Structure And Characteristics Of Fe-based Single Phase Room-temperature Multiferroic Ceramics

Due to the rich physics and the great potential applications,multiferroic materials have attracted increasing scientific interests in the recent years.In the present thesis,two types of Fe-based single phase multiferroic materials(BiFeO3 and hexagonal-LuFeO3)have been investigated throughly,and the following primary conclsions have been achieved.The structure of(1-x)BiFeO3-xSr0.5Ca0.5TiO3 solid solution ceramics changes from noncentrosymmetric(R3c)system to centrosymmetric(Pnma)system with increasing substitution content.The P-E hysteresis loops measured with DLCC method show the significantly enhanced ferroelectric properties for x=0.25 and 0.3 samples,which can be attributed to the suppressed leakage current.The best ferromagnetic characteristics are achieved in the composition of x=0.25,where the saturated M-H loop is determined with a maximum remanent magnetization?Mr=34.8 emu/mol.The enhanced magnetism originated from the partial substitution of Fe3+ by Ti4+,which destroyed its previous spiral structure to allow the appearance of a macroscopic magnetization.The(Sr,Ca)TiO3-modified BiFeO3 multiferroic ceramics have been investigated throughly by changing Sr/Ca ratio.With increasing Sr/Ca ratio,the morphotropic phase boundary in Bi1-x(Sr,Ca)xFe1-xTixO3 system shifts toward(Sr,Ca)TiO3 side.The best ferroelectric properties are obtained nearby the phase boundary.However,the best magnetic properties are obtained around x=0.2?0.25 compositions for Bi1-x(Sr,Ca)xFe1-xTixO3 ceramics.The magnetic properties are mainly influenced by the substitution of B-site,and the Sr/Ca ratio has little effects on them.The structure and multiferroic properties of the(1-x)BiFeO3-x(0.5CaTiO3-0.5SmFeO3)system have been investigated.The refinement results confirm the different phase assemblages with varying amounts of polar rhombohedral R3c and non-polar orthorhombic Pbnm as a function of the substitution content.In the compositions range of 0.2?x?0.5,polar R3c and non-polar Pbnm coexist,which is referred to polar-to-non-polar morphotropic phase boundary(MPB).According to the dielectric and DSC analysis results,the ceramics with x?0.2 change to diffused ferroelectric,and the ferroelectric properties are enhanced significantly.Two dielectric relaxations are detected in the temperature range of 200?300K and 500?700,respectively.The high temperature dielectric relaxation is attributed to the grain boundary effects.While the low temperature dielectric relaxation obtained in the samples with x=0.3?0.5 is related to the charge transfer between Fe2+ and Fe3+.The magnetic hysteresis loops measured at different temperature indicate the enhanced magnetic properties in the present ceramics,which could be attributed to the suppressed cycloidal spin magnetic structure by Ti ions.In addition,the rare-earth Sm spin moments might also affect the magnetic properties at relatively lower temperature.The hexagonal structure can be obtained in LuFeO3 ceramics through the In substitution of In for Lu.The XRD refinement results indicate the structure evolution from noncentrosymmetric P63cm(x=0.4?0.6)to centrosymmetric P63/mmc(x=0.75).The SAED patterns also confirm this structure transformation.The ferroelectric domains can be obtained in polar P63cm phase.Two magnetic transitions have been determined for all compositions,which are identified as the paramagnetic to antiferromagnetic transition at Neel temperature TN,and the antiferromagnetic to weak-ferromagnetic transition at spin-reorientation temperature TSR.The Neel temperature is higher than room temperature for polar P63cm phase,indicates the anti-ferromagnetic(AFM)orders at room temperature.With increasing In content,the spin-reorientation transtion temperature TSR increases.Low-temperature magnetic transition with a dielectric anomaly can be attributed to the spin-reorientation,indicates the magnetoelectric coupling effect in Lu1-xInxFeO3 ceramics.Two dielectric relaxations are observed in Lu1-xInxFeO3 ceramics.The low temperature dielectric relaxation is attributed to the charge transfer between Fe3+ and Fe2+ ions.However,the high temperature dielectric relaxation is the typical Debye-type relaxation,which is originated from the oxygen vacancies.At higher temperature about 500K,a phase transition from semiconductor phase to metal phase is determined for all compositions due to the extremely huge conductivity.The XPS analysis results confirm the coexistence of Fe3+ and Fe2+ ions at room temperature,which can be responsible for the large conductivity in Lu1-xInxFeO3 ceramics.