Magnetic field control of insulator-metal crossover in cobaltite films via thermally activated percolation

Competition between the double-exchange hopping and Coulomb repulsion underlies fertile phenomena in mixed-valent manganites, such as colossal magnetoresistance associated with a magnetic-field-driven insulator-metal transition (IMT). In contrast, an analogous double-exchange system, perovskite cobaltites, however, shows much smaller magnetoresistance, and the insulator-to-metal phase evolution with field stimulus remains elusive. Here, we unveil a submicrometer-scale phase separation and magnetic-field-controlled crossoverlike IMT in a tensile-strained ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Co}{\mathrm{O}}_{3}$ film. Our transport, magnetization, and magnetic-force-microscopy measurements reveal that, although the IMT is barely induced under an isothermal field sweep, it emerges under field cooling through a thermally activated ferromagnetic domain percolation. Such thermodynamic-history-dependent properties signify a nonergodic feature associated with the microscopic phase separation. By further comparing the transport properties among ${\mathrm{La}}_{0.7}\mathrm{A}{\mathrm{E}}_{0.3}\mathrm{Co}{\mathrm{O}}_{3}$ films with AE from Ca, Sr, to Ba, we reveal that the nonergodic behavior prevails in these films and that the mixed spin states are a key factor to enhance the energy barrier for domain-wall motions during domain percolation. Such a spin-state degree of freedom is absent in manganites, probably resulting in the large difference of the phase evolution kinetics in the magnetic-field-induced IMT between cobaltites and manganites.