Dependence On Sress Of The Ferromagnetic And Ferroelectric Domains In Multiferroic BiFeO₃Ceramic

Pure BiFeO₃powders were synthesized by homogeneous coprecipitationmethod, and BiFeO₃ceramics were prepared in O2atmosphere by the method ofpressureless sintering. It was analyzed that the influences of the stress onferroelectric and ferromagnetic domains in BiFeO₃ceramics. There were twoaspects of the research, performance and structure. On one hand, the effects ofimpurities and defects were analyzed on ferroelectricity and ferromagnetism inmaterials, through measurement the magnetization magnetic field, polarizationelectric feld hysteresis loops and dielectric constant. On the other hand, theinfluences of impurities and defects were observed in ferroelectric domains ofBiFeO₃ceramics, through methods of scanning electron microscopy （SEM）, andtransmission electron microscopy ferroelectric （TEM）. The deflections offerromagnetic domains were also observed under the action of magnetic field in theway of atomic force microscopy （AFM）.The71,109and180ferroelectric domains were identified in BiFeO₃ceramics. All of them could not pass through the grain boundary, impurities andother defects, limited inside the grains, and the grain size influenced the width andsurface density of ferroelectric domain. The domain width of domains wasproportional to the square root of the grain size, and the surface density increasedwith the increase of grain size. However, the domain width of or domainsincreased, and the surface density decreased, as the increase of grain size. Thedomain width and surface density were changed resulted from the strain or stress,which were calculated as0.2849%,4.5108Nrespectively.The magnetic domains in BiFeO₃ceramics were the sheet-shaped. The width ofmagnetic domains was1.7μm, and the length was10μm-15μm.The magnetic domainsspanned grain boundaries, impurities and other defects, pinned in these defects. Theenergy of domain wall is small compared with BiFeO₃film, the energy density,energy and Gibbs free energy of the domain wall is w5.4109J/m2,F（D2.6106w）=J/m2and F5.2106J/m2, respectively. The magnetic domainschanged from no-regulation arrangement to approximately straight line arrangementalong the external field direction, through the deflection of the magnetic momentsand the displacement of domain walls under the magnetic field. The electric hysteresis loops of the BiFeO₃ceramics displayed the residual polarization ofPr=0.02μC/cm2, the coercive field of E c150V/cm,and the magnetic hysteresis loopshowed the remanent magnetization ofM r=0.4emu/g, the coercive field of H c142Oe.Those ions could ferromagnetism was increased, ferroelectricity decreased, and thehysteresis or hysteresis loop were asymmetric, because of the existence of Fe2+andFe3+. The dielectric constant and loss were great in the lower frequency region,tended to stable at122and0.016, when the frequency is higher than105Hz, becauseof the interface polarization.