| Multiferroic materials, which show both the coexistence of at least two ferroic (anti-/ferri-/ferromagnetic, anti-/ferroelectric or ferroelastic) orders and the existence of coupling interaction between the different order parameters, have become one of the hot topics because of both their intrinsic scientific interest and possible technological applications in multifunctional devices.Multiferroics are classified into two types, single-phase multiferroics and composite multiferroics, according to their composition. Single-phase multiferroics have abundant physical contents, but their Curie temperature is too low and their magnetoelectric effect is too weak to use in practical applications. Giant magnetoelectric effect is obtained in composite multiferroics with more design freedom.First, BaTiO3(BTO)/FeBSi composite multiferroic films consisting of ferromagnetic/magnetostrictive FeBSi film and extensively investigated ferroelectric/piezoelectric BTO film are studied. High-permeability FeBSi alloy has large piezomagnetic coefficient, and lead-free BTO film is environment friendly. There are no reports from other research groups on BTO/FeBSi composite films until now. Then the Fe:BTO films, which were prepared by implanting Fe ions into BTO films, are investigated. The investigation on Fe:BTO films builds a foundation for the next research. At last Fe:BTO/FeBSi composite films were prepared, and the magnetoelectric effects of Fe:BTO/FeBSi and BTO/FeBSi composite films are comparatively studied.1. BTO film, FeBSi film and BTO/FeBSi composite films were prepared by pulsed laser deposition (PLD) and ion beam sputtering technologies. The room temperature Raman spectrum shows that the BTO film is tetragonal and the room temperature X-ray diffraction (XRD) pattern shows that the FeBSi film is amorphous. No additional peaks appeared except for the peaks from the substrate and the BTO film in the XRD pattern of the BTO/FeBSi composite films. The BTO and FeBSi film did not react chemically and no impurity phases were detected in the BTO/FeBSi composite films within the detection limits of the XRD measurements. A cross-sectional scanning electron microscope image showing clear interface between the top FeBSi film and the bottom BTO film demonstrates that the BTO/FeBSi composite films have a (2-2) connectivity. The BTO/FeBSi composite films simultaneously showed both ferromagnetic and ferroelectric order at room temperature. The stress induced magnetoelectric coupling was also observed in the BTO/FeBSi composite films. The BTO/FeBSi composite films are a promising multiferroic material, and they have potential applications in multifunctional devices.2. A series of Fe:BTO films were prepared by PLD and metal vapor vacuum arc (MEVVA) technologies. BTO films about 590 nm in thickness were prepared by PLD, and then implanted with Fe ions at different doses by MEVVA technology. X-ray photoelectron spectrum shows that the implanted Fe ions are in the Fe3+ state in the Fe:BTO films. The SRIM (the Stopping and Range of Ions in Matter) result shows that all the Fe ions were implanted into the BTO film and none reached the Si substrate in the Fe:BTO film. Room temperature ferromagnetic 3×1016 Fe:BTO and 5×1016 Fe:BTO films, in which the doses of implanted Fe ions were 3×1016 cm-2 and 5×1016 cm-2 respectively, kept ferroelectric at room temperature, and they exhibited cluster-glass magnetic behavior at low temperature. The complex exchange interactions between magnetic Fe3+ ions, which randomly distributed to a certain extent in the BTO film, are responsible for the complex magnetic behavior in 3×1016 Fe:BTO and 5×1016 Fe:BTO film. More importantly, the exchange interactions between magnetic Fe3+ ions changed as the positions of magnetic Fe3+ ions changed during the ferroelectric/tetragonal-paraelectric/cubic phase transition in 3×1016 Fe:BTO and 5×1016 Fe:BTO film, so distinct change in the magnetization was observed around the ferroelectric Curie temperature, which suggests that magnetoelectric coupling exists in 3×1016 Fe:BTO and 5×1016 Fe:BTO film. The larger the dose of implanted Fe ions, the stronger the magnetoelectric coupling in the Fe:BTO film is. When the dose of implanted Fe ions is larger than 1×1016 cm-2 and smaller than 8×1016 cm-2, the Fe:BTO film is a multiferroic material, and it has potential applications in multifunctional devices such as multiple-state memory and magnetic field sensor. 3. BTO/FeBSi,3×1016 Fe:BTO/FeBSi and 5×1016 Fe:BTO/FeBSi composite films were prepared by PLD and multifunctional ion implantation and ion beam sputtering system. The magnetoelectric coupling in 3×1016 Fe:BTO/FeBSi and 5×1016 Fe:BTO/FeBSi composite films is not stronger than that in BTO/FeBSi composite films. The implanted Fe ions randomly distributed to a certain extent in the BTO film, and they were not always incorporated into the lattice of BTO film. The implantation of Fe ions reduced the structural quality of the tetragonal BTO film. Ion implantation, which is a non-equilibrium process, is not suitable to modify the tetragonality of the BTO film. Other equilibrium processes, for example PLD, can be used to prepare Ba(Ti,Fe)O3 film using Ba(Ti,Fe)O3 target. Then Ba(Ti,Fe)O3/FeBSi composite films can be prepared by sputtering FeBSi film on the top of Ba(Ti,Fe)O3 film. Perhaps the magnetoelectric coupling in Ba(Ti,Fe)O3/FeBSi composite films is stronger than that in BTO/FeBSi composite films.
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