We apply neutron diffraction, high-resolution synchrotron x-ray diffraction, magnetization measurements, electronic structure calculations, and quantum Monte-Carlo simulations to unravel the structure and magnetism of (CuCl)LaTa 2O 7. Despite the pseudo-tetragonal crystallographic unit cell, this compound features an orthorhombic superstructure, similar to the Nb-containing (CuX)LaNb 2O 7 with X = Cl and Br. The spin lattice entails dimers formed by the antiferromagnetic fourth-neighbor coupling J 4, as well as a large number of nonequivalent interdimer couplings quantified by an effective exchange parameter J eff. In (CuCl)LaTa 2O 7, the interdimer couplings are sufficiently strong to induce the long-range magnetic order with the Néel temperature T N7 K and the ordered magnetic moment of 0.53μ B, as measured with neutron diffraction. This magnetic behavior can be accounted for by J eff/J 41.6 and J 416 K. We further propose a general magnetic phase diagram for the (CuCl)LaNb 2O 7-type compounds, and explain the transition from the gapped spin-singlet (dimer) ground state in (CuCl)LaNb 2O 7 to the long-range antiferromagnetic order in (CuCl)LaTa 2O 7 and (CuBr)LaNb 2O 7 by an increase in the magnitude of the interdimer couplings J eff/J 4, with the (CuCl)LaM 2O 7 (M = Nb, Ta) compounds lying on different sides of the quantum critical point that separates the singlet and long-range-ordered magnetic ground states.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - 29 Aug 2012|