Oscillatory changes in the tunneling magnetoresistance effect in semiconductor quantum-dot spin valves

We experimentally study the tunneling magnetoresistance (TMR) effect as a function of bias voltage $\left(V_{\mathrm{SD}}\right)$ in lateral $\mathrm{Ni}∕\mathrm{In}\mathrm{As}∕\mathrm{Ni}$ quantum-dot (QD) spin valves showing Coulomb blockade characteristics. With varying $V_{\mathrm{SD}}$, the TMR value oscillates and the oscillation period corresponds to conductance changes observed in the current-voltage $\left(I\text{−}{V}_{\mathrm{SD}}\right)$ characteristics. We also find an inverse TMR effect near $V_{\mathrm{SD}}$ values where negative differential conductance is observed. A possible mechanism of the TMR oscillation is discussed in terms of spin accumulation on the QD and spin-dependent transport properties via excited states.

Figure 1

(Color online) (a) A schematic diagram of our lateral quantum-dot spin-valve device. (b) A two-dimensional plot of differential conductance $dI∕d{V}_{\mathrm{SD}}$ as a function of $V_{\mathrm{SD}}$ and $V_{\mathrm{G}}$ in parallel magnetic configuration at $B=0.1\mathrm{T}$ and $T=50\mathrm{mK}$. (c) $I\text{−}{V}_{\mathrm{SD}}$ curve measured at $V_{\mathrm{G}}=0.1\mathrm{V}$.

Figure 2

(Color online) The representative MR curves for various $V_{\mathrm{SD}}$, (a) $12\mathrm{mV}$, (b) $15\mathrm{mV}$, (c) $17\mathrm{mV}$, (d) $21\mathrm{mV}$, (e) $30\mathrm{mV}$, and (f) $39\mathrm{mV}$, measured at $V_{\mathrm{G}}=0.1\mathrm{V}$ and $T=50\mathrm{mK}$.

Figure 3

(Color online) (a) TMR value as a function of $V_{\mathrm{SD}}$ at $V_{\mathrm{G}}=0.1\mathrm{V}$. The inset shows schematic diagrams of tunneling processes via excited states. An energy window between the occupied in the right and empty in the right electrodes is opened by applying $V_{\mathrm{SD}}$. The solid blue and dashed pink lines exhibit the ground and excited states in the QD, respectively. (b) The corresponding $dI∕d{V}_{\mathrm{SD}}\text{−}{V}_{\mathrm{SD}}$ curve at $V_{\mathrm{G}}=0.1\mathrm{V}$ and $B\sim 0.1\mathrm{T}$.