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Quantum interference effect in electron tunneling through a quantum-dot-ring spin valve.

Ma JM, Zhao J, Zhang KC, Peng YJ, Chi F - Nanoscale Res Lett (2011)

Bottom Line: It is shown that the magnitudes of these quantities are sensitive to the relative angle between the leads' magnetic moments and the quantum interference effect originated from the inter-lead coupling.We pay particular attention on the Coulomb blockade regime and find the relative current magnitudes of different magnetization angles can be reversed by tuning the inter-lead coupling strength, resulting in sign change of the TMR.For large enough inter-lead coupling strength, the current spin polarizations for parallel and antiparallel magnetic configurations will approach to unit and zero, respectively.PACS numbers:

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Physics, Bohai University, Jinzhou 121000, China. chifeng@semi.ac.cn.

ABSTRACT
Spin-dependent transport through a quantum-dot (QD) ring coupled to ferromagnetic leads with noncollinear magnetizations is studied theoretically. Tunneling current, current spin polarization and tunnel magnetoresistance (TMR) as functions of the bias voltage and the direct coupling strength between the two leads are analyzed by the nonequilibrium Green's function technique. It is shown that the magnitudes of these quantities are sensitive to the relative angle between the leads' magnetic moments and the quantum interference effect originated from the inter-lead coupling. We pay particular attention on the Coulomb blockade regime and find the relative current magnitudes of different magnetization angles can be reversed by tuning the inter-lead coupling strength, resulting in sign change of the TMR. For large enough inter-lead coupling strength, the current spin polarizations for parallel and antiparallel magnetic configurations will approach to unit and zero, respectively.PACS numbers:

No MeSH data available.


Current, current polarization and TMR each as a function of the inter-lead coupling strength for different values of φ and fixed PL = PR = 0.3. The other parameters are as in Fig. 2.
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Figure 4: Current, current polarization and TMR each as a function of the inter-lead coupling strength for different values of φ and fixed PL = PR = 0.3. The other parameters are as in Fig. 2.

Mentions: Finally we study how the inter-lead coupling strength tLR influence these quantities. In Figure 4 we show their characteristics each as a function of tLR with fixed bias voltage eV = U and εd = 0.5, which means that we are focusing on the Coulomb blockade region. It is shown in Figure 4(a) that in the case of weak inter-lead coupling, typical spin valve effect holds true, i.e., the current magnitude is decreased with increasing φ as was shown in Figure 2(a) and 2(d) (see the Coulomb blockade region in them). With the increase of tLR, reverse spin valve effect is found, in other words, current magnitudes of larger angles become larger than those of smaller angles. This phenomenon can be understood by examining the spin-dependent line-width function. The basic reason is that in this Coulomb blockade region, the relative magnitudes of the currents through the QD of different angle will keep unchanged regardless of the values of tLR (see Figure 2). But the current through the bridge between the leads, which is directly proportional to the inter-lead line-width function , will be drastically varied by the angle. In the parallel configuration, for example, spin-up inter-lead line-width function is larger than the spin-down one since ρL↑ = ρR↑ = ρ0 (1 + Pβ) and ρL↓ = ρR↓ = ρ0 (1 - Pβ). So the current polarization will increase with increasing tLR as shown by the solid curve in Figure 4(b). As the polarization of the leads is fixed, both spin-up and spin-down line-width functions will be enhanced with increasing tLR, resulting in increased total current as shown in Figure 4(a). For the antiparallel case (φ = π), the current magnitude will also be enhanced for the same reason. But the current spin polarization is irrelevant to the tunnel process through the bridge since ρL↑ = ρR↓ = ρ0 (1 + Pβ) and ρL↓ = ρR↑ = ρ0 (1 - Pβ). The inter-lead line-width functions of both spin components are equal . The current spin polarization is mainly determined by the transport process through the QD. From the above discussion we also know that the current magnitude of the parallel configuration through the bridge is larger than that of the antiparallel alignment. With the increase of tLR, current through the bridge play a dominant role as compared with that through the dot, and the reverse spin valve effect may emerge accordingly. For the case of 0 <φ < π, the behavior of the current can also be understood with the help of the above discussions. Due to the reverse spin valve effect, the TMR in Figure 4(c) is reduced with increasing tLR, and becomes negative for high enough inter-lead coupling strength.


