Correlated electron materials exhibit complex phase diagrams with a variety of competing phases of very different electronic and magnetic properties. Employing field-effect principles to electrostatically modulate the charge density of correlated electron systems without introducing disorder provides an opportunity to explore the effect of changing the carrier concentration on the properties of these systems. We have developed a platform for electrostatic charging of correlated electron materials using a thinned SrTiO3 single crystal, and employing this platform, we were able to achieve charge modulation approaching 1014 carriers/cm2 at low temperatures. This platform was used to study the response of ultrathin La0.8Ca0.2MnO3 films to gate electric fields and magnetic fields at low temperatures. An applied gate voltage produces a large ambipolar decrease in resistance at low temperature. The nature and magnitude of the gate effect suggest the development of a pseudogap in the density of states at the lowest temperatures. We also find that we are able to alter the magnetic moment of the La0.8Ca0.2MnO 3 films with the application of a gate electric field. In addition, the films exhibit a hierarchical and glass-like response to external perturbations by either electric or magnetic fields. We have developed a consistent interpretation of the gate effect, magnetoresistance data, and magnetization measurements at low temperatures within the framework of a mixed phase scenario where phase boundaries move through a hierarchical pinning landscape.
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