Research Of Tunnel Magnetoresistance In Coupling Double Magnetic Molecule System

Quantum transport through mesoscopic system has been studied for decades. There are a lot of interesting effects found in simple quantum dot system, for example Kondo effect, Coulomb blockade, Pauli spin blockade, negative differential conductance, tunnel magnetoresistance effect, spin-orbital coupling, thermoelectric effect and so on. In recent years, people pay more attention on molecular magnets, where spin exchange interaction between the molecular magnets and electrons may result in some novel phenomena. Particularly, much attention was paid on tunnel magnetoresistance effect induced by imbalance of currents in a magnetic tunnel junction.Firstly, by means of the Rate equation approach, we study spin-polarized transport through series double quantum dots weakly coupled to collinear ferromagnetic leads with particular attention on the effect of interdot spin exchange interaction. For the asymmetric double quantum dots giant negative differential conductance is realized, which depends on the energy-level spacing between two dots. It is demonstrated that the voltage dependencies of the tunnel magnetoresistance and the shot noise are sensitive to the SEI, which leads to the additional imbalance between spinpolarized currents. The super-Poissonian statistics is enhanced in the parallel leads'configuration by the ferromagnetic spin exchange interaction, which favorites the spin bunching, while it is suppressed by stronger antiferromagnetic spin exchange interaction in antiparallel configuration for a symmetric double quantum dots. The voltage dependencies of the TMR and shot noise may be used to probe the SEI.Secondly, we study spin-polarized transport through a molecule magnet dimer of both ferromagnetic and antiferromagnetic types, which is weakly coupled to collinear ferromagnetic leads. We pay particular attention to the effect of spin exchange interactions between electron spin and the molecule magnet on the tunnel magnetoresistance. Giant positive TMR is found in the positive bias with the AFM dimer. Particularly we are able to realize the negative TMR of theoretical lowest limit. And thus the molecule dimer device acts as an ideal valve to blockade completely the current channel of parallel configuration of electrode polarizations.Finally, We study the quantum transport through a tunnel junction embedded with a uniaxial molecule-magnet-dimer non-collinear with the magnetization of ferromagnetic electrodes. The non-collinear angle dependence of spin polarized currents and tunnel magnetoresistance is analyzed based on the rate equation approach in sequential tunneling regime. For the ferromagnetic dimer the majority component of spin-polarized current decreases with the increase of non-collinear angle, while the minority component is almost unchanged. As a consequence the spin polarization inversion takes place at a critical value of angle. A high spin-polarization and low-density current is realized when the anisotropy axis of molecule magnet is perpendicular with the magnetization of electrodes. We moreover demonstrate the tunable tunnel magnetoresistance from negative to positive values in the antiferromagnetic dimer junction. Our observation may have technical applications in the spintronic device.