Magnetic Properties Of Binary Doped Bismuth Ferrite Materials

Multiferroicity refers to ferromagnetism（antiferromagnetism,ferromagnetism）,ferroelectricity（antiferroelectricity）,ferroelasticity,and multiferroic materials have two or more of these properties.In recent years,many researchers have paid more attention to multiferroic materials because of their multiferroic properties and potential magnetoelectric coupling effect.However,Bi Fe O₃,as the only single-phase multiferroic material that can exist at room temperature,and this is attributed to its higher ferroelectric-to-paraelectric phase transition Curie temperature（T_C～1103 K）and its higher antiferromagnetic-to-paramagnetic phase transition Nair temperature（T_N～643 K）than the room temperature,so that they have both ferroelectric and G-type antiferromagnetism.Therefore,it can be applied to information storage,energy conversion,magnetoelectric sensing,integrated circuits and other fields,and has a broad application prospect.At present,it has become one of the hot research materials at the forefront of materials science and condensed matter physics.Over a long period of time,a wide range of Bi Fe O₃-based materials have been studied in various forms,including bismuth ferrite films,ceramics and nanoparticle structures.However,it is difficult to accurately grasp various conditions in the process of controlling synthesis of the high quality Bi Fe O₃ in pure phase,so that defects and heterophase often appear in the prepared materials.However,these defects as well as Fe change price and other problems will lead to large leakage current,which greatly limits the development of Bi Fe O₃-based materials.In order to break through this limitation,researchers have made many attempts to improve its properties in recent years,including improving the preparation technology and doping modification.It has been proved that doping modification is a very effective way to improve its properties.This thesis mainly studies the structure and properties of bismuth ferrite doped with（La,Co）and（Eu,Co）.The specific research content is as follows:（1）Bismuth ferrate samples with different lanthanum（La）doping concentrations and fixed cobalt（Co）doping concentrations were prepared.The structure,surface morphology,magnetic changes and local structure of the samples were studied.The results show that in（La,Co）co-doped Bi Fe O₃ samples,the structural transition from rhombohedral to orthogonal structure is observed by X-ray diffraction,and this structural transition significantly improves the magnetic properties.SEM analysis showed that the grain size decreased with the increase of La concentration.The results of hysteresis loop confirms significant intensification of magnetization in all samples.According to XRD and XAFS results,it can be concluded that the magnetic changes of the samples are caused by the local structural changes and structural transformation.（2）Bismuth ferrite samples with different europium（Eu）and fixed cobalt（Co）doping concentrations were prepared.The structure,local electronic structure,magnetic and electrical properties of the samples were studied.The X-ray diffraction data showed（104）,（110）bimodal merging and high angular offset,indicating a continuous change in the crystal structure of the sample as Bi was continuously replaced by Eu.The hysteresis loop and ZFC/FC curve show the relationship between magnetization and magnetic field intensity and temperature.Through XRD and XAS results,it can be concluded that the magnetic variation of the sample can be attributed to the structural phase transition and the change of valence state of Fe.