The coupling of magnetic and electric order parameters, i.e., the magnetoelectric effect, has been widely studied for its intriguing physical principles and potentially broad industrial applications. The important interactions between ferroic orderings -- ferromagnetism, ferroelectricity and ferroelasticity -- will enable the manipulation of one order through the other in miniaturized materials, and in so doing stimulate emerging technologies such as spintronics, magnetic sensors, quantum electromagnets and information storage. By growing ferromagnetic-ferroelectric heterostructures that are able to magneto-electrically couple via interface elastic strain, the various challenges associated with the lack of single-phase multiferroic materials can be overcome and the magnetoelectric (ME) coupling effect can be substantially enhanced. Compared with magnetic field-controlled electric phenomena (i.e., the direct magnetoelectric coupling effect), the converse magnetoelectric effect (CME), whereby an electric field manipulates magnetization, is more exciting due to easier implementation and handling of electric fields or voltages. CME also affords the possibility of fabricating highly-efficient electric-write/magnetic-read memories. This study involved two avenues of inquiry: (a) exploring the strain-mediated electric field manipulation of magnetization in ferroelectric-ferromagnetic heterostructures, and (b) investigating coupling and switching behaviors at the nanoscale. Accordingly, a series of magnetoelectric heterostructures were prepared and characterized, and their electric field tunability of magnetic properties was explored by various techniques and custom-designed experiments. Firstly, the relevant properties of the individual components in the heterostructures were systematically investigated, including the piezoelectricity and ferroelectric/ferroelastic phase transformations of the ferroelectric substrates, lead magnesium niobate-lead titanate, or Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT). This investigation revealed significant information on the structure-property relationships in crystals oriented at <110>, as well as shed light on the effect of ferroelectric phase transformation on magnetoelectric coupling. This investigation of electric field controlled strain, in contrast to many prior studies, enables a more rational and detailed understanding of the magnetoelectric effect in complex ferroelectric-ferromagnetic heterostructures. The magnetoelectric thin film heterostructures were fabricated by depositing ferromagnetic iron-gallium (Fe-Ga) or cobalt ferrite (CoFe2o4 or CFO) films on top of differently-oriented ferroelectric PMN-PT substrates. Through significant electric field-induced strain in the piezoelectric substrate, the magnetic remanence and coercive field, as well as the magnetization direction of the ferromagnetic overlayer, can be substantially tuned. These goals were achieved by the interfacial strain modification of the magnetic anisotropy energy profile. The observation and analysis of the electric field tunability of magnetization and the establishment of novel controlling schemes provide valuable directions for both theoretical development and future application endeavors. / Master of Science