Current Oscillation and Low-Field Colossal Magnetoresistance Effect in Phase-Separated Manganites

Current-induced switching from a metallic to an insulating state is observed in phase-separated states of $({\mathrm{La}}_{1-y}{\mathrm{Pr}}_y{)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ ($y=0.7$) and ${\mathrm{Nd}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{1-z}{\mathrm{Cr}}_z{\mathrm{O}}_3$ ($z=0.03$) crystals. The application of magnetic fields to this current-induced insulating state causes a pronounced low-field negative magnetoresistance effect [$\rho (H)/\rho (0)={10}^{-3}$ at $H=1\mathrm{kOe}$]. The application of a constant voltage also causes the breakdown of the Ohmic relation above a threshold voltage. At voltages higher than this threshold value, oscillations in currents are observed. This oscillation is well reproduced by a simple model of local switching of a percolative conduction path.

Figure 1

(a) A $V-I$ curve of LPCMO-0.7 at 30 K. (b) $V-I$ curves of Cr3%-NCMO at various temperatures. It takes about 7 min to obtain one $V-I$ curve. Insets of (a) and (b) show temperature dependence of the resistivity of LPCMO-0.7 and Cr3%-NCMO measured at $I=10\mu \mathrm{A}$.

Figure 2

(a) Magnetic field dependence of $V-I$ curves of LPCMO-0.7 at 30 K. The inset shows field dependence of $I_{\mathrm{th}}^{\mathrm{up}}$ and $I_{\mathrm{th}}^{\mathrm{down}}$. (b) Magnetic field dependence of resistivity at various currents. To make the sample insulating at zero field, a current of 15 mA was applied prior to each measurement. The dotted line indicates the magnetoresistance with $I=10\mu \mathrm{A}$ at 130 K. Note that the horizontal scales for the dotted line and solid lines are different.

Figure 3

Temporal variation of the resistivity of LPCMO-0.7 at 30 K after the application of various values of currents. The inset shows the relation between total energy put into the sample volume between voltage terminals ($W_{\mathrm{J}}$) and time needed to complete the transition ($t_{\mathrm{th}}$) (see text).

Figure 4

(a) An $I-V$ curve of LPCMO-0.7 taken at 30 K. The experimental configuration for this measurement is schematically drawn in the upper inset. The lower inset shows a differential MO image between $V_{\mathrm{out}}=+6\mathrm{V}$ and $-6\mathrm{V}$. White broken lines are drawn to show the location of the sample edges. (b) The time dependence of the current at $V_{\mathrm{out}}=4$ and 6 V (solid lines). The dash-dotted line is the measured voltage that is actually applied to the sample ($V_{\mathrm{s}}$) for $V_{\mathrm{out}}=6\mathrm{V}$. The dotted line represents the result of simulation described in the text.