Preparation And Characterization Of Lead-free BCT/CFO Magnetoelectric Composite Thin Films

Magnetoelectric composite thin films can not only display ferromagnetic and ferroelectric ordering, but also a magnetoelectric coupling effect, and thus have widely potential applications in memories, ME devices, etc. Now, lead-based materials were used as ferroelectric phases in the magnetoelectric composite films. A serious environmental pollution of lead-based materials restricts the practical application of magnetoelectric composite films. Ba0.9Ca0.1TiO3 (BCT) ferroelectric films, CoFe2O4 (CFO) ferromagnetic films and 2-2 type layered BCT/CFO magnetoelectric composite films were prepared by the sol-gel method.The inpact of preparation process on the ferroelectric, ferromagnetic and magnetoelectric coupling properties of composite thin films was also investigated..Firstly, BCT powders were prepared by sol-gel method in this paper. The XRD results indicate that the BCT powders annealed at 700℃ were composed of BCT phase without second phases. BCT films were deposited on Pt/Ti/SiO2/Si substrate.The impact of different annealing processes on the phase structures, microstructures and properties of BCT films were studied. The results show that, all of the films annealed under three different annealing processes are composed of pure BCT phase. Compared with those prepared under ordinary annealing process and rapid annealing process, the BCT film prepared under complex annealing process have better crystallinity, more compact surface, and more clear interface between films and substrate. And the ferroelectric test results indicate that, the BCT film prepared under complex annealing process have higher remnant polarization (Pr=10.78uC/cm2), smaller coercive field (Ec=94.86kV/cm), higher dielectric constant(ε=338), smaller dielectric loss(tanδ=0.08).Secondly, the CFO powders and films were prepared by different preparation methods. The results show that, compared with the CFO film prepared by citric acid method, the CFO film prepared by acetic acid method contain pure phase at lower temperature, have more compact surface without pores and cracks, higher saturation magnetization (Ms=540 emu/cm3) and remanent magnetization (Mr=149 emu/cm3). But compared with BCT films, the interface between CFO film and substrate is not clear and smooth.Therefore the BCT films were firstly deposited on the substrate when composite films were prepared.Finally, the BCT ferroelectric thin film and CFO ferromagnetic film were deposited on Pt/Ti/SiO2/Si substrate alternately by sol-gel method to prepare magnetoelectric composite films. The impacts of different annealing processes on the microstrctures, ferroelectric, ferromagnetic and magnetoelectric coupling properties of composite films were investigated. The results show that, all of the composite films prepared under three different annealing processes exhibit 2-2 layered structure. Compared with the composite films prepared under ordinary annealing process and rapid annealing process, the composite films prepared under complex annealing process possess better crystallinity, more compact surface, more clearly interface and there is no diffusion between ferroelectric and ferromagnetic phase. And the composite films prepared under complex annealing process display better ferroelectric properties(Pr=6.45 uC/cm2), ferromagnetic properties(Ms=373 emu/cm, Mr=103 emu/cm) and higher magnetoelectric coupling coefficient (aE=82 mV/(cm · Oe)).