Preparation And Multiferroics Properties Research Of BiFeO₃ Nanoceramics

Multiferroic materials are a class of materials of processing two or more ferroic ordering,such as ferroelectric,ferromagnetic,and ferroelasticity.Meanwhile these ferroic ordering can couple with each other.BiFeO₃ is a well known room temperature multiferroic material with ferroelectric ordering below 1103 K and G-type antiferromagnetic ordering below 643 K,which offered a wide range of potential applications such as multi-states storage,and non-volatile magnetoelectric memory.Based on this situation,we have prepared BiFeO₃ nanoceramics by spark plasmas sintering method,and improved its magnetic properties by breaking the antiparallel spin with helical repeat of?62 nm.Dielectric properties,magnetic properties,and defects types were investigated in BiFeO₃ ceramics with different grains size.On the basis of the abovementioned research,the properties of ferroelectric,dielectric,and ferromagnetic were investigated in Ti doped BiFeO₃ nanoceramics(BiFe_0.95Ti_0.05O₃).The influence of grain size and structural distortion on multiferroic properties of Ti doped BiFeO₃nanoceramics were analysied.Further,we compared the ferroelectric and magnetic properties of different lanthanide(Gd³⁺and Ho³⁺)doped BiFe_0.95Ti_0.05O₃ nanoceramics samples.The results are shown as follows:1.The study of grain-size-effect and preparing of BiFeO3 nanoceramics.Single-phased BiFeO₃ nanoceramics were obtained by spark plasmas sinering method.Medium-grained?grain size=500 nm?and coarser-grained?grain size=1.2?m?BiFeO₃ceramics were prepared for comparison using the same method.The XRD patterns demonstrated that all samples contained a single phase without other phases.The effects of grain size on the electric and ferromagnetism of pure phase BiFeO₃ ceramics were investigated.The results indicated that the decreasing grain size helped to decrease the content of Fe²⁺ions and dielectric loss in BiFeO₃ samples.Thus the dielectric properties in BiFeO₃ nanoceramics have been improved..For the meauremnt of ferromagnetic properties,BiFeO₃ nanoceramics?S1?possess a good performance of ferromagnetic?M_r=0.01 emu/g?,which was much larger than the coarser-grained BiFeO₃ ceramics?M_r=0.0057 emu/g?.This phenomenon can be explained as the broken of spin spiral structure with the decrease of grain size.A large value of exchange bias(H_EB=500 Oe)was observed for S1 samples at 5 K,which can be explained as an exchange interaction between the small grains?below 62 nm?with ferromagnetic state and the big grains?above62nm?withantiferromagneticstate.Themeasurementof magnezation-temperature indicated the existence of spin glass state in BiFeO₃nanoceramics.2.Preparing of BiFe_0.95Ti_0.05O₃ nanoceramics and their properties improved by Ti doping.Based on the study of phase pure BiFeO₃ nanoceramics,a strategy of doping Ti⁴⁺ion at B-site was adoped,to promote the property of multiferroic in BiFeO₃ based ceramics.Many test methods have been carried in Ti-doped BiFeO₃ nanoceramics to study their microstructure,ferroelectric and ferromagnetic properties.Ti-doped BiFeO₃nanoceramics with an average grain size less than 100 nm exhibited a rhombohedral phase.Comparing to the single phase BiFeO₃ nanoceramics,the dielectric loss of BiFe_0.95Ti_0.05O₃was remarkably reduced?tan?