Imaging of Percolative Conduction Paths and Their Breakdown in Phase-Separated $({\mathrm{L}\mathrm{a}}_{1-y}{\mathrm{P}\mathrm{r}}_y{)}_{0.7}{\mathrm{C}\mathrm{a}}_{0.3}{\mathrm{M}\mathrm{n}\mathrm{O}}_3$ with $y=0.7$

Local magnetization and current distribution in $({\mathrm{L}\mathrm{a}}_{1-y}{\mathrm{P}\mathrm{r}}_y{)}_{0.7}{\mathrm{C}\mathrm{a}}_{0.3}{\mathrm{M}\mathrm{n}\mathrm{O}}_3$ ($y=0.7$) crystals are studied by a magneto-optical (MO) imaging technique. MO images below 120 K visualize inhomogeneous magnetization and conduction paths that manifest the percolative conduction originated from the mesoscopic phase separation into ferromagnetic metals and antiferromagnetic insulators. Application of large amounts of current switches the current distribution from inhomogeneous to homogeneous concomitantly with a steep increase in resistivity. These phenomena are discussed in view of current induced collapse of the phase separation through a local heating.

Figure 1

(a) A polarizing microscope image of the observed surface. Areas with different color correspond to different crystallographic domains. (b) Temperature dependence of resistivity and average magnetization in heating and cooling processes. MO images without current at (c) 34 K and (d) 133 K. Dashed lines represent edges of the sample.

Figure 2

(a) Differential MO image for $I=\pm 15\mathrm{m}\mathrm{A}$ at $T=33\mathrm{K}$. Bright and dark parts correspond to the regions with positive and negative magnetic field caused by the current. The direction of the average current is shown by an arrow. (b) Current distribution calculated from magnetic image of (a). The light gray (yellow) region corresponds to the area where amplitude of the current is large. (c) Differential MO image for $I=\pm 15\mathrm{m}\mathrm{A}$ at $T=146\mathrm{K}$. (d) Differential MO image for $I=\pm 25\mathrm{m}\mathrm{A}$ at $T=33\mathrm{K}$.

Figure 3

(a) $V$-$I$ curves at various temperatures between 33 and 146 K. The inset shows the $V$-$I$ curve at 33 K in logarithmic scale. (b) Temperature dependence of the threshold current $I_{\mathrm{t}\mathrm{h}}$ in current up (closed circles) and down (open circles) procedures defined as marked by closed and open arrows in (a).

Figure 4

Differential MO images at 33 K and $I=0\mathrm{A}$ between before and after application of temporal current $I_{\mathrm{t}\mathrm{e}\mathrm{m}\mathrm{p}}$ of (a) 25 mA and (b) 15 mA.