Molecular Spintronics

In organic molecules and molecular solids the weak spin-orbit and hyperfine interactions result in extremely long spin-lifetimes reaching up to the second mark. However the same are characterized by a generally poor mobility, so that the spin-diffusion lengths are rather short. These peculiar characteristics position organic molecules in a unique space within Spintronics and one should envision applications where the spins are manipulated close to where they are injected [1]. In this contribution I will review the current state of the art of the theory of spin-transport and manipulation in organic molecules. I will start the discussion by presenting a new mechanism, the electrostatic spin crossover effect, for manipulating electrically the magnetic state of a molecules without calling for current-driven spin-transfer torques [2]. This is based on the fact that the different spin states of a molecule Stark-shift differently and it is mostly effective when inversion symmetry is broken. Then I will move to discuss the consequences of such an effect on the transport properties of a molecule presenting two magnetic centers and demonstrate that there exist a critical voltage at which the current becomes temperature-independent [3]. Finally I will present results for spin-transport in Mn$_{12}$ and demonstrate that the magnetic state of the molecule can be read electrically with a single $I$-$V$ read-out obtained by using non-magnetic electrodes [4]. \\[4pt] [1] G. Szulczewski, S. Sanvito and J.M.D. Coey, Nature Materials {\bf 8}, 693 (2009).\\[0pt] [2] N.~Baadji, M.~Piacenza, T.~Tugsuz, F.~Della~Sala, G.~Maruccio and S.~Sanvito, Nature Materials {\bf 8}, 813 (2009).\\[0pt] [3] S.K.~Shukla and S. Sanvito, Phys. Rev. B, in press; also at arXiv:0905.1607.\\[0pt] [4] C.D.~Pemmaraju, I.~Rungger and S.~Sanvito, Phys. Rev. B {\bf 80}, 104422 (2009).