Nanoscale Magnetization Curve Measurement And Atomic Scale Electron Energy-loss Sperctrum Imaging

The transmission electron microscope provides a good experimental platform for scientific researchers to characterize the crystal structure,electronic structure and chemical composition of the material on the basis of high spatial resolution.With the development of electron energy loss magnetic chiral dichroism（EMCD）technique,it is possible to analyze the magnetic structure of the material on the scale of a few nanometers.This technique serves as a powerful tool for researchers to study the microscopic magnetic properties of materials.At present,on the one hand,the practical applications of EMCD technique still has the potential to be further explored.On the other hand,the improvement of spatial resolution of electron energy loss spectrum（EELS）and EMCD is also necessary to realize the interpretation of magnetic information of materials at atomic scale.This thesis will focus on the improvement of EELS and EMCD technique.First of all,we have developed a magnetization curve measurement technique,which is based on site-specific EMCD technique and connects with the microstructure of the material.This technique compensates for the weak imaging capability,and the limited ability of only acquiring the in-plane or surface magnetization information of some microscopic measurement methods.We selected the specific NiFe₂O₄ nanograin from NiFe₂O₄-BiFeO₃ multiferroic composite thin film,on which we measured local magnetization curve and found significant exchange bias effect in the magnetization process.We have analyzed the sample’s interface structure and conducted corresponding micromagnetic simulation,verifying the source of the observed exchange bias effect.Therefore,we have succeeded in directly linking the microstructure of the material to the behavior of its magnetization process.Secondly,we have developed the atomic plane resolved-electron energy loss spectroscopy（APR-EELS）technique under parallel incident electron beam.We have investigated the CaTiO₃/SrTiO₃ superlattices and observed the difference in the fine structure of EELS extracted from different atom planes and extracted from atom planes and positions that are between atom planes.This result is also consistent with the one calculated by Bloch-waves method.Hence,we have achieved precise analysis of the chemical composition and electronic structure of different atom planes in the material using APR-EELS.This technique has unique advantages over EFTEM and STEM-EELS technqiue,and potentially lays the foundation for atomic plane resolved-EMCD technique in the future.In addition,we have further explored the APR-EELS technique and found that the quality of APR-EELS was affected by some factors such as defocus,sample thickness,diffraction condition and the size of the entrance aperture.We also studied the optimization approaches of the APR-EELS results.This work hopefully provides helpful guidance for atomic plane resolved-EELS/EMCD technique and EFTEM imaging.At last,we have prepared 5Mn steel samples with characteristic regions by focused ion beam（FIB）milling and the in-situ characterizations of the phase transformation in different phase regions of 5Mn steel have been carried out.The microstructural evolutions of 5Mn steel at atomic level and the changes in chemical composition corresponding to the microstructural evolutions have been acquired.This work is of instructional significance for using in-situ transmission electron microscopy to study the microstructure of materials and to explain the properties of materials in the future.