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 insulatormetal 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 La0.7Sr0.3CoO3 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 thermodynamichistory-dependent properties signify a nonergodic feature associated with the microscopic phase separation. By further comparing the transport properties among La0.7AE0.3CoO3 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.