Multiferroicity In Ca₂FeAlO₅-based Ceramics At Room Temperature

Recently,magnetoelectric multiferroic materials have attracted great attention from academic and industrial circles due to their rich physical connotation and attractive application prospect,it is a hot topic to explore the new structure of multiferric materials.In the present thesis,based on the Ca2FeAlO5 with the structure of brownmillerite,the crystal structures,ferroelectric,thermal,magnetic and magnetoelectric properties of Ca2Fe(Ali-xGax)O5(x=0,0.05,0.10,0.15)ceramics,in which Al3+ cations were partially replaced by Ga3+,were investigated to explore their experimental evidences of multiferroics.The influence of contents of Ga3+ on the properties of ceramics were deeply analyzed.Moreover,the crystal structures and ferroelectric properties of Ca2Fe2-x,AlxO5,O5(x = 0.6,0.7,0.8,0.9)ceramics,in which Al3+ are partialy replaced by Ga3+ based on the Ca2FeAl5,were investigated,and the influence of Fe3+ on the ceramics were analyzed.The space groups of Ca2Fe(Ali-x,Gax)O5(x = 0,0.05,0.10,0.15)ceramics at room temperature were Ibm2,which was a polar point group.Different degrees of deviation along the c axis occurred on the octahedral and tetrahedral center ions,and the spontaneous polarization was induced along the c axis.Typical hysteresis loops were found in the ceramics of x= 0.05,0.10 and 0.15 at room temperature.It was found that the Ca2Fe(Alo.95Ga0.05)O5 had the maximum residual polarization,0.35 ?C/cm2.The coercive field was too large to reverse polarization when x was zero,while the coercive field decreased rapidly with the increase of x.The change of symmetry was induced by partial oxygen tetrahedral order orientation of Ca2Fe(Al0.95Ga0.05)O5 disordered at 1073K.The Neel temperature located on 350K,the weak ferromagnetism induced from the DM interaction of Fe3+ and uncancellated net magnetic moments between Fe3+ and Fe4+ ions.The magnetization decreased first and then increased with the increasing of x,and the minimum occurred when x equaled to 0.10.Obvious magnetoelectric effect was observed in Ca2Fe(Al1-xGax)O5(x=0.05,0.10,0.15)ceramics,while the change percent of magnetization after polarized declined overall with the increase of x.The space groups of Ca2Fe2-xAlxO5(x = 0.7,0.8,0.9)ceramics at room temperature were also Ibm2.The dielectric strengths of ceramics were enhanced by using spark plasma sintering(SPS)process.In addition,the dielectric strength was enhanced by increasing of x.The Pr and P max of ceramics increased non-linearly when the applied field was larger than 160 kV·cm-1,which was the characteristic of ferrToelectrics.However,the polarization was too small to be detected because it was covered by other factors such as electric conductivity.In order to confirm the ferroelectricity in the present ceramics,further work should be done.