Magnetic pyroxenes LiCrGe₂O₆ and LiCrSi₂O₆: Dimensionality crossover in a nonfrustrated S = 3/2 Heisenberg model

The magnetism of magnetoelectric S = 3/2 pyroxenes LiCrSi₂O₆ and LiCrGe₂O₆ is studied by density functional theory calculations, quantum Monte Carlo (QMC) simulations, neutron diffraction, as well as low- and high-field magnetization measurements. In contrast with earlier papers, we find that the two compounds feature remarkably different, albeit nonfrustrated magnetic models. In LiCrSi₂O₆, two relevant exchange integrals, J₁ ≃ 9 K along the structural chains and J_ic1 ≃ 2 K between the chains, form a two-dimensional anisotropic honeycomb lattice. In contrast, the spin model of LiCrGe₂O₆ is constituted of three different exchange couplings. Surprisingly, the leading exchange J_ic1 ≃ 2.3 K operates between the chains, while J₁ ≃ 1.2 K is about two times smaller. The additional interlayer coupling J_ic2 ≃ J₁ renders this model three dimensional. QMC simulations reveal excellent agreement between our magnetic models and the available experimental data. Underlying mechanisms of the exchange couplings, magnetostructural correlations, as well as implications for other pyroxene systems are discussed.