Quantum interference effect in electron tunneling through a quantum-dot-ring spin valve.

Ma JM, Zhao J, Zhang KC, Peng YJ, Chi F - Nanoscale Res Lett (2011)

Current, current polarization and TMR each as a function of the inter-lead coupling strength for different values of φ and fixed PL = PR = 0.3. The other parameters are as in Fig. 2.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211328&req=5

Figure 4: Current, current polarization and TMR each as a function of the inter-lead coupling strength for different values of φ and fixed PL = PR = 0.3. The other parameters are as in Fig. 2.
Mentions: Finally we study how the inter-lead coupling strength tLR influence these quantities. In Figure 4 we show their characteristics each as a function of tLR with fixed bias voltage eV = U and εd = 0.5, which means that we are focusing on the Coulomb blockade region. It is shown in Figure 4(a) that in the case of weak inter-lead coupling, typical spin valve effect holds true, i.e., the current magnitude is decreased with increasing φ as was shown in Figure 2(a) and 2(d) (see the Coulomb blockade region in them). With the increase of tLR, reverse spin valve effect is found, in other words, current magnitudes of larger angles become larger than those of smaller angles. This phenomenon can be understood by examining the spin-dependent line-width function. The basic reason is that in this Coulomb blockade region, the relative magnitudes of the currents through the QD of different angle will keep unchanged regardless of the values of tLR (see Figure 2). But the current through the bridge between the leads, which is directly proportional to the inter-lead line-width function , will be drastically varied by the angle. In the parallel configuration, for example, spin-up inter-lead line-width function is larger than the spin-down one since ρL↑ = ρR↑ = ρ0 (1 + Pβ) and ρL↓ = ρR↓ = ρ0 (1 - Pβ). So the current polarization will increase with increasing tLR as shown by the solid curve in Figure 4(b). As the polarization of the leads is fixed, both spin-up and spin-down line-width functions will be enhanced with increasing tLR, resulting in increased total current as shown in Figure 4(a). For the antiparallel case (φ = π), the current magnitude will also be enhanced for the same reason. But the current spin polarization is irrelevant to the tunnel process through the bridge since ρL↑ = ρR↓ = ρ0 (1 + Pβ) and ρL↓ = ρR↑ = ρ0 (1 - Pβ). The inter-lead line-width functions of both spin components are equal . The current spin polarization is mainly determined by the transport process through the QD. From the above discussion we also know that the current magnitude of the parallel configuration through the bridge is larger than that of the antiparallel alignment. With the increase of tLR, current through the bridge play a dominant role as compared with that through the dot, and the reverse spin valve effect may emerge accordingly. For the case of 0 <φ < π, the behavior of the current can also be understood with the help of the above discussions. Due to the reverse spin valve effect, the TMR in Figure 4(c) is reduced with increasing tLR, and becomes negative for high enough inter-lead coupling strength.

Bottom Line: It is shown that the magnitudes of these quantities are sensitive to the relative angle between the leads' magnetic moments and the quantum interference effect originated from the inter-lead coupling.We pay particular attention on the Coulomb blockade regime and find the relative current magnitudes of different magnetization angles can be reversed by tuning the inter-lead coupling strength, resulting in sign change of the TMR.For large enough inter-lead coupling strength, the current spin polarizations for parallel and antiparallel magnetic configurations will approach to unit and zero, respectively.PACS numbers:

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Physics, Bohai University, Jinzhou 121000, China. chifeng@semi.ac.cn.

ABSTRACT
Spin-dependent transport through a quantum-dot (QD) ring coupled to ferromagnetic leads with noncollinear magnetizations is studied theoretically. Tunneling current, current spin polarization and tunnel magnetoresistance (TMR) as functions of the bias voltage and the direct coupling strength between the two leads are analyzed by the nonequilibrium Green's function technique. It is shown that the magnitudes of these quantities are sensitive to the relative angle between the leads' magnetic moments and the quantum interference effect originated from the inter-lead coupling. We pay particular attention on the Coulomb blockade regime and find the relative current magnitudes of different magnetization angles can be reversed by tuning the inter-lead coupling strength, resulting in sign change of the TMR. For large enough inter-lead coupling strength, the current spin polarizations for parallel and antiparallel magnetic configurations will approach to unit and zero, respectively.PACS numbers:

No MeSH data available.