Tunneling magnetoresistance in phase-separated manganite nanobridges

The manganite $\left(\text{La},\text{Pr},\text{Ca}\right){\text{MnO}}_3$ is well known for its micrometer-scale phase separation into coexisting ferromagnetic metallic (FMM) and insulating regions. Fabrication of bridges with widths smaller than the phase-separation length scale has allowed us to probe the magnetic properties of individual phase-separated regions. At the onset of phase separation, a magnetic field induced insulator-to-metal transition among a discrete number of domains within the narrow bridges gives rise to abrupt, low-field colossal magnetoresistance steps at well-defined switching fields. At lower temperatures when the FMM phase becomes energetically favorable, the insulating regions shrink to form thin insulating strips separating adjacent FMM regions with different coercive fields. Tunneling magnetoresistance is observed across the naturally occurring intrinsic insulating strips (tunnel barriers) spanning the width of the bridges. The presence of such intrinsic tunnel barriers introduces an alternative approach to fabricating novel nanoscale magnetic tunnel junctions.

Figure 1

Temperature-dependent resistivity of an unpatterned LPCMO thin film grown on ${\text{NdGaO}}_3$ (011) substrate. Vertical lines and labels identify the various phase-transition temperatures and boundaries discussed in the following sections. Beginning from the higher temperatures, $T_{\text{CO}}=240\text{K}$ marks the approximate charge-ordering temperature (values vary in the literature), $T_C$ marks the Curie temperature, $T_{\text{IM}}^0=105\text{K}$ marks the onset of large-scale phase separation and the maximum of the resistivity peak for unpatterned thin films, $T_{\text{IM}}=64\text{K}$ marks the onset of a stable ferromagnetic state (a metastable insulating state) and the maximum of the resistivity peak for the patterned submicron bridges, and lastly $T_G=48\text{K}$ marks the fully ferromagnetic metallic state of the sample.

Figure 2

(Color online) Temperature-dependent resistance for both cooling and warming (indicated by arrows) of a $2.5\text{−}\mu \text{m}$-wide bridge (blue, lower curve) and a $0.6\text{−}\mu \text{m}$-wide bridge (green, upper curve) patterned from 30-nm-thick LPCMO thin films reveal the evolution of pronounced steplike changes and the insulator-to-metal transition temperature (see text) as the bridge width becomes comparable to $\left(2.5\mu \text{m}\right)$ and then smaller than $\left(0.6\mu \text{m}\right)$ the micron-size regions of coexisting COI and FMM phases. A scanning electron micrograph of a $0.2\text{−}\mu \text{m}$-wide bridge [with the protective polymer and metal layers still present (Ref. 5)] taken shortly after the FIB process is shown in the inset together with the orientations of the applied fields: $H_x$, $H_y$, or $H_z$.

Figure 3

(Color online) For temperatures, $T>T_{\text{IM}}$ for the $0.6\text{−}\mu \text{m}$-wide bridge, repeated magnetic field sweeps at the indicated temperatures (73, 70, 67, and 64 K) reveal reproducible hysteretic temperature-dependent colossal resistance jumps that are more sensitive to in-plane $\left(H_x,H_y\right)$ rather than perpendicular ($H_z$, black lines) fields. The critical switching field from high to low resistance reduces with decreasing temperature.

Figure 4

(Color online) Unpatterned thin-film $R\left(H_y\right)$ data in the range $T_C>T=120\text{K}>T_{\text{IM}}$. The low- to high-field transitions shown expanded in the inset are smooth when compared to the bridge data in Fig. 3. A distribution of the first-order transitions shown in Fig. 3 over many domains may account for this.

Figure 5

(Color online) For the $0.6\text{−}\mu \text{m}$-wide bridge, for temperatures above $T_G$ and below $T_{\text{IM}}$ magnetic field induced phase transitions terminate in a predominantly low-resistance FMM state that exhibits TMR. (a) A field-induced insulator-metal transition at 57 K of a zero-field-cooled sample showing a pronounced resistance drop at $H_z=6\text{kOe}$ and subsequent entrance into a low-resistance phase that is stable with respect to repeated field sweeps between $\pm 20\text{kOe}$. The inset schematically depicts the coalescence of FMM (white) regions at the expense of insulating (black) regions with applied fields or lowering temperatures, with a rectangular overlay depicting the $0.6\mu \text{m}$ bridge. (b) Magnification of the TMR region, as described in the text.

Figure 6

(Color online) Waterfall plot of repeated magnetic field sweeps in the temperature range, $T_{\text{IM}}>T>T_G$, showing the temperature-dependent evolution of TMR peaks and their disappearance below $T_G=48\text{K}$.

Figure 7

A $T\text{−}{H}_y$ phase diagram summarizing the low- and high-resistance switching fields, similar to bulk (Ref. 13) and thin-film (Ref. 19) $T\text{−}H$ phase diagrams. The bold boundary (right) indicates the switching field from the high- to low-$R$ state for increasing field while the thinner boundary (left) is for an increase back to a high-$R$ state for decreasing field. At low enough temperature, the FMM phase is energetically favorable while the insulating phase is metastable in an applied magnetic field, and thus the sample remains in the low-$R$ state for all increasing and decreasing field sweeps following the initial field increase (see Fig. 5).