magnetization plateau. Recent neutron diffraction experiments on this compound evidenced long-range magnetic order, inconsistent with the previously suggested phenomenological magnetic model of isolated dimers and spin chains. Based on extensive density functional theory band structure calculations, exact diagonalizations, quantum Monte Carlo simulations, third-order perturbation theory as well as high-field magnetization measurements, we find that the magnetic properties of CdCu 2(BO 3) 2 are accounted for by a frustrated quasi-2D magnetic model featuring four inequivalent exchange couplings: the leading antiferromagnetic coupling J d within the structural Cu 2O 6 dimers, two interdimer couplings J t1 and J t2, forming magnetic tetramers, and a ferromagnetic coupling J it between the tetramers. Based on comparison to the experimental data, we evaluate the ratios of the leading couplings J d:J t1:J t2:J it=1:0.20:0.45:-0.30, with J d of about 178K. The inequivalence of J t1 and J t2 largely lifts the frustration and triggers long-range antiferromagnetic ordering. The proposed model accounts correctly for the different magnetic moments localized on structurally inequivalent Cu atoms in the ground-state magnetic configuration. We extensively analyze the magnetic properties of this model, including a detailed description of the magnetically ordered ground state and its evolution in magnetic field with particular emphasis on the 1/2-magnetization plateau. Our results establish remarkable analogies to the Shastry-Sutherland model of SrCu 2(BO 3) 2, and characterize the closely related CdCu 2(BO 3) 2 as a material realization for the spin-1/2 decorated anisotropic Shastry-Sutherland lattice. © 2012 American Physical Society.