Spin-Dependent Quantum Transport Properties Of Single-Molecule Magnets

The research of spintronic devices is closely related to quantum computing and memory.The single-molecule magnet(SMM)has become a popular nano-material for information storage and quantum computing,due to the uniaxial anisotropy and large spin.Based on the rate equation method,the dynamic properties,thermoelectric effect and tunneling anisotropy of SMM are studied in this work.Here spin polarization transport,spin flip,pure spin current generation and tunneling anisotropic magnetoresistance under different tunneling conjunction structures are discussed.Specific research contents are as follows:First,we investigate the non-equilibrium quantum transport through a single-molecule magnet with ferromagnetic electrodes.The lead magnetization direction is non-collinear with the uniaxial anisotropy easy-axis of molecule-magnet.The magnetization reversion of molecule-magnet is induced by the back action of spin-polarized current in the sequential tunnel regime.It is found that the antiparallel configuration of the ferromagnetic electrodes and molecular anisotropy easy-axis is an effective structure to reverse both the magnetization of molecule-magnet and spin-polarization of the transport current.The dependence of transport properties on non-collinear angle provides useful knowledge for the quantum manipulation of molecule-magnet and spin polarized electron-transport.Second,we study the generation of spin-current in a single-molecule magnet(SMM)tunnel junction with Coulomb interaction of transport electrons and external magnetic field.In the absence of field the spin-up and-down currents are symmetric with respect to the initial polarizations of molecule.The existence of magnetic field breaks the time-reversal symmetry of system,which leads to unsymmetrical spin currents of parallel and antiparallel polarizations.Both the amplitude and polarization direction of spin current can be controlled by the external magnetic field.Particularly when the magnetic field increases to a certain value the spin-current with antiparallel polarization is reversed along with the magnetization reversal of the SMM.The two-electron occupation indeed enhances the transport current compared with the single-electron process.However the increase of Coulomb interaction results in thesuppression of spin-current amplitude at the electron-hole symmetry point.We propose a scheme to compensate the suppression with the external magnetic field.Finally,we simulate the tunneling anisotropic magnetoresistance(TAMR)in a single-molecule-magnet(SMM)dimer tunnel-junction with metal and ferromagnetic(FM)electrodes.The non-collinear polarization of electrode with respect to the uniaxial anisotropy-axis of magnet results in both the FM and anti-ferromagnetic(AFM)TAMR respectively for the FM and AFM inter-molecule couplings.In terms of the spin coherent state representation of electron spin the non-collinear tunneling is able to be analyzed with the usual rate equation approach in a sequential tunneling regime.The ferromagnetic TAMR varies with the non-collinear angle and the tunneling magnetoresistance(TMR)is just a special case of the angle ? = ?.With the FM dimer we obtain the higher TMR up to 400% and the high polarization rate(79%)of spin current as well.The angle dependence of TAMR for the AFM dimer is also presented along with the spin current.In both FM and AFM configurations,the Coulomb and Pauli spin expulsions generally exist and have obviously effects on TAMR.