Preparation And Magnetoelectric Properties Of BiFeO₃-based Solid Solutions

The development of electronic and magnetic materials has penetrated in all areas of modern technology,and the devices miniaturization trend also promotes the development of multifunctional materials with integrating electric and magnetic properties.Multiferroic material is a new type of multifunctional material,which simultaneously exhibit two or more"ferroic" in one phase.The "ferroic" containing ferroelectric,ferromagnetic or ferroelastic order might produce coupling effects between different order parameters,which has attracted considerable attention.Among these potential multiferroic materials,BiFeO3 and its derived materials show simultaneous coexistence of ferroelectric and magnetic polarization at room temperature and good piezoelectric properties with high ferroelectric Curie temperature,and thus are regarded as promising candidates for high temperature piezoelectric,lead-free ferroelectric and magnetoelectric applications.However,its development has been mainly limited by the difficulties in the observation of intrinsic ferroelectric and piezoelectric properties caused by the formation of the second phase,the valence fluctuation of Fe,and the formation of oxygen vacancies during a sintering process.Moreover,magnetization is rather small because of the nature of G-type antiferromagnetism with spatially modulated cycloidal-spin structures in BiFeO3.Therefore,we prepared(1-x)BiFeO3-x(Ba0.85Ca0.15)TiO3 solid solution by ball milling,and investigated the effects of different sintering aids on the magnetoelectric properties of the system.The main results are as follows:1.We prepared(1-x)BiFeO3-x(Ba0.85Ca0.15)TiO3(0.2≤x≤0.5)solid solutions with a conventional solid-state reaction method.The composition range of morphotropic phase boundary(MPB)in(1-x)BiFeO3-x(Ba0.85Ca0.15)TiO3(0.2≤x≤0.5)solid solutions is determined to be 0.24≤x≤0.3.The samples with the composition near MPB exhibit saturated hysteresis loops and optimal piezoelectric properties.2.0.7BiFeO3-0.3(B0.85Ca0.15)TiO3(BF-BCT)ceramics were prepared by two-step sintering together with ball milling.The results manifest that the samples prepared by ball milling are denser than those by mortar milling.Moreover,the sample milled for 40 hours possesses the maximum remanent polarization of 52 μC/cm2,which is in good agreement with the PUND test.This implies that the ferroelectricity is intrinsic,excluding the contribution of leakage current.The large remanent polarization is due to the decrease of oxygen vacancies.3.The effect of CuO and MnO2 as sintering additives on the structural,ferroelectric,and magnetic properties of BF-BCT was investigated.The addition of CuO can inhibit the grain growth and enhance the magnetic properties.However,the addition of MnO2 can promote grain growth and the ferroelectric properties.Nevertheless,the addition of both CuO and MnO2 can markedly enhance the magnetic properties originating from the enhanced superexchange interaction between Fe2+and Fe3+because of the increased Fe-O-Fe bond angle.4.We prepared BF-BCT ceramics with different amount of MnO2 and found that the remanent polarization of the x=0.8 sample is almost three times as much as the end compound.The piezoelectric coefficient of the x=1.2 sample can reach as high as 137 pC/N.The dielectric results show that the samples with the addition of MnO2 undergo non-relaxor ferroelectrics to relaxor ferroelectrics.The optimal piezoelectric properties are mainly due to the enhanced ferroelectric and dielectric properties,as well as the coexistence of polar nano-regions and long-range ferroelectric domains in relaxor ferroelectrics.