Toroidal quantum states in molecular spin-frustrated triangular nanomagnets with weak spin-orbit coupling: Applications to molecular spintronics

We theoretically investigate the ground-state spin texture and spin transport properties of triangular rings with on-site spins Sq=12(q=1-3). In the limit of strong antiferromagnetic exchange coupling and weak spin-orbit coupling, we find it is possible to prepare a noncollinear degenerate ground state with a zero magnetic moment and a nonzero toroidal moment τ=gμBqrq×Sq, aligned along the C3-symmetry axis. These pure toroidal states can be prepared: (i) within the fourfold degenerate spin-frustrated ground state even without any spin-orbit coupling; (ii) within the ground Kramers doublet resulting from weak spin-orbit splitting of the fourfold degenerate frustrated manifold via Dzyaloshinskii-Moriya antiferromagnetic exchange coupling. We also investigate the relationship between toroidal states and chiral spin states, characterized by the eigenvalues of the scalar spin chirality operator χ=43Ŝ1·Ŝ2×Ŝ3, and find that, since [τ,χ]≠0, it is not possible to prepare states that are both toroidal and chiral simultaneously. Finally, by setting up a quantum transport model in the Coulomb blockade regime, we find that a spin current injected through a spin-polarized source electrode into the triangle is partially reversed upon scattering with the molecular toroidal states. This spin-switching effect is, in fact, a signature of molecular spin-transfer torque, which can be harnessed to modify the nonequilibrium populations of the +τ- and -τ-toroidal states, thus, to induce a net toroidal magnetization in the device using a spin current.