Structure And Properties Of Fe-Based Multiferroic Ceramics And Thin Films

Because of the rich physical contents and great potential applications in the field of spintronics,the magnetoelectric multiferroic materials with the co-presence of ferroelectric and magnetic orders have been attracting the increasing scientific interests.In the present work,Fe-based multiferroic materials such as Ba4Pr2Fe2Nb8O30 ceramics,La1-xYxFeO3 ceramics and SmFeO3 ceramics and thin films have been systematically investigated,and the following primary conclusions have been reached.The typical relaxor ferroelectric nature is indicated in the Ba4Pr2?Fe2Nb8O30 ceramics,where the broaden dielectric constant and dielectric loss peaks with strong frequency dispersion feature are observed.The peak temperatures for both dielectric constant and dielectric loss are significantly increased together with much stronger frequency dispersion by N2-annealing,while the O2-annealing results in the decreased peak temperatures and the suppressed frequency dispersion.These phenomena are closely related to the valence states of Fe and Pr ions.The hopping between Fe2+ and Fe3+ inside the grains causes the dielectric relaxation in as-sintered and N2-annealed samples.The enhanced ferroelectricity near room temperature is attributed to the increased ion radius difference between Al and A2 ions because of the increasing content of the Pr4+ after N2-annealing.Besides,intrinsic room-temperature weak ferromagnetism is observed.Near room temperature multiferroicity is determined and it could be modified by annealing.In perovskite-type La1-xYxFeO3(x=0,0.25,0.5,0.75,1.0)ceramics,the composition of x=0.5 adopts orthorhombic Pbnm and P21nm mixed phases,while other compositions have the Pbnm single phase structure.La0.5Y0.5FeO3 ceramics with La and Y ions partially ordered as rocksalt type have been systematically investigated.Such ordering leads to a polar P21nm phase,which might be consistent with a particular type of hybrid improper ferroelectricity(HIF)that has rarely been observed.The saturated polarization-electric field loops are determined at room temperature with a remnant polarization of about 0.11 μCcm2,together with a room-temperature predominantly antiferromagnetic order accompanied by a weak magnetization,which renders our La0.5Y0.5FeO3 ceramics a novel example of a room-temperature multiferroic compound.The multiferroic phase P21nm leads to a magnetoelectric coefficient of about 0.38 mV/cm Oe.The dielectric constant can be modified to vary in 3 orders of magnitude by Y-substitution.Two dielectric anomalies are found due to the increase of DC conductivity and semiconductor to metal phase transition in all La1xYxFeO3 ceramics.The high temperature DC conductivity is dominated by long-range motion of double ionized oxygen vacancies.With increasing Y-substitution,the magnetic transition temperature drops gradually from 742 to 642 K due to the decrease of Fe-O-Fe bond angles.Weak ferromagnetism has been observed by M-H hysteresis loops and magnetic domains from MFM phase images.Enhanced magnetic property with increased remnant magnetization is achieved by Y-substitution.Room-temperature multiferroicity is achieved in both SmFeO3 ceramic and thin films with a remnant electrical polarization of about 0.07 and 1.5 μC/cm2,which is caused by different mechanisms.The ferroelectricity of SmFeO3 ceramic should be originated from different mechanism from SmFeO3 single crystal,for which is 1 order of magnitude larger than reported in the SmFeO3 single crystal.The ferroelectricity is likely to be a result of the co-presence of Pbn21 polar phase together with Pbnm non-polar phase.Furthermore,weak ferromagnetism is obtained in both SmFeO3 ceramics and thin films originated from the V2 spin structure of the antiferromagnetic orders.The grain boundary effects lead to the dielectric relaxation of the SmFeO3 ceramics.The nanoscale ferroelectric domains with 1800° switchable polarization are detected by the piezoelectric force microscopy(PFM),and a considerable piezoelectric coefficient(d33～25.1 pm/V)is obtained.The ferroelectric transition temperature(TC～306 K)and the spin-reorientation transition temperature(TSR～480 K)can be indicated according to the dielectric anomalies.Strong magnetoelectric coupling with magnetoelectric coefficient of about 87.6 mV cm-1 Oe-1 is achieved as a consequence of the coexistence of weak ferromagnetic behavior and robust ferroelectricity.