Study On The Magnetic Properties And Mossbauer Spectrum Of Eu Or Sr Doped BiFeO₃

Up to now,BiFeO3 is one of few multiferroics with room temperature ferroelectric(Curie temperature TC?1103 K)and antiferromagnetic properties(Néel temperature TN?643 K),so it shows potential applications at room temperature.However,the antiferromagnetic ordering with a spatially modulated spin structure in BiFeO3 doesn't allow the linear magnetoelectric effect.Therefore,a necessary condition for displaying the magnetoelectric effect is to break the spatially modulated spin structure in BiFeO3.Because of the distinctions in ion radii and electronic structures between rare earth elements or alkaline earth metal elements and Bi element,the spatially modulated spin structure of BiFeO3 can be suppressed by replacing Bi elements with rare earth elements or alkaline earth metal elements,leading to the release of locked magnetization and the improvement of multiferroic properties in these doped BiFeO3 compounds.(1)Eu3+ ions are selected to dope BiFeO3 at A-site by solid phase sintering method in this paper,and the magnetic properties and Mossbauer spectrum were studied.The research contents and main conclusions are as follows:Solid phase sintering method was used to prepare undoped sample BiFeO3 and Eu3+ ions doped sample Bi0.9Eu0.1Fe O3.Both Bi0.9Eu0.1Fe O3 and BiFeO3 samples are rhombohedral R3 c structure with a very small amount of impurity phase.Doping of Eu3+ ions causes a slight lattice distortion,which also leads to the reduction of lattice parameters and unit cell volume.Doping of Eu3+ ions enhances the magnetization and remanent magnetization(Mr)of BiFeO3 sample.This is because the lattice distortion caused by doping of Eu3+ ions suppresses the spiral spin modulation and releases the locked magnetization.Doping of Eu3+ ions improves the Curie-Weiss temperature and effective magnetic moment(?eff)of BiFeO3 sample,which can be attributed to Eu3+ ion's magnetism,the change of bonding interaction and the appearance of exchange interaction.By fitting and analyzing the Mossbauer spectrum of the samples,it can be found that only Fe3+ ions exist in both two samples,the impurity phase in two samples was determined to be Bi2Fe4O9,and it is further confirmed that the doping of Eu3+ ions produces distortion in the lattice of BiFeO3.(2)Sr2+ ions are selected to dope BiFeO3 at A-site by sol-gel method in this paper,and the magnetic properties and Mossbauer spectrum were studied.The research contents and main conclusions are as follows:Bi1-x Srx Fe O3(x=0,0.05 and 0.1)samples were prepared by sol-gel method.With the increase of Sr2+ ions doping,the structure of BiFeO3 sample changes from rhombohedral structure to cubic phase structure,and the content of impurity phase in the samples are decreased.This is because the Sr2+ ions compensate for the Bi3+ ions that were evaporated during the preparation process.The hysteresis loops of Bi1-x Srx Fe O3(x=0,0.05 and 0.1)samples are symmetric,indicating that Bi1-x Srx Fe O3(x=0,0.05 and 0.1)samples are magnetically ordered materials but don't reach saturation.The increase of Sr doping also enhances the magnetization,remanent magnetization(Mr)and coercive field(Hc)of BiFeO3 sample.By fitting and analyzing the Mossbauer spectrum of the samples,it can be found that only Fe3+ ions exist in three samples,the impurity phase in samples were determined to be Bi2Fe4O9 and Bi25 Fe O39,and it is further confirmed that when Sr2+ ions replaced Bi3+ ions,BiFeO3 balanced its valence by generating oxygen vacancy instead of changing the valence state of Fe,and the doping of Sr2+ ions disturbed the lattice of BiFeO3.