Quantitative atomic resolution characterisation of internal interfaces in doped bismuth ferrite

Q Science (General);QC Physics

Salih, Jalal Mohammed ; MacLaren, Ian

University:University of Glasgow

Department:School of Physics and Astronomy

关键词: Doped Bismuth ferrite, HRSTEM, NCSI HRTEM, EELS, anti-phase boundaries, non-rigid registration.;   

Others  :  http://theses.gla.ac.uk/8283/1/2016jalalphd.pdf

来源: University of Glasgow

【 摘 要 】

Since bismuth ferrite-based perovskites display magnetic ordering as wellas ferroelectric behaviour, they have been widely researched due to the possibility to fabricate multiferroic devices. This thesis is focused on investigating structures formed in bismuth ferrite (BFO) as a consequence of Ti doping using atomic resolution transmission electron microscopy (TEM). This is performed both using the negative spherical aberration (NCSI) imaging technique in TEM and scanning transmission electron microscopy (STEM) with simultaneous dark and bright field imaging.A quantitative study was made of the diference between high resolutionTEM (HRTEM) and high resolution STEM (HRSTEM) for quantitative polarisation mapping around antiphase boundaries (APBs) is observed and reported. Although similar trends in the structures are found in the two techniques, quantitative diferences are noted. For HRTEM, using NCSI gives 60% lower polarisation values in comparison to HRSTEM in the case studied here. It is shown that the sample tilt has no in uence on the polarisation measurements in this case as the direction of the sample tilt in our case is perpendicular to the direction of polarisation. It is shown that the ultra-thin sample used in HRTEM had a reduced polarisation due to the effects of surfaces, but that also the polarisation was underestimated from the imagesdue to one oxygen column appearing in the images at a position slightly displaced from its real position due to the effects of electron channeling in the material. For the case of HRSTEM, it was found that the polarisation was overestimated due to a similar effect, where an oxygen atom image is slightly displaced from the column position in the opposite direction at this specifc sample thickness. Thus, it is clear that the use of simulations is essential to any atomic resolution quantifcation of polarisation by either technique.In addition to this, , the structure and chemistry of the crosses and cornerson APBs were determined using a combination of high angle annular darkfield (HAADF) combined with electron energy loss spectroscopy spectrum imaging (EELS-SI) in STEM. The 3D structures of the APB cross structure were fully determined and these were verified by multislice frozen phonon image simulations. It was suggested on the basis of the structure of some of the edge-sharing octahedral structural units in this cross that they would support a permanent magnetic ordering, since their structure was similar to that of maghemite. In analysing this structure, the advantages of non-rigid registration (alignment of a sequence of short exposure images as well as removing local distortions in each image prior to summation) for removing line noise distortions from STEM images are investigated quantitatively.Two APB corners were also investigated using NCSI HRTEM and HRSTEM. Whilst some details of each structure were impossible to determine unambiguously, most parts of these APB corners have structural units similar to those in steps and in the APB cross. It should also be noted that the two APB corners show some detailed differences in structure suggesting that there is a range of possible structures on such antiphase boundaries, but all constructed from a limited number of structural units.