Nonmonotonic crossover in electronic phase separated manganite superlattices driven by the superlattice period

Studying manganite superlattices ${[{(\mathrm{LCMO})}_{2n}/{(\mathrm{PCMO})}_{n}]}_{t}$ made of ${\mathrm{La}}_{0.625}{\mathrm{Ca}}_{0.375}\mathrm{Mn}{\mathrm{O}}_{3}$ (LCMO) and ${\mathrm{Pr}}_{0.625}{\mathrm{Ca}}_{0.375}\mathrm{Mn}{\mathrm{O}}_{3}$ (PCMO), we found an unexpected behavior varying the period $n$. At small $n$, the ensemble is a three-dimensional ferromagnetic metal due to interfacial charge transfer. At large $n$, the LCMO layers dominate transport. However, rather than a smooth interpolation between these limits a sharp transport and magnetic anomaly is found at an intermediate critical PCMO thickness ${n}^{*}$. Magnetic force microscopy reveals that the phase-separation length scale also maximizes at ${n}^{*}$ where, unexpectedly, it becomes comparable to that of the ${({\mathrm{La}}_{1--y}{\mathrm{Pr}}_{y})}_{0.625}{\mathrm{Ca}}_{0.375}\mathrm{Mn}{\mathrm{O}}_{3}$ (LPCMO) alloy. We conjecture the phenomenon originates in a disorder-related length scale: Large charge-ordered clusters as in LPCMO can only nucleate when Pr-rich regions reach a critical size related to ${n}^{*}$.