M. J. Ma

Spin-flip effects in a ferromagnetic/normal-metal island/ferromagnetic double tunnel junction system

We theoretically study the spin dependent transport through a magnetic double tunnel junction system, which consists of a nonmagnetic metal island sandwiched by two ferromagnetic electrodes. The transport model in both cotunneling and sequential tunneling regimes is based on the master equation, and systematically incorporates the effects of both intraisland spin-flip (SF) and SF during tunneling between the electrode and the island. We found that the tunnel magnetoresistance (TMR) in the cotunneling regime is impervious to either the intraisland SF or the tunneling SF effect. On the other hand, in the sequential tunneling regime, the TMR decreases with the increasing intraisland SF rate Γ and tunneling SF probability η (η 0.5, the TMR is enhanced by increasing the tunneling SF probability. The increase in the temperature suppresses the cotunneling TMR, whereas the sequential tunneling TMR is found to be more robust to temperature variation.

Sequential tunneling through a two-level semiconductor quantum dot system coupled to magnetic leads

Sequential transport through a semiconductor quantum dot (QD) weakly coupled to two collinear magnetic leads is analyzed theoretically based on the master equation method. The transport model considers two discrete energy levels in the dot, i.e., the lowest unoccupied ϵe (electron) and the highest occupied ϵh (hole) energy levels, with associated Coulomb correlation energies Ue and Uh, respectively, and the spin-flip effect within the dot. The charge and spin currents and the tunneling magnetoresistance are evaluated in the sequential tunneling regime. It was found that when both energy levels, ϵe and ϵh, contribute to the tunneling transport, there is enhancement of the current as well as tunneling magnetoresistance, compared to the case of a single-level QD transport considered previously. The tunneling transport through the two levels of the QD also results in additional bias-dependence: it was observed that spin accumulation and magnetoresistance of the system are particularly suppressed by spin flip ...

Spin-bias driven field effect transistor

We propose a spin field effect transistor driven by spin biases which are externally generated in the source and drain electrodes. We employed the Keldysh non-equilibrium Green’s function formalism to evaluate the charge and spin currents through the transistor, and verify the operation of the transistor as predicted by a semiclassical model. Our calculations show that in the “off” state, both the charge and spin currents are suppressed. In the “on” state, the device allows only the spin current to pass through but not charge current, thus potentially improving the energy efficiency of the device.

Electron Transport at the Interface Between a Ferromagnetic Insulator and a Topological Insulator

We perform a theoretical study of the electron transport through a normal metal-topological insulator-normal metal (NM-TI-NM) system, where a ferromagnetic insulator (FI) layer is deposited on top of the TI. The spin conductance of the system is analyzed as a function of parameters such as the strength of exchange coupling between the surface states of the TI and the magnetic moments of the FI layer, as well as the dimension of the TI channel. We find that the strength of the spin conductance can be optimized by tuning the parameters.

Spin current generator based on topological insulator coupled to ferromagnetic insulators

We propose a spin current generator based on a topological insulator current-in-plane spin valve, consisting of a 3D topological insulator sandwiched between two ferromagnetic insulator layers. The “on” and “off” states of the spin current can be toggled by switching the magnetization configuration of the two ferromagnetic insulator layers which are coupled to the surface states of the topological insulator.

Spin-flip associated sequential tunnelling through a magnetic double tunnel junction system

Spin-flip (SF) associated sequential tunnelling through a semiconductor quantum dot (QD) sandwiched by two collinear ferromagnetic leads is theoretically analysed based on the Master equation method. The transport model considers two discrete energy levels in the dot, i.e. the lowest unoccupied and the highest occupied energy levels, with associated intra-level Coulomb correlation energies. The tunnelling current and the tunnelling magnetoresistance (TMR) are evaluated in the presence of SF effects both within the quantum dot (SF-QD) and during sequential tunnelling across a junction (SF-TJ). It was shown that the presence of both SF mechanisms only affects the tunnelling current when the leads are in the antiparallel configuration. The antiparallel current increases monotonically with increasing strength of SF-QD resulting in a suppression of the TMR. The increase occurs for both low and high bias regions corresponding to singly occupied and freely occupied QD states, respectively. For the SF-TJ effect, the increase in the antiparallel current and resulting suppression of TMR is more pronounced for high bias corresponding to the freely occupied state. The TMR suppression occurs for SF probability within the range of 0 ≤ η ≤ 0.5, and reaches a maximum at η = 0.5 where TMR vanishes completely. However, at higher SF probability of 0.5 ≤ η ≤ 1, the trend in the antiparallel current is reversed leading to an enhancement of the TMR. Overall, the η-dependence of the TMR is symmetric about η = 0.5 and roughly proportional to (1 − 2η)2. In the presence of both SF mechanisms, it was found that SF-QD has a stronger suppressive effect on the TMR compared with SF-TJ.

