In ceramic samples of the material, (BiFe1-xCoxO3)0.4-(Bi1/2K1/2TiO3)0.6 (BFCBKT), a new kind of nano-sized region, so called multiferroic clusters (MFC) were found at room-temperature. These exhibit the largest reported magnetoelectric (ME) coupling coefficient, α ≈ 1.0 x 10-5 s/m (corresponding to a Voltage coefficient dE/dH ≈ 1300 V/(cm Oe)) for a single phase multiferroic so far, to the best of the author’s knowledge. Furthermore, they are ferroelectric and ferrimagnetic at the same time, which has not yet been observed directly in one material. Using a broad range of experimental techniques, formation of the MFC has been understood in terms of the nonergodic relaxor properties and ferrimagnetism in inherent Bi(Fe,Co)O3 rich regions. Furthermore, the presence of magnetic nanoregions (MNR) was confirmed and their formation and role in ME coupling are discussed. Solid solutions of the system (PbZr0.52Ti0.48O3)1-x-(PbFe2/3W1/3O3)x (PZTPFW) were prepared which, however, did not show ME effects. Nevertheless, analogies to BFC-BKT concerning occurrence of characteristic magnetic phases were established. Since piezoresponse force microscopy (PFM) played an important role in this study, the method was subject to detailed investigations concerning e.g. the signal stability. Among others, it was found that it is highly beneficial to use diamond coated tips in PFM. In addition, a useful and easily applicable technique for distinguishable marking of micro-sized areas has been developed. It is expected that especially the findings on BFC-BKT will have wider implications, as they enable an entirely new perspective for such relaxortype ferrimagnetic multiferroics, which were barely considered in research so far. The relaxor properties are expected to play an important role in the strong ME coupling. Since the MFC are well-separated and offer large ME coupling, they are suitable to serve as bits in an electrically controlled magnetic nanodot storage device (MERAM). Various possibilities, the for realization of such a device are discussed.