=0.02?.For the measurement of impedance spectrum,the valence fluctuation of Fe ions in BiFe_0.95Ti_0.05O₃ nanoceramics was significantly supressed,and the diffusion of oxygen ion was the major reason for dielectric relaxation at high temperature.The remenant polarization and P-E loops confirmed the intrinsic ferroelectric in BiFe_0.95Ti_0.05O₃ nanoceramcis.The PFM measurements showed nanoscaled ferroelectric domains for BiFe_0.95Ti_0.05O₃ nanoceramics.Those results demonstrated the exisintance of ferroelectric properties in BiFe_0.95Ti_0.05O₃ nanoceramics.Meanwhile,high remnant magnetization?M_r=0.1 emu/g,M_s=0.5 emu/g?were observed.Comparing to the results of BiFeO₃ nanoceramics,the ferroelectric and ferromagnetic properties were significant improved in BiFe_0.95Ti_0.05O₃ nanoceramics with a rhombohedral phase.3.Preparing of Bi_0.8Gd_0.2Fe_1-xTi_xO₃ nanoceramics and their properties improved by Gd³⁺and Ti⁴⁺doping.The prior studied BiFeO₃ system was mainly rhombohedral phase,to further improve the ferromagnetic properties,a change in crystal structure was necessary.Results show that Ln³⁺doping at A-sites could introduce a orthorhombic structure in BiFeO₃ system.An increase in the induced magnetic moment was caused by the structure evolution.Hence,to obtain orthorhombic phase with good magnetic property,Bi_0.8Gd_0.2Fe_1-xTi_xO₃?x=0,0.03,0.05?nanoceramcis were prepared by spark plasma sintering method,and their structure,ferroelectric and ferromagnetic behaviors were investigated.Bi_0.8Gd_0.2Fe_0.97Ti_0.03O₃ and Bi_0.8Gd_0.2Fe_0.95Ti_0.05 samples showed a mixture structure with three kinds of space group?R3c,Pnma,and Pn2₁a?.And there were two kinds of space group?R3c and Pnma?in Bi_0.8Gd_0.2FeO₃ samples.Relaxor ferroelectric-paraelectric transition peaks were observed.The relaxor behavior in BGFO ceramics comes from the disorder distributions of Bi³⁺and Gd³⁺(or Fe²⁺and Fe³⁺)ions on the crystallographically equivalent sites.The ferroelectric measurement of Bi_0.8Gd_0.2Fe_0.97Ti_0.03O₃ and Bi_0.8Gd_0.2Fe_0.95Ti_0.05 samples showed an obvious ferroelectric polarization owing to a low leakage current density.For the measurement of ferromagnetic properties,the improved magnetism in Bi_0.8Gd_0.2Fe_1-xTi_xO₃ nanoceramics?M_s?1.0emu/g?can be explained as:?a?The increase in the induced magnetic moment caused by the structure evolution;?b?the broken of the antiparallel spin.This paper has verified the existence of good multiferroic properties in co-doped samples.4.Preparing of orthorhombic structure Bi_0.8Ho_0.2Fe_0.95Ti_0.05O₃ nanoceramics and their magnetic properties improved with Ho³⁺and Ti⁴⁺doping.The previous section has described the structure and multiferroic properties in Bi_0.8Gd_0.2Fe_1-xTi_xO₃ nanoceramics.As a comparison,we selected smaller ion radius Ho³⁺,the properties of ferroelectric and ferromagnetic in Bi_0.8Ho_0.2Fe_0.95Ti_0.05O₃ nanoceramics were studied.Ho³⁺has higer magnetic moment for larger number of 4f electrons than Gd³⁺,and Ho³⁺doped BiFeO₃ has larger crystal distortion for smaller ionic radius than Gd doped BiFeO₃,thus Bi_0.8Ho_0.2Fe_0.95Ti_0.05O₃ ceramics possessed of better magnetic properties than Bi_0.8Gd_0.2Fe_0.95Ti_0.05O₃ nanoceramics.Meanwhile,a total orthorhombic structure with the introduction of Ho³⁺led to weakened ferroelectric properties in Bi_0.8Ho_0.2Fe_0.95Ti_0.05O₃nanoceramics,and Bi_0.8Ho_0.2Fe_0.95Ti_0.05O₃ nanoceramics maintained a low dielectric loss.