Correlated electron oxides,  like colossal magnetoresistive manganites and high  temperature superconductors, have diverse  electronic  and  magnetic  phase  diagrams.  Electrostatic  modification  of  the  carrier  density  in  correlated electronic systems can be used to effectively tune and switch between their electronic and magnetic properties without inducing  further  chemical  disorder.  Therefore,  in  order  to  test  the  fascinating  concept  of field  effect modulation  of carrier population, we have fabricated heterostructures composed of ferroelectric Pb(Zr_0.2Ti_0.8)O₃ (PZT) and magnetic La_1-xSr_xMnO₃ (LSMO) thin films by pulsed laser deposition on (001) SrTiO₃. These doped perovskite manganites have extremely high carrier density. That  is why  the high polarization of 80 μC/cm²  in our PZT  thin  films  is  found  to be of great  importance  to effectively alter  the conduction  in  the vicinity of  the  interface between  the  two  layers. We have observed a clear metal-insulator-transition by changing the direction of PZT polarization, resulting in a huge resistance modulation up  to  three orders of magnitude.  In  this  talk,  I will be presenting a study on how effectively  the  transport and magnetic properties can be modulated by ferroelectric polarization affecting the LSMO films.