Magnetoelectric Properties Of Multiferroic BaTiO₃-BaFe₁₂O₁₉ Based Composite Ceramics

In multiferroic composite materials consisting of ferroelectrics and ferromagnetics,the magnetoelectric coupling effect is realized by strain between the two different phases.In a “0-3” typed structure of ferroelectric/ferromagnetic composite ceramics,amounts of charge accumulate in the interface and form conductive paths along the ferromagnetic phase due to its relatively lower resistivity under applied electric field,giving rise to a large leakage current and deterioration in ferroelectric properties.Therefore,how to increase the content of ferromagnetic phase while maintaining the ferroelectricity of the composite has been an obstacle for the development of multiferroic magnetoelectric composites.In this work,multiferroic BaTiO₃-BaFe₁₂O₁₉ based composite ceramics of “0-3” typed structure were prepared by microwave hydrothermal and microwave sintering methods.In order to obtain an excellent magnetoelectric performance,the resistivity of BaFe₁₂O₁₉ phase was adjusted through ion-doping in A site or B site,and the composite proportion of ferroelectric phase and ferromagnetic phase was optimized as well.Firstly,the multiferroic Bi₂Fe₄O₉-BaFe₁₂O₁₉ composite ceramics were obtained to enhance the ferromagnetism of Bi₂Fe₄O₉.The impact of resistivity on magnetoelectric properties in multiferroic composite ceramics was studied.The results of dielectric and complex impedance exhibited the presence of grain and grain boundary effects,and their conductivity is corresponding to the first and second ionization of oxygen vacancies,respectively.Due to the exchangecoupling interaction of neighboring grains,the Mr/Ms for all the samples is higher than 0.5（the maximum was up to 0.89）.The obtained multiferroic Bi₂Fe₄O₉-BaFe₁₂O₁₉ composite ceramics are expected to have promisingly applications such as magnetic recording media of high density and permanent magnets.The BaTiO₃-Ba_1-xSr_xFe₁₂O₁₉ composite ceramics were obtained through doping Sr²⁺ at the A site of BaFe₁₂O₁₉.The results showed that the resistivity was enhanced by doping Sr²⁺,and the highest resistivity was obtained when x = 0.6.The two relaxation peaks derived from Debye-like relaxation and hopping of electrons between Fe²⁺/Fe3+ at the octahedral sites.The results of activation energy indicated that the hopping of electrons between Fe²⁺/Fe3+ was a direct hopping behavior.All the magnetic hysteresis loops presented a waisted shape.On the one hand,it is because the grain size of Ba_1-xSr_xFe₁₂O₁₉ is larger than single-domain critical size which leads to a stronger coercive field.On the other hand,it is due to the suppressed magnetic domain rotating and shifting by the non-magnetic phase BaTi O₃.Besides Sr²⁺-doping in A site,BaTiO₃-BaFe_12-xMn_xO₁₉ composite ceramics were obtained through doping Mn²⁺ at the B site of BaFe₁₂O₁₉.The resistivity was enhanced by Mn²⁺-doping and it is higher than the undoped one（for x = 0）by two orders of magnitude.The largest Ps was obtained for x = 0.6.The P-E loops showed slimmer loops,and the magnetic hysteresis loops showed an excellent ferromagnetic behavior as well.In a word,a weaker leakage conductance and saturated P-E loops were obtained by doping Mn²⁺ in the BaTiO₃-BaFe₁₂O₁₉ composite ceramics successfully while the good ferromagnetism was well maintained.In addition to ion-doping,the phase compositions of the ferroelectric/ferromagnetic composite ceramics were optimized as well to further improve its magnetoferroelectric properties.The（1-x）BaTi O₃-xBaFe₁₂O₁₉ composite ceramics were prepared,and it was found that the resistivity sharply declined by three orders of magnitude with decreasing x.There could be two reasons accounting for this.On the one hand,BaFe₁₂O₁₉ phase which defined the resistivity of composite ceramics decreased.On the other hand,the probability of surrounding BaFe₁₂O₁₉ phase by BaTiO₃ phase to form an “island structure” increased,and the probability of forming conductive paths（“island chain”）by low-resistivity BaFe₁₂O₁₉ phase reduced.With the decreasing of x,both the conductive leakage current and the dielectric loss decreased.Accordingly the shape of P-E loops changed from “fat” to “slim”.The maximum of Ps was up to 40 μC/cm2 at 7.5 kV/mm for x = 0.1.