Spin-dependent Electron Transport Through Single-molecule Magnets

In molecular spintronics, single-molecule magnets (SMMs) possess high spins and uniaxial magnetic anisotropy with an easy axis, which have potential applications in information storage and processing, and electron transport through SMMs has received much attention. In both experimental and theoretical aspects, since the spin-exchange interaction between the tunneling electron and the molecule, many interesting properties have been found, such as Berry-phase blockade, the magnetization reversal of SMM, and thermoelectric effect induced pure spin-current. Spin-polarized transport is at the heart of spintronics, and thus in this thesis, within a rate-equation approach we focus on the spin-dependent transport through a SMM weakly coupled to magnetic electrodes.First, we theoretically study the charge and spin currents through a SMM between ferromagnetic leads, where the magnetizations of two magnetic electrodes are collinear with the magnetic easy-axis of the SMM. In both sequential and cotunneling regimes, we systematically study the cases of ferromagnetic and antiferromagnetic coupling between a tunneling electron and the local molecular spin in the parallel and antiparallel lead magnetizations. We exhibit the charge and spin current, the differential conductance, and the current spin polarization as functions of bias voltage for all cases. An interesting observation is that the polarization reversal of spin-current can be realized and manipulated by the variation of bias voltage (but not inverting) in the case of antiferromagnetic exchange-coupling with antiparallel lead-configuration. The above effect may be useful in the development of spintronic devices, which can reverse the direction of spin current.Secondly, we investigate the effects of intrinsic spin relaxation on the spin-dependent transport through a SMM with ferromagnetic leads in both the sequential and cotunneling regimes, and discuss mainly the effect on the tunneling magnetoresistance (TMR). It is shown that the TMR disappears almost under the fast spin-relaxation in the sequential regime while can vary from large positive to small negative values in the cotunneling regime. Moreover, when a magnetic field is applied along the easy-axis of SMM, a large negative TMR is obtained with the increase of the relaxation strength. Finally in the large-bias voltage limit, the TMR for the negative bias is slightly larger than its characteristic value in the sequential regime, but for the positive bias it becomes a negative value due to the fast spin-relaxation.Then, we study electron transport through SMM junctions with one ferromagnetic lead and one normal-metal lead in the sequential regime, and find the typical tunneling anisotropic magnetoresistance (TAMR) effect, which varies with the angle between the magnetization direction of ferromagnetic lead and the easy-axis of SMM. The angular dependence of TAMR can serve as a probe to determine experimentally the easy-axis of SMM. Moreover, it is demonstrated that both the magnitude and sign of TAMR are tunable by the bias voltage, suggesting a new spin-valve device with only one magnetic electrode in molecular spintronics.Finally, we present a proposal for a voltage-tunable SMM based spin current injector. The device consists of a SMM that is weakly coupled to one ferromagnetic electrode and one nonmagnetic electrode. We show that the current polarization can be dramatically enhanced. Moreover, the sign of spin-polarization can be changed by the gate or the bias voltage.