Electrostatic modulation of interface conduction between semiconductors and insulating oxides is the foundation of semiconductor technology. This field effect concept can be applied on complex oxides, such as high temperature superconductors and colossal magnetoresistive manganites, in order to create new electronic and magnetic phases. Competition and coexistence of multiple nanoscale phases make them exciting to study around phase transitions. This study on hole doped La1-xSrxMnO3 systems has a two-fold purpose. One is the demonstration of the field effect on La1-xSrxMnO3 (x = 0.125, 0.2, 0.3, 0.5) thin films. It is an important step towards electrostatic control of material properties; however, a challenging task because of their charge carrier densities of 0.01-1 hole/unit cell, a few orders of magnitude larger than in doped semiconductors. Control by linear dielectrics needs huge, constantly applied bias. Energy efficient tuning with low voltages requires highly polar ferroelectric. Pb(Zr0.2Ti0.8)O3 was chosen, whose remanence provides 0.5 charge carrier/unit cell on the manganite/ferroelectric interface. La1-xSrxMnO3/Pb(Zr0.2Ti0.8)O3 heterostructures were synthesized by pulsed laser epitaxy and remarkable conduction modifications were observed in the La1-xSrxMnO3. This can be a strong foundation of a new tool to research electronic oxides. The second purpose of this work is to utilize the phase separation in manganites. There has been extensive research on multiferroic materials, in which dielectric and magnetic responses are controlled by magnetic and electric field, respectively. In order to demonstrate magnetically tuned capacitance, insulating La7/8Sr1/8MnO3 was studied. Drastic capacitance change in magnetic field was shown through a phase transitions and explained in the framework of electronic phase separation. It makes this material eligible for high frequency magnetoelectric applications. Modulating charge carriers, mobility and magnetism in magnetic oxides, superconductors and superlattices has a great impact on the emerging field of oxide electronics. These compounds overcome the scaling limitations of conventional semiconductors; using low operation voltage oxide ferroelectrics lowers energy consumption. This thesis shows that changing fundamental physical properties of complex oxides on the atomic scale is possible by ferroelectric field effect. This technique is proposed as a tool to study thin films, artificially stacked structures and to induce and optimize novel phases and phenomena.