Electric field driven dynamic percolation in electronically phase separated (La${}_{0.4}$Pr${}_{0.6}$)${}_{0.67}$Ca${}_{0.33}$MnO${}_3$ thin films

Competing ferromagnetic metallic (FMM) and insulating phases in the manganite (La${}_{1-y}$Pr${}_y$)${}_{1-x}$Ca${}_x$MnO${}_3$ lead to a phase separated state in which micrometer scale FMM regions behave in a fluidlike manner over a narrow temperature range. Here we show that an electric field can realign the fluidlike FMM phases embedded in an insulating matrix, resulting in anisotropic in-plane resistance in microstructures of (La${}_{0.4}$Pr${}_{0.6}$)${}_{0.67}$Ca${}_{0.33}$MnO${}_3$ thin films. Time and voltage dependent resistance and magnetization measurements show that the dynamic percolation of the FMM regions leads to an insulator to metal transition due to electric-field induced realignment of the FMM regions, which is analogous to the dielectrophoresis of metallic particles suspended in fluid media. In-plane strain anisotropy plays an important role in determining the speed of dynamic percolation of the FMM regions by modifying the local electric fields in the phase separated state.

Figure 1

(a) A 1 $\times$ 1-$\mu$m${}^2$ atomic force microscope image of a (La${}_{0.4}$Pr${}_{0.6}$)${}_{0.67}$Ca${}_{0.33}$MnO${}_3$ (LPCMO) thin film on an NdGaO${}_3$ substrate. The vertical scale is 5 nm and the height of each step is about 0.4 nm. (b) Magnetization hysteresis loops along the hard axis and easy axis at 50 K for a 30-nm-thick LPCMO thin film. (c) Optical microscope image showing the dimensions of the cross-shaped microstructure of an LPCMO thin film. The orientation of the substrate and the relative directions of the magnetic hard and easy axes are shown schematically above the image of the microstructure. (d) $R\left(T\right)$ curves of the LPCMO microstructure along the magnetic easy axis and hard axis. The arrows show the direction of temperature change. (e) Isothermal time dependent resistance curves along the magnetic easy axis and hard axis at 68.5 K.

Figure 2

$M\left(t\right)$ and $R\left(t\right)$ curves along the easy axis at 68 K in an 800-Oe field showing that the electric-field induced drop in resistance has a negligible effect on the magnetization.

Figure 3

$R\left(T\right)$ curves for two voltages along the (a) easy axis and (b) hard axis. Voltage dependence of the $R\left(t\right)$ curves along the (c) easy axis and (d) hard axis at 68.5 K. The arrow in (d) marks the anomalous increase in resistance along the hard axis. Breakdown time ($t_{\text{BD}}$) is shown as a function of applied voltage ($V_A$) along the (e) easy axis and (f) hard axis. Insets: Semilog plots along the (e) easy axis and (f) hard axis.

Figure 4

(a) Simultaneous $R\left(t\right)$ measurements at 68.5 K along the hard and easy axes. The arrow marks the initial anomalous resistance increase along the hard axis. The dashed box highlights the resistance increase along the easy axis simultaneously with the resistance drop along the hard axis, a magnified view of which is shown in the bottom inset. Top inset: Schematic diagram of the simultaneous transport measurement setup. $R_{s1}$ and $R_{s2}$ are standard resistors. Schematic diagrams show the effect of the anisotropic shapes of the nucleating FMM phase on the local electric field when the external voltage is applied (b) along the magnetic easy axis and (c) along the hard axis.