Abstract:  Room temperature multiferroic BiFeO₃, material has recently become the focus of research for its potential applications in multifunctional devices. It is meaningful to systematically investigate the thickness-dependent properties of BiFeO₃, films since the polarization of ferroclectrics have been reported to be size-dependent and the special cycloidal spin structure with a period of 62 nm could bring about additional complexity to BiFeO₃.BiFeO₃ films with various thicknesses arc prepared on Si substrates by metal organic decomposition method.The microstructure, ferroclectricity and magnetism of the films are investigated by X-ray diffraction, the piczoresponsc force microscope, and superconducting quantum interference device，respectively. It is found that the polycrystalline
films exhibit ferroclectric properties even with thickness as low as 30 nm.The grain size increases rapidly first with the film thickness increasing from 30 nm to 120 nm, and then increases slowly. Similarly, the ferroclectric domain size also increases with increasing film thickness，which implies the probable enhancement of the ferroclectric polarization. The single-domain state is predominant for the grains with size about 47一104nm, while multi-domain state can be found for larger grains. Moreover, the ferroclectric domain can be switched by voltages applied through the probe of piczoresponse force microscope.The magnetism of BiFeO₃ also shows thickness dependence.The saturation magnetization Ms decreases abruptly with the film thickness increasing up to 120 nm, and then it
decreases much slower, which could be attributed to the variation of surfacrto-volume ratio.