Field effect in manganite ultrathin films:: Magnetotransport and localization mechanisms

In this paper, we report on field-effect experiments in La0.7Sr0.3MnO3 side-gate channels patterned on ultrathin epitaxial films having thickness ranging from 12 to 4 unit cells. Transport mechanisms and competition between phases, under the effect of electric and magnetic fields, as well as of other perturbations such as disorder and proximity to the interface with substrate are explored. We observe, in a 7 unit cells thick sample, a shift of the metal-insulator transition temperature as high as 43 K and a resistivity modulation up to 250% at low temperatures. In striking contrast, the 6-4 unit cells thick samples result to be insulating and almost insensitive to field-effect modulation. Such a finding indicates that for films thinner than 7 unit cells, a strong localization mechanism develops, which cannot be healed by band refilling. On the other hand, our results are compatible with a Mn e(g) orbital rearrangement driven by the broken translational symmetry at the surface and/or interface, which suppresses the double-exchange mechanism and localizes the carriers.