Intrinsic Tunneling in Phase Separated Manganites

We present evidence of direct electron tunneling across intrinsic insulating regions in submicrometer wide bridges of the phase-separated ferromagnet $(\mathrm{La},\mathrm{Pr},\mathrm{Ca}){\mathrm{MnO}}_3$. Upon cooling below the Curie temperature, a predominantly ferromagnetic supercooled state persists where tunneling across the intrinsic tunnel barriers (ITBs) results in metastable, temperature-independent, high-resistance plateaus over a large range of temperatures. Upon application of a magnetic field, our data reveal that the ITBs are extinguished resulting in sharp, colossal, low-field resistance drops. Our results compare well to theoretical predictions of magnetic domain walls coinciding with the intrinsic insulating phase.

Figure 1

$R$ vs $T$ upon zero-field cooling and warming (ZF-C/W) of bridges patterned from the same LPCMO film are labeled for the unpatterned film (gray), the $2.5\mu \mathrm{m}\times 8\mu \mathrm{m}$ bridge (black) and the $0.6\mu \mathrm{m}\times 8\mu \mathrm{m}$ bridge (green), respectively. A FC curve for the $0.6\mu \mathrm{m}\times 8\mu \mathrm{m}$ bridge (blue) is also shown. Insulator-metal transitions for the two bridges are indicated by the vertical color-coded dashed lines. Lower inset: schematic of the four-terminal configuration along with the applied field directions. Upper inset: scanning electron micrograph of the $0.6\mu \mathrm{m}$ wide bridge.

Figure 2

$I\mathrm{\text{−}}V$ characteristics for the $0.6\mu \mathrm{m}$ wide bridge at the three indicated temperatures all measured during one cooling cycle. Top inset: $dI/dV\mathrm{\text{−}}V$ curve at 15 K with a fit (red curve) to the Simmons’ model. Bottom inset: $R(T)$ curves obtained at the indicated currents.

Figure 3

(a) ZFC and FC resistance transitions are labeled by the field directions defined with respect to bridge orientation in Fig. 1. Inset: $R$ measured at 5 K when the bridge is cooled in separate runs at the indicated fields. (b) $I\mathrm{\text{−}}V$ characteristics of the $0.6\mu \mathrm{m}$ wide bridge zero-field-cooled to 15 K, measured at the indicated values of $H_z$ and (c) normalized to the voltage, $V_{\max }$, measured at maximum applied current.

Figure 4

Field sweeps ($H$) between $\pm 20\mathrm{kOe}$ at temperatures (a) $T=57\mathrm{K}$ (below $T_{\mathrm{IM}}$) and (b) $T=70\mathrm{K}$ (above $T_{\mathrm{IM}}$) for both increasing (bold curve) and decreasing (thin curve) fields: $H_y$ (black) and $H_z$ (blue). Inset: schematic showing the coalescence of FMM (white) regions at the expense of insulating (black) regions with applied fields or lowering temperatures. The rectangular overlay depicts the $0.6\mu \mathrm{m}$ bridge.