Interplay between lattice distortion and spin-orbit coupling in double perovskites

Abstract

We develop anisotropic pseudo-spin antiferromagnetic Heisenberg models for monoclinically distorted double perovskites. We focus on these A 2BB′O6 materials that have magnetic moments on the 4d or 5d transition metal B′ ions, which form a face-centered cubic lattice. In these models, we consider local z-axis distortion of B′-O octahedra, affecting relative occupancy of t2g orbitals, along with geometric effects of the monoclinic distortion and spin-orbit coupling. The resulting pseudo-spin-1/2 models are solved in the saddle-point limit of the Sp(N) generalization of the Heisenberg model. The spin S in the SU(2) case generalizes as a parameter κ controlling quantum fluctuation in the Sp(N) case. We consider two different models that may be appropriate for these systems. In particular, using Heisenberg exchange parameters for La2LiMoO6 from a spin-dimer calculation, we conclude that this pseudo-spin-1/2 system may order, but will be very close to a disordered spin liquid state. © 2011 American Physical Society.