The Fabrication And Properties Of Magnetoelectric Composite Films With Fe-doped Bi₄Ti₃O₁₂ As Ferroelectric Phase

In multiferroic magnetoelectric（ME）materials,the coexistence of ferromagnetic and ferroelectric properties provides a possibility to obtain“magnetoelectric（ME）effect”,by which an induced electrical polarization and magnetization can be controlled by applying a magnetic and electric field,respectively.It would play an important role in the novel multifunctional devices such as sensors,electric field-controlled magnetic data storage,actuators,spintronics,and microelectro-mechanical systems.In this paper,using Bi₄Ti_3-xFe_xO₁₂（BITF_x,x=0.05,0.1,0.15,0.2）as a ferroelectric phase,using La_0.67Sr_0.33Mn O₃（LSMO）or Co Fe₂O₄（CFO）as a ferromagnetic phase,BITF/LSMO and BITF/CFO ferromagnetic-ferroelectric composite films with 2-2 layered structure were successively deposited onto the（00l）-oriented La Ni O₃（LNO）buffered Si substrate via all sol-gel method.The crystalline,microstructure,ferroelectric and ferromagnetic properties,and ME coupling response of the as-prepared ferromagnetic-ferroelectric composite films were also analyzed by X-ray diffraction（XRD）,scanning electron microscope（SEM）,TF Analyzer（TFA）and so on.The specific research contents and results were as follows:（1）The BITF_x/LSMO magnetoelectric composite films were fabricated,the effects of Fe³⁺doping contents on the ferroelectric and leakage current properties of the composite films were investigated.The results indicated that the composite films with the Fe-doping content of x=0.1 exhibited the maximum remnant polarrization（Pr=28.74μC/cm²）,the minimum coercive field（Ec=213 k V/cm）and the lowest leakage current density(J=5.3×10^-7A/cm²).Furthermore,the ferromagnetic properties of three kinds of BITF_0.1/Si,BITF_0.1/LNO/Si and BITF_0.1/LSMO/LNO/Si films were investigated.The results indicated that the doped BITF_0.1film had both ferroelectricity and ferromagnetic properties.The BITF_0.1/LNO/Si film with c-axis orientation had the greater saturation magnetization compared with the BITF_0.1/Si film with random orientation,and the c-aixs oriented BITF_0.1/LSMO/LNO/Si film had the largest saturation magnetization（Ms）of 500.8 emu/cm³.The composite films had excellent magnetoelectric coupling properties,and the magnetoelectric coupling coefficientα_Eincreased first and then decreased with the increasing of DC bias magnetic field H_bias.When H_bias=49.63Oe,the value ofα_Eis maximum.The composite film also exhibits a strong ME response in the absence of DC bias magnetic field.Its ME voltage coefficient,α_E,gradually increases from zero with the increasing of the AC magnetic field frequency,and eventually reaches 47.5V/cm·Oe at 100 k Hz.（2）The CFO films were deposited on（00l）-oriented LNO/Si films at different crystallization temperatures.The results indicated that all the CFO films were randomly oriented,and the CFO films crystallized at 700℃had the best ferromagnetic properties and showed the highest Ms of 585 emu/cm³.（3）The BITF_0.1ferroelectric layers was deposited on CFO/LNO/Si with different hating rate,and the BITF_0.1/CFO/LNO/Si with different c-axis oriented degree were obtained.The effects of heating rate on the orientation,surface morphology and the ferroelectricity of the films were investigated.The results demonstrated that the coposite film prepared at the heating rate of 2400℃/min exhibited improved ferroelectric and ferromagnetic properties and had the biggest Pr of 28.1μC/cm²,the lowest J of 6.68 x 10^-9A/cm²,the higest Ms of 703 emu/cm³.The composite films also showed the highestα_E20.1 V/cm Oe.