Secondary insulator-to-metal transition and magnetic properties in epitaxial La0.92Sr0.08MnO3 films

${\mathrm{La}}_0.92{\mathrm{Sr}}_0.08{\mathrm{MnO}}₃$ films with the thickness ($t$) ranging from 8 to 157 nm, grown on (001)-oriented ${\mathrm{SrTiO}}₃$ substrates by rf magnetron sputtering, undergo a secondary metal-insulator transition (SMIT) below a critical temperature when $t\ensuremath{\ge}63\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. The transition temperature gradually increases from 25 K at 63 nm to 108 K at 157 nm with increasing film thickness. Raman spectra demonstrate that the occurrence of the SMIT is attributed to the reduction of the Jahn-Teller distortion. Further investigations indicate that the SMIT can take place in the ferromagnetic $(63\phantom{\rule{0.16em}{0ex}}\mathrm{nm}\ensuremath{\le}t<98\phantom{\rule{0.16em}{0ex}}\mathrm{nm})$ and $A$-type antiferromagnetic ($t\ensuremath{\ge}98\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$) spin structures, which is ascribed to the generation of two-dimensional conduction bands in the ${\mathrm{MnO}}₂$ planes through the double-exchange interaction. This work suggests that the SMIT provides a pathway to study multiple metal-insulator transitions and demonstrates the potential applications in spintronic devices.