Interfacial transport properties between a strongly correlated transition metal oxide and a metal: Contact resistance in ${\text{Fe}}_3{\text{O}}_4/M$ ($M=\text{Cu}$, Au, Pt) nanostructures

Multiterminal measurements have typically been employed to examine electronic properties of strongly correlated electronic materials such as transition metal oxides without the influence of contact effects. In contrast, in this work we investigate the interface properties of ${\text{Fe}}_3{\text{O}}_4$ with different metals, with the contact effects providing a window on the physics at work in the correlated oxide. Contact resistances are determined by means of four-terminal electrical measurements as a function of source voltage and temperature. Contact resistances vary systematically with the work function of the electrode metal, $\varphi \left(M\right)$, $M=\text{Cu}$, Au, and Pt, with higher work function yielding lower contact resistance. This trend and the observation that contact resistances are directly proportional to the ${\text{Fe}}_3{\text{O}}_4$ resistivity are consistent with modeling the oxide as an effective $p$-type semiconductor with hopping transport. The jumps in contact resistance values at the bias-driven insulator-metal transition have a similar trend with $\varphi \left(M\right)$, consistent with the transition mechanism of charge gap closure by electric field.

Figure 1

(a) NPGS pattern of different contact metal devices on the same piece of ${\text{Fe}}_3{\text{O}}_4$ film. Gray squares are $300\mu \text{m}\times 300\mu \text{m}$ pads leading to source and drain electrodes while pads colored in white lead to the voltage probes. The very small box in the center of the upper left device defines a region magnified in (b). Colored in black and gray, the ⌈- and ⌋-shaped source/drain leads are the ones made of different contact metals. (c) and (d) are SEM images of four-terminal devices with Au-Au and Cu/Au-Pt source-drain leads, respectively.

Figure 2

(Color) Schematics of (a) electrical circuit for four-probe measurements. Letters S and D denote source and drain contacts, respectively, and (b) how the source voltage, $V_{\text{out}}$, is distributed along the channel. (c) Temperature dependence of the low bias device resistance, $R_{\text{DEV}}$, and contact resistance $R_{\text{C}}\left(M\right)$, $M=\text{Cu}$, Au, and Pt, demonstrating $R_{\text{C}}\left(\text{Pt}\right)<R_{\text{C}}\left(\text{Au}\right)<R_{\text{C}}\left(\text{Cu}\right)$, and that for all cases $R_{\text{C}}\sim R_{\text{DEV}}$. Inset shows the weak dependence of $R_{\text{C}}\left(M\right)$ on source voltage in the (nearly) linear regime around 0 V at 80 K.

Figure 3

(Color online) Schematic of the suggested energetics at the metal/oxide interface below the Verwey temperature. Various parameters are defined in the text. Charge transport takes place between the single-particle states at the metal Fermi level and localized states in the broadened “valence” band of the oxide. Above the Verwey temperature, the situation is similar, with transport in the oxide still taking place via the hopping of localized carriers. The trends in the contact resistance data suggest that in our samples dominant carriers are holelike throughout the whole temperature range.

Figure 4

(Color) $I\text{−}{V}_{\text{out}}$ (black) and $I\text{−}\Delta V$ (red) characteristics at 80 K (a) in nonlinear regime before transition and (b) if applied voltage exceed a critical switching value, $V_{\text{sw}}$, inducing resistive switching.

Figure 5

(Color) (a) Calculated differential $R_{\text{C}}\left(M\right)$, $M=\text{Cu}$, Au, and Pt, and resistance of the magnetite channel, $R_{\text{DEV}}$, at 82 K before and after transition point. (b) Temperature dependence of the absolute values of jumps in contact resistances, $\left|R_{\text{C}}^{jump}\left(M\right)\right|$. Error bars represent a standard deviation over several independent measurements (different devices, left/right voltage pairs, different grounds, positive/negative switching branches).