Structure And Properties Of Bismuth Ferrite Based Multiferroic Ceramics

Magnetoelectric muliferroicshave recently been a research hotspot in the fields both of condensed matter physics and materials science due to their physical significance and great application potential.As so far the most important room temperature single phase multiferroic material,BiFeO3 has attracted great attention during the last decade,but it still suffers from several drawbacks including poor electric properties,unclear magnetoelectric coupling mechanism and weak magnetoelectric coupling.In the present thesis,the preparation,evolution of symmetry/structure,variation of ferroelectric/magnetic characteristics were systematically studied together with the possibility of enhancing the magnetoelectric coupling effect.The following primary conclutions have been obtained.Rapid cooling,as well as a small amount of Ti4+-substitution,can greatly enhance the ferroelectric property in BiFeO3-based ceramics by modifying the behavior of defect dipoles.Rapid cooling can avoid the alignment of defect dipoles and thus suppress the pinning effect on the ferroelectric domains,while Ti4+-substitution is able to ’soften’ the defect dipoles by lower the barrier of their switching process,which furtherly weaken the pinning effect of these defect dipoles.By a combined use of these two strategies,the remnant polarization of Bi0.86Sm0.14FeO3 ceramics is successfully increased by nearly ten times.In the Bi1-xSmxFeO3 ceramics,with increasing Sm content,a sequential phase transition takes place from rhombohedral R3c phase(ferroelectric with polarization along[111]pc)to a bridging Pna21 phase(ferroelectric with polarization along[001]pc)and finally to the orthorhombic Pbnm phase(paraelectric).In the vicinity of the morphotropic phase boundary between R3c and Pna21 phase(with a Sm content of approximately 0.10-0.12),the ferroelectric and piezoelectric properties of the system are greatly improved due to the low energy path of ferroelectric domain switching induced by the appearance of another polarization state,which is of great significance to obtain strong electrical control of magnetism.The ferroelectric Curie temperature(TC)of BiFeO3 decreases with Sm substitution while the antiferromagnetic Neel temperature(TN)slightly increases.When these two critical temperatures come across,a step-like drop of TN occurs,which corresponds to a magnetoelectric coupling energy of about 1.25 meV and a magnetism-coupled parasitic polarization of about 96 μC/m2,indicating a magnetoelectric coupling effect comparable to that of the typical type-Ⅱ multiferroics.Such strong magnetoelectric coupling effect causes the cycloidal spin structure in BiFeO3,but it vanishes in the bridging Pna21 phase and leads to the appearance of weak ferromagnetism.At the phase boundary between R3c and Pna21 phase,an electric field can switch the Pna21 phase back to the R3c phase,accompanied by a magnetic state change from the weak ferromagnetic state to the cycloidal spin state,thus realizing strong electrical control of magnetism.Although the rare-earth-ion-substituted BiFeO3 systems share a similar phase transition from the initial R3c phase to the final Pbnm phase,the chemical pressure induced by the difference of ion radius greatly affects the phase transition path.When the chemical pressure is relatively low,as in the case of Sm and Gd substituted systems,the substitution induced phase transition shows a typical second order phase transition behavior.The phase transition between R3c phase and Pbnm phase is continuous and a bridging phase is adopted to ensure such continuity.The phase coexistence in the phase transition process is not intrinsic,but a result of inhomogeneity.The ferroelectric Curie temperature continuously decreases with increasing substitution amount and the diffused behavior also causes by inhomogeneity.When the chemical pressure is high,as in the case of Lu substituted system,the substitution induced phase transition presents a typical first order phase transition behavior.As the substitution amount increases,Pbnm phase suddenly appears among the R3c phase matrix.No bridging phase is involved and the coexistence of R3c phase and Pbnm phase is intrinsic.The ferroelectric Curie temperature barely decreases by substitution and the ferroelectric phase transition also barely diffuses.Such first order phase transition behavior suggests the possibility of intrinsic electric field induced phase transition in this system,which is a promising path to colossal magnetoelectric effect.When the chemical pressure is on a critical level,as in the case of Ho substituted system,the phase transition path becomes dependent on the preparation method.To simultaneously adjust Tc and TN of BiFeO3 down to room temperature and obtain enhanced magnetoelectric effect,Sr and Mn co-substitution is conducted.Since the Mn ions can change their valence during sintering,the amount of Sr and Mn can be regulated independently.When the ratio between Sr and Mn is 1:1,a continuous phase transition from Rhombohedral R3c phase to cubic Pm-3m takes place around the substitution amount of 10%-20%.But Tc decreases far more rapidly than TN,thus these two critical temperatures cannot be simultaneously adjusted to room temperature in this case.Fixing the content of Sr to be 10%and increasing the amount of Mn,a phase transition takes place around the Mn content of 30%-40%,from the R3c phase to a Tetragonal phase similar to that of Bi0.5Sr0.5MnO3.In the composition with the content of Mn around 35%,both Tc and TN are near temperature,which is expected to possess good magnetoelectric property.