Enhanced magnetoresistance due to charging effects in a molecular nanocomposite spin device

We have investigated a Coulomb-blockade effect and a higher order cotunneling effect in rubrene-Co nanocomposite spin devices, where the Co nanoparticles were uniformly embedded into the rubrene matrix and a large magnetoresistance (MR) effect appeared. A clear Coulomb gap was observed between $\pm 1.1\text{V}$ at low temperature. Within the gap, the enhancement of the MR ratio was observed up to $\sim 50\%$, which has not been explained by previous theoretical studies. The enhancement is induced by higher-order cotunneling. Power-law dependence of an electric current for a bias voltage ($I\sim V^{2N-1}$; $N$ is an order of cotunneling) was observed in the Coulomb gap, which corroborates that at maximum fifth-order cotunneling is attributed to the enhancement of the MR ratio.

Figure 1

(Color online) Schematic illustration of a sample and a plot of the temperature dependence of current-bias voltage curve. (a) A schematic structure and a cross-sectional view of a rubrene-Co nanocomposite with a 600 nm gap length. (b) The temperature dependence of current $\left(I\right)$-bias voltage $\left(V\right)$ characteristics of the rubrene-Co nanocomposite. A clear CB (indicated by arrows) can be seen at $\pm 1.1\text{V}$.

Figure 2

(Color online) Spin-transport properties of the rubrene-Co nanocomposites. The bias voltage dependence of the MR ratio (blue circles) and the electric current without an external magnetic field (red lines) at (a) 80, (b) 15, and (c) 3.2 K. The MR ratio is defined as $\frac{I\left(B=3T\right)-I\left(B=0T\right)}{I\left(B=0T\right)}$, and it is calculated from the $I\text{−}V$ curves at 0 and 3 T. (d) Log-log plots of the $I\text{−}V$ characteristics of rubrene-Co nanocomposite at different temperatures. The $I\text{−}V$ characteristic in the Coulomb gap is $I\propto V^9$ at 3.2 K and $I\propto V^3$ at 15 K, and the power of the bias voltage decreases monotonously as the temperature increased. The black lines ($I\propto V^9$ and $I\propto V^3$) are guides for the eyes. The inset is the number of junctions $\left(N\right)$ which contributes to cotunneling at different temperatures. It decreases as temperature increases. (e) The dependence of the MR ratio on the number of junctions at $V=0.6\text{V}$ in the low-temperature region. The monotonous increase in the MR ratio with the number of junctions implies that cotunneling governs the MR enhancement.

Figure 3

(Color online) Spin transport property of rubrene-Co (4:1) nanocomposite. The bias voltage dependence of the MR ratio (blue circles) and the electric current in the absence of an external magnetic field (a red line) at 3.2 K in a rubrene-Co nanocomposite with a composition ratio of 4:1 for rubrene:Co. The vertical axis scale is the same as those in Figs. 2a, 2b, 2c. The current from $-2$ to 2 V appears to be almost zero, implying that CB is dominant in this bias voltage region. The inset is a log-log plot of the (electric current)-(bias voltage) characteristic and it shows that the current depends on the power of the bias voltage. Cotunneling is dominant for the MR enhancement between $-2$ and 2 V. Enhancement due to CB is not observed in this bias region.