Spin Polarisation Effects in Electron Energy Loss Spectroscopy
Electron energy loss spectroscopy (EELS) is a powerful analytical technique, able to provide detailed information on the bonding, chemical structure and electronic structure of materials over extremely small (<1nm) length scales. Ionisation edges occur when the electron beam causes core electrons in the atoms in a sample to be excited into unoccupied energy levels. The energy loss region up to ~50 eV above this edge is termed the energy-loss near-edge structure (ELNES) and is closely related to the local density of states (DOS) of the material. In this thesis a study into the effect of spin polarisation on ELNES is presented.
It has recently been shown that the inclusion of antiferromagnetic order in simulations of the oxygen K-edge ELNES of magnesium chromite greatly increased the level of agreement with experiment when compared to a non-magnetic calculation (McComb et al, 2003). In this thesis this observation is extended to a systematic study of chromite and ferrite spinels. Calculations are carried out using two simulation codes, FEFF8.2 and WIEN97/2k. These codes are in popular use within the scientific community and are accessible to non-specialist modelling groups. The use of appropriate spin polarisation parameters in calculations of oxygen K-edge ELNES improves agreement with experiment in all cases for chromite spinels, although the ferrite spinel case is more ambiguous. Density of states and total energy calculations are presented to aid the discussion of the theoretical and experimental spectra. A consideration of the effect of paramagnetism on ELNES is also presented.
The need for inclusion of spin polarisation in the ELNES calculations of ferrite and chromite spinels is unusual since the magnetic ordering temperatures of spinels are typically much less than 100K. A possible explanation for this comes from the fact that magnetic short range order (SRO) has been seen to persist to temperatures well above the magnetic ordering temperature in zinc ferrite using neutron powder diffraction (NPD) (Kamazawa et al, 2003). There is no information available in the literature concerning the temperatures to which magnetic SRO may be detected in chromite spinels using NPD. Therefore, in this thesis, NPD has been used to investigate magnetic SRO in magnesium chromite and zinc chromite, with a view to explaining the requirement of spin polarisation in ELNES calculations. The SRO is observed to persist at or close to room temperature in both chromite spinels, supporting the case for the inclusion of an ordered magnetic system in ELNES calculations. Calculations of magnetic exchange energies between cation pairs are