Research On The Preparations And Magnetoelectric Properties Of ?1-x?SrTiO_3-xNiFe₂O₄ Composite Ceramics

Multiferroic magnetoelectrics possess great prospect of applications in sensors,transducers and memories due to the coexistence of ferroelectricity and ferromagnetism as well as magnetoelectric effect.As to this research,the systems of?1-x?SrTiO₃-x NiFe₂O₄?0<x?0.3?were chosen.To be specific,SrTiO₃ is incipient ferroelectric which is without ferroelectricity at room temperature and NiFe₂O₄ exhibits ferrimagnetism.The emphasis focused on inducing the ferroelectricity of SrTiO₃ by means of the stress action of NiFe₂O₄.In addition,it was very feasible to reduce the electric leakage through establishing the core-shell inlaid structure,namely,SrTiO₃ was the shell and NiFe₂O₄ was the core.We adopted sol-gel method to synthesize different components of SrTiO₃-NiFe₂O₄ and investigated the heat-treatment processes.Moreover,the ferroelectricity and ferromagnetism under ambient temperature were discussed in detail.The homogeneous stable composites were prepared with glacial acetic acid as a solvent and ethylene glycol as a stabilizer on the basis of appropriate proportion and dose.The optimal preparation conditions were variable as for different components.Besides,the Ti ion was extremely sensitive to the external factors,which contributed to precipitation easily.Ultimately,the phase-pure?1-x?SrTiO₃-x NiFe₂O₄ powders and ceramics were successfully synthesized under the conditions of 1050?-2h calcination and 1350?-5h sintering.The relative densities of all acquired ceramics surpassed 95% with the 0.9SrTiO_3-0.1NiFe₂O₄ being supreme.The test results indicated that the samples possessed ferroelectricity at room temperature as a result of the induction of ferromagnetic NiFe₂O₄.Concretely speaking,the ferroelectricity became first better and then worse with the increasing of NiFe₂O₄.Equally,the property of 0.1NiFe₂O₄ system was best.The remanent polarization?Pr?and coercive field?Ec?of this system were 1.65?C/cm2 and 19.18 k V/cm,respectively when applying an electric field?E?of 220 k V/cm and the current leakage was just 4.05×10-6A/cm2 with E=100k V/cm.However,the sample with x=0.2 showed enormous leakage,namely 0.177A/cm2 even though the applied electric field was just 30 k V/cm.There was no doubt that the ferroelectricity was inferior and the hysteresis loop was in shape of round.As to x=0.3,it didn't exhibit ferroelectricity.As XRD and Raman demonstrated,the paraelectric cubic SrTiO₃ was converted into tetragonal phase related to Ti ion deviation from Ti O6 octahedral.And the result of SEM indicated that the mosaic structure as mentioned above was gained successfully.The dielectric properties of 0.9SrTiO_3-0.1NiFe₂O₄ ceramics with different sintering temperatures?1200?,1250 ?,1300? and 1350??were studied.It was found that the relative permittivity??r?and dielectric loss?tan??were gradually increased and decreased,respectively with the enhancement of sintering temperature as for x=0.1 system.At 1350 ?,?r came up to 205.71 and tan? was only 0.035.Particularly,the growth rate was maximal at 1300 ?,which corresponded to the relative density.As we can see from the dielectric-temperature graph,it showed relaxation phenomenon obviously,which was due to Oxygen vacancy.Moreover,the maximum of ?r decreased with the increasing of frequency and the peak removed to higher temperature.But the dielectric loss?tan??was slowly raising when the frequency increased though its maximum also removed to higher temperature.In addition,the magnetization increased with the NiFe₂O₄ content: when x=0.2,the saturation magnetization?Ms?was 5.57emu/g;but Ms was only 0.068emu/g with x=0.04.However,the test pieces of different sintering temperatures had no evident trend of regularity.Furthermore,the magnetroelectric effect characterized by the magnetic dielectric rate?MD?was found successfully in SrTiO₃-NiFe₂O₄.As to the system sintered at 1350 ? with 0.1NiFe₂O₄,which possessed the best ferroelectricity,the MD was equal to 1.03% when applying 10000Oe-15 min magnetic field.Also,the clustering extent of magnetic phase and different sintering atmospheres were taken into account.It showed that the clustering of NiFe₂O₄ was harmful to the ferroelectric and dielectric properties.Comparing to O2 sintering atmosphere,N2 significantly reduced the ferroelectric and dielectric properties as a result of the emerging of oxygen vacancy and Ti3+ simultaneously,which was verified by XPS.Beyond the above mentioned,the similar system of 0.9SrTiO_3-0.1Co Fe2O4 was discussed and the results also exhibited analogous regularity.