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Under applied bias V, only the QWS with energies E σ n that lie within the bias window from -eV/2 to +eV/2, denoted by
the shaded area in Figure 2. contribute to the resonant tunneling. This can be seen clearly in Figure 3a and Figure 3b, where
we display the low-temperature (T=5K) bias dependence of the spin-polarized currents, I ↑ (up triangles) and I ↓ (down
triangles), for N C =4AS and N C =7AS, respectively, and for θ=π/2. The spin-polarized currents switch on at V σ on=2|E σ n-E F |,
when the n σ QWS enters the bias energy window. For N C =7AS, the switch on for both spin-polarized currents occur at about
the same bias, V ↑ on≈ V ↓ on due to the fact that |E 1 ↓ -E F | ≈ |E 3 ↑ -E F | as one can see in Figure 2. Both currents decrease with
increasing bias because the density of states of the minority band in the leads decreases. In Figure 3c and Figure 3d we
display the low-temperature (T = 5K) bias behavior of the parallel, T i,|| , (squares) and perpendicular T i,┴ (circles) components
of the local spin torque on the first site in the central FM, for θ=π/2 and for N C =4AS and N C =7AS, respectively. The local
spin-transfer component, T i,|| exhibits a switch on bias behavior at V σ on, similar to that of the spin polarized currents in Figure
3a. On the other hand, T i,┴ which is non-zero for zero bias, displays a non-monotonic bias dependence, changing sign
Figure 3: Low-temperature (T=5K) bias dependence of the majority (up triangles) and minority (down triangles) currents in
the central FM for (a) N C =4AS and (b) N C =7AS, respectively, and for $\theta=\pi/2$. Bias dependence of T i,|| , (squares) and
perpendicular T i,┴ (circles) on the first site in the central FM, for (c) N C =4AS and (d) N C =7AS. The bias V σ on denote the
switch-on bias.
between V ↑ on and V ↓ on similar to that of the exchange field in quantum dots connected to FM leads [11]. It is important to
note in Figure 3d the scale which shows that both T i,|| and T i,┴ are strongly enhanced for N C =7AS, even though the
corresponding spin-polarized currents are smaller than those for N C =4AS. This clearly demonstrates that the underlying
mechanism that controls the enhancement of the efficiency of the local spin-transfer torque is the close proximity of the
majority and minority QWS energies of different quantum number within the bias energy window. This in turn enhances the
spin mixing σ↔σ’ in the central FM, when electrons tunnel resonantly through the spin polarized QWS. The enhancement of
the local spin-transfer torque is independent of the parity of the QWS wavefunctions. This spin-transfer torque enhancement
may have important technological applications since the CIMS may be facilitated under such conditions in DBMTJ.
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