Spin polarized transport in an asymmetric ferromagnetic/quantum dot/ferromagnetic system

We investigate the tunnel magnetoresistance (TMR) of the double barrier magnetic tunnel junction, where a quantum dot (QD) with discrete electron and hole energy levels is sandwiched between ferromagnetic leads. The effects of the symmetry of the coupling between the leads and the dot on both the TMR and spin accumulation (SA) are studied for voltage ranges corresponding to the QD’s single and double occupancies. When the QD is singly occupied, both the TMR and SA assume at their minimum values for symmetrical junctions with identical coupling strengths. For the doubly occupied QD, the opposite occurs with the highest TMR and SA being observed for symmetrical junctions with identical coupling strengths. The TMR is found to be strongly correlated with the spin accumulation in the QD.

Effect of interface spin-flip scattering on the spin polarized transport through a quantum dot: Master equation approach

We investigate the spin-flip scattering effects on the tunnel magnetoresistance (TMR) through the double barrier magnetic tunnel junction, where a two-energy level quantum dot is sandwiched by two ferromagnetic leads. The spin-flip scattering, which occurs at the interface between the lead and the dot, suppresses the TMR in the bias voltage regions corresponding to the singly occupied (SO) and freely occupied (FO) quantum dot state, respectively. In the FO state, the dot can be occupied by up to two electrons or holes. The suppression of the TMR in the SO region is more significant than that in the FO region in the weak spin-flip regime, i.e., when spin-flip probability η 0.5, negative TMR is observed in both occupied regions, with the magnitude occurring in the FO region greater. High asymmetry between the spin-flip strengths of spin-up and spin-down electrons can result in an enhancement in the TMR.

Enhanced magneto-Coulomb effect in asymmetric ferromagnetic single electron transistors

We investigate the magneto-Coulomb (MC) effect in a ferromagnetic single electron tunneling transistor (FM-SETT), with asymmetric junction resistances and FM electrodes. The MC effect enables the conductance of the FM-SETT by an applied magnetic B-field in addition to the usual gate-bias modulation. Under optimal biasing of the asymmetric FM-SETT near the sawtooth edge of its gate oscillation, the sensitivity γB=dI∕dB can be enhanced by a factor α, where α=R1∕R2 denotes the junction resistance asymmetry. The enhanced B-field modulation is, however, susceptible to thermal smearing effects. Finally, an asymmetry in the magnetic properties of the FM leads results in a complex magnetoconductance response, with distinct conductance states.

Boltzmann transport study of bulk and interfacial spin depolarization effects in spin valves

A theoretical model is proposed to analyze both bulk and interfacial spin depolarization effects on the magnetoresistance (MR) of nanoscale spin valves (SVs). The model is based on the spin coupled Boltzmann transport equations where the momentum spin relaxation arising from spin flip and non-spin-flip scattering are considered. In the boundary conditions we include the parameter q which denotes the interfacial spin flip probability, while bulk spin depolarization is characterized by the ratio r of spin flip to non-spin-flip scattering times. We consider a typical Fe∕Cr∕Fe pseudo-SV trilayer, and calculate the current for parallel and antiparallel alignments, to deduce the MR. A decreasing trend in MR is observed with an increase in either r or q, with the dependence on q being more pronounced. We also studied the combined effect of interfacial diffusive scattering (described by parameters Ns and D↑) and spin flip scattering. We found that although diffusive scattering generally results in an improvement ...