Magnetic Field Effects on Tunnel Magnetoresistance of a Coupled Carbon-Nanotube-Molecular-Magnet System

The effects of external magnetic field on the transport properties of a coupled carbon-nanotube-molecular-magnet system weakly attached to ferromagnetic leads are studied theoretically. The calculations are performed using the real-time diagrammatic technique, including the lowest order terms of expansion with respect to the tunnel coupling. It is shown that the magnetoresistive properties of the device depend on both the ground state of isolated nanotube, which in the two-electron regime can be either a spin singlet or a spin triplet, as well as the type of exchange interaction between the molecular magnet and the nanotube. Furthermore, external magnetic field was found to give rise to highly nontrivial behavior of the tunnel magnetoresistance (TMR). First of all, it results in an asymmetry of TMR with respect to the bias reversal. Moreover, it can lead to both enhancement or suppression of TMR, depending on the model parameters and the applied bias voltage.