Effect of strain and electric field on the electronic soft matter in manganite thin films

We have studied the effect of substrate-induced strain on the properties of thin films of the hole-doped manganite ${\left({\mathrm{La}}_{1-y}{\mathrm{Pr}}_y\right)}_{0.67}{\mathrm{Ca}}_{0.33}\mathrm{Mn}{\mathrm{O}}_3$ ($y=0.4$, 0.5, and 0.6) grown on $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3$ (NGO) substrates, in order to distinguish between the roles played by long-range strain interactions and quenched atomic disorder in forming the micrometer-scale phase separated state. We show that a fluid phase separated (FPS) state is formed at intermediate temperatures similar to the strain-liquid state in bulk compounds, which can be converted to a metallic state by applying an external electric field. In contrast to bulk compounds, at low temperatures a strain stabilized ferromagnetic metallic (FMM) state is formed in the $y=0.4$ and 0.5 samples. However, in the $y=0.6$ sample a static phase separated (SPS) state is formed similar to the strain-glass phase in bulk compounds. Our results suggest that the substrate-induced strain is a function of temperature. Hence, we show that the temperature induced variation of the long-range strain interactions plays a dominant role in determining the properties of thin films of phase-separated manganites.

Figure 1

(a) Resistivity vs temperature curves for thin films of ${\left({\mathrm{La}}_{1-y}{\mathrm{Pr}}_y\right)}_{0.67}{\mathrm{Ca}}_{0.33}\mathrm{Mn}{\mathrm{O}}_3$ ($y=0.4$, 05, and 0.6) on NGO substrates. The open black squares show the resistivity behavior of an LPCMO $(y=0.5)$ film on STO. Cooling and warming directions are indicated by arrows. The dotted lines mark the range of temperatures for the $I\text{−}V$ curves shown in Fig. 2. The upper inset shows the variation of the transition temperatures and low temperature resistivity $\left({\rho }_0\right)$ with $y$. The lower inset shows the setup for measuring the two probe resistance using a constant voltage source. (b), (c), and (d) $R$ vs $H$ curves for the $y=0.6$ sample in the cooling cycle. (e) The $T\text{−}H$ phase diagram for the $y=0.6$ thin film in the cooling cycle.

Figure 2

(a) $I\text{−}V$ curves of the ${\left({\mathrm{La}}_{1-y}{\mathrm{Pr}}_y\right)}_{0.67}{\mathrm{Ca}}_{0.33}\mathrm{Mn}{\mathrm{O}}_3$ $(y=0.6)$ thin film in the cooling cycle with the voltage being ramped up. The dashed curve is the $I\text{−}V$ curve at $59\mathrm{K}$ with the voltage being ramped down. (b) The $T\text{−}V$ phase diagram for the $y=0.6$ thin film in the cooling cycle. (c) Schematic representation of the phase coexistence in the $T\text{−}V$ plane.

Figure 3

$I\text{−}V$ curves of the ${\left({\mathrm{La}}_{1-y}{\mathrm{Pr}}_y\right)}_{0.67}{\mathrm{Ca}}_{0.33}\mathrm{Mn}{\mathrm{O}}_3$ $(y=0.6)$ thin film in the warming cycle with the voltage being ramped up. The $I\text{−}V$ curve at $56.5\mathrm{K}$ for the cooling cycle is shown for comparison.