Making Spin Current in Organic Magnets

Abstract

We show that a class of organic antiferromagnets with checker-plate type molecular arrangements, such as a typical strongly correlated system κ-(BEDT-TTF)₂ X, can serve as a new type of spin current generator, even without relying on the atomic spin-orbit coupling. The mechanism relies on a peculiar spin splitting of the energy bands and a real space anisotropy of the magnon or electron transfers owing to another type of “spin-orbit coupling”. This is activated by glide symmetry breaking of the molecular arrangement due to the antiferromagnetic ordering. Based on the multi-site Hubbard model and the effective Heisenberg model, we analyze the spin current transport properties. When a thermal gradient is applied to the antiferromagnetic insulating state, the up- and down-spin magnons drift to opposite ways due to the anisotropic transfer integrals, resulting in a spin current perpendicular to the thermal gradient. In the doped antiferromagnetic metallic phase, a similar spin current generation occurs by replacing the magnons and the external field to electrons and an electric field. We find that the spin current conductivities are given by symmetric tensors, in stark contrast to the conventional spin Nernst and spin Hall effects described by antisymmetric tensors. Our findings provide another route to generate a spin current and open a new field of spintronics based on organic magnets.