The Investigation And Tuning Of Multiferroicity In Magnetoelectric Materials

In the past two decades,multiferroics and magnetoelectric(ME)coupling effect have received considerable attention due the rich physics and potential application in storage and novel ME devices.Great progress has been made in this area,including promising materials that have been synthesized and the underlying mechanisms that have been discussed in detail.For type-I multiferroics,the magnetism and the ferroelectricity originate from independent mechanisms.In this kind of materials,the coexistence of large electric polarization and moderate magnetization can be realized above room temperature.But unfortunately,the ME coupling in these materials is usually weak.For type-II multiferroics,special magnetic structures break the space inversion symmetry and induce the electric polarization.Thus,in type-II multiferroics the magneto-control of ferroelectricity is naturally realized.However,in these materials the ME coupling is usually realized at extremely low temperatures,and the electric polarization is small.Besides,the magneto-response to the electric field in the type-II multiferroics is extremely weak.For linear ME materials,remarkable ME effect is likely to emerge at high temperatures.But up to now,for most linear ME materials the performance is poor,and the investigation on new materials is limited.This is more or less due to the limited understanding of the underlying physics.To overcome the difficulties,we focus our work on the following several aspects,and the dissertation includes several chapters as described in the following:The first chapter can be divided into two parts.In the first part,we briefly introduce the magnetic materials and ferroelectric materials.In the second part,we first review the history of magnetoelectric materials,followed by the demonstration of the underlying physics and recent progress in the type-I multiferroics,type-II multiferroics and linear magnetoelectric materials,respectively.Finally we summarize the advantages and disadvantages of these materials and point out the challenges in the future investigation.In the second chapter,we briefly describe the physics of the crystal growth,which includes the driving force of phase transition and the theory of nucleation.Then we introduce three different strategies of crystal growth that are widely used in material exploration.In the third chapter,we focus on the honeycomb Co₄Nb₂O₉,and the variation of structure,magnetism and ME coupling with Mn doping is investigated.It is found that the antiferromagnetic transition temperature increases rapidly with Mn content and reaches above 100K.Besides,the ME coupling coefficient decreases moderately with increasing Mn content,but still remains around 10ps/m.Through the detailed analysis,we believe that this is due to the weakening of spin-orbit interaction via Mn doping,and the Dzyaloshnskii-Moriya(DM)interaction plays an important role in the linear ME coupling in Co₄Nb₂O₉.The present work clarifies the mechanism of linear ME effect in Co₄Nb₂O₉ and provides a paradigm of tuning the linear ME effect.In the fourth chapter,we focus on the Mn₄Nb₂O₉ and report the systematic measurements of structure,magnetism,heat capacity,dielectric constant,electric polarization and ME coupling using the polycrystalline samples and single crystal samples,respectively.It is found that Mn₄Nb₂O₉ forms a long range antiferromagnetic order below 109K,and the linear ME effect emerges simultaneously.The behavior of electric polarization observed in Mn₄Nb₂O₉ polycrystalline samples is different from that observed in Co_4-xMn_xNb₂O₉ in the third chapter,suggesting a different ME coupling mechanism.In the investigation of single crystal samples,it is found that the coupling tensors fit the magnetic point group-3?m?.We discuss the ME coupling in Mn₄Nb₂O₉ and point out that the coupling mainly originates from the exchange striction dependent mechanism.The present work contributes to the understanding of linear ME effect and the exploration of the high temperature ME materials.In the fifth chapter,we investigate the influence of different electro-poling processes on multiferroicity using GdMn₂O₅ single crystal samples.It is demonstrated that even if the electro-poling process is carried out within the paramagnetic-paraelectric(PM-PE)region at high temperatures,one can still observe a negative electric polarization at low temperature.Through the combination of first-principle calculation and analysis,we proposed that electronic polarization exists at PM-PE region and it is polarized under the electric field.Moreover,the polarized electronic polarization is able to influence the alignment of the ferroelectric domains and thus the electric polarization.The present work provides a new route to control the multiferroic properties in type-II multiferroics.The sixth chapter is summary and prospect of the thesis.