The metal-insulator (MI) transition in epitaxial thin films of ${\text{La}}_{0.75}{\text{Ca}}_{0.25}{\text{MnO}}_3$ (LCMO) is accompanied by the appearance of an intrinsic electrical nonlinearity. The latter, probed by electrical third harmonic voltage, $U_{3\omega }$, or resistance, $R_{3\omega }=d{U}_{3\omega }/dJ$, is drastically enhanced in the vicinity of the MI transition, $T_{\text{MI}}=267\text{K}$. Applied magnetic field, $B=5\text{T}$, suppresses the nonlinearity, resulting in a huge “nonlinear” ${\text{CMR}}_{3\omega }\left(T_{\text{MI}}\right)\sim {10}^5\%$. $R_{3\omega }$ shows a peculiar low-frequency $\left(\omega \le 1\text{kHz}\right)$ dependence, $R_{3\omega }\sim {\left(\omega \text{−}{\omega }_0\right)}^n$, with exponent, $n$, changing across the MI transition from $n\sim 1,5–2$ for $T\ge T_{\text{MI}}$ to $n=1$ $\left(T<T_{\text{MI}}\right)$. The observed electrical nonlinearity in LCMO reflects the behavior of correlated polarons, the number of which dramatically enhances in the vicinity of $T_{\text{MI}}$. We argued that correlated polarons, considered as electric-elastic quadrupoles, provide a nonlinear (quadratic) coupling to the electric field, yielding a third harmonic electric nonlinearity in LCMO. The reference film of ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_3$ (LSMO), a prototypic double exchange system with second-order phase transition, is characterized as a linear metallic material in the whole range of temperatures $\left(T=10–400\text{K}\right)$, magnetic fields $\left(B=0–5\text{T}\right)$, and frequencies $\left(\omega =1–1000\text{Hz}\right)$.

• Received 15 August 2008

DOI:https://doi.org/10.1103/PhysRevB.79.134413