Crystal-field, exchange interactions and magnetism in pyrochlore ferromagnet R2V2O7 (R3+=Y, Lu)

Abstract

The temperature dependence of the observed bulk magnetic susceptibility, magnetization, paramagnetic Curie temperature θCW, magnetic specific heat of ferromagnetic semi-conducting pyrochlore-based vanadate compounds Y2V2O7 and Lu2V2O7, which are the simplest of R2M2O7 pyrochlore series of oxides, are simulated and analyzed, simultaneously and consistently, within the frame work of the appropriate crystal-field (CF) theory and a mean-field approximation by introducing effective anisotropic molecular-field tensors and also taking account of appreciable spin–orbit coupling. The electronic and magnetic properties are correlated to the structural parameters. Ten-fold degenerate 2D term of 3d1 V4+-ions is split into five Kramers doublets with overall CF splitting Δ1≈2eV and the total splitting of the 2T2g state Δ0≈0.4eV under combined actions of octahedral CF, trigonal (D3d) distortion at V-site and spin–orbit coupling. The ground doublet is a well-isolated effectively spin s=1/2 state, characterized by the anisotropic g-tensors and directional magnetic moments. The degeneracy of the ground state is lifted by the spin–spin correlations among V4-tetrahedra at T∼170K, which causes the formation of ferromagnetic clusters in these pyrochlores. The temperature dependence of the calculated directional site-susceptibilities shows that the V4+ ions have a substantial easy-axis single-ion anisotropy along local 〈111〉 axis of a given V4-tetrahedron in the magnetic phase where ferromagnetic clusters coexist with paramagnetic phase.