Ferromagnetism And Defect Properties In Multiferroic Materials Studied By Positron Annihilation Technique

Multiferroics are multifunctional materials that possess ferroelectric and magnetic ordering properties simultaneously.They have attracted much attention due to their importance to both fundamental physics and significant technological applications in novel multifunctional devices.Many experimental and theoretical studies on multiferroic materials have been carried out in recent years.Theoretical explanations on the origin of magnetism,the magnetoelectric effect,and the origin of ferroelectric are emerging.Great efforts have been made to realize substantial room temperature multiferroic and magnetoelectric properties,especially to improve the ferromagnetic performances.It also raises questions regarding what caused the enhancement of the magnetic performance,and how we can take advantage of the factors to get a further improvement.It is already well known that vacancy defects are playing a major role in determining room temperature magnetic property of different oxides.It is thus desirable to investigate the defects in multiferroic materials and its correlation with ferromagnetism.Positron annihilation spectroscopy is a technique that applies particle detection technology to the field of solid physics and materials.It includes two major parts,positron annihilation related measurements and positron annihilation theoretical calculations.Positron annihilation is very sensitive to the defects on the atomic scale,and can provide the information on species,concentration,and state distribution of defects as well as other structure information.It is an irreplaceable tool in exploring material defects.In this thesis,several typical multiferroic materials,including "star"multiferroic material BiFe03,the layered Bi-containing oxides,and nanomaterial SrTiO3,are systematically studied by means of positron annihilation technology and theoretical calculations.The intrinsic relationship between the microcosmic defect structure and the macroscopic properties of the material is discussed.The main research results are as follows:1)The defects in the La doped BiFeO3 materials are systematically investigated by using positron annihilation lifetime spectroscopy(PALS),coincidence Doppler broadening spectroscopy(CDBS)and positron lifetime calculations.On this basis,combined with X-ray diffraction(XRD),Raman spectroscopy and magnetic test results,the origin of ferromagnetic properties is revealed.(a)The experimental results indicate that in Bi1-xLaxFeO3 materials,the concentration of Fe monovacancies and Fe-related divacancies increases when x>0.20 as evidenced by a increase of position lifetime.Meanwhile,Bi monovacancies disappear gradually andsome larger size vacancy clusters appear.Magnetic measurements indicate that the remnant magnetization increases greatly when x>0.20,which coincides with the variation of the concentration of the cation defects,suggesting that the concentration of divacancies or larger vacancy clusters is related to the remnant magnetization.(b)For the Bi0.7La0.3FeO3 samples obtained under different calcination conditions,it is found that the magnetic properties of Bi0.7La0.3FeO3 have been changed obviously only for the samples which were calcined at high temperature for long time.The concentration of defects in such sample is the highest,and the defect species is mainly compound vacancy defects such as divacancies or trivacancies.It shows that after full calcination the grains inside the sample can be fully developed.It results in the loss of large number of vacancy clusters and the increase of the concentration of divacancies,which will enhance the RT ferromagnetic properties,(c)In exploring the influence of Fe vacancies on ferromagnetic properties of Bi0.7La0.3FeO3 samples,we try to change Fe vacancies by reducing the content of Fe element.The results indicate that Fe vacancy may have an effect on RT ferromagnetic properties.2)The defects in layered Bi-containing oxides are systematically investigated by using positron annihilation lifetime spectroscopy(PALS)and coincidence Doppler broadening spectroscopy(CDBS).Combined with X-ray diffraction(XRD),scanning electron microscopy(SEM)and magnetic test results,we have discussed the effects of defects on RT ferromagnetic properties.(a)By exploring the correlation between ferromagnetism and the concentration of interfacial defects in multiferroic Bi7Fe2.75Co0.25Ti3O21(BFCTO)nanoplates,it is found that the variation of saturation magnetization in annealed BFCTO samples is consistent with the disappearance process of interfacial defects.The results suggest that the interfacial defects indeed play a role in the ferromagnetism.It can be also inferred that the concentration of interfacial defects is crucial to the saturation magnetization in BFCTO sample.(b)Er substituted for Bi at the A-site in Bi7Fe2.9Co0.1Ti3O21 sample has also been studied.It is found that both the defect species and defect concentration have changed.The variation trend of the defect concentration in the material is not consistent with the change of saturation magnetization.It shows that the effect of defect on the magnetic properties of the material is not only related to the concentration of the defect,but also the defect species.3)By using positron lifetime spectroscopy and positron lifetime calculations,combined with XRD,SEM and magnetic tests,the origin of magnetism of the nanomaterial SrTi03 is investigated.The experimental results show the magnetism disappears with the significant decrease of internal defects,indicating that the magnetism is related to the defects.Furthermore,we have found that Ti vacancy defects are the main components of the disappearing defects.It might suggest that the Ti vacancy defect could be an important factor of magnetism of SrTiO3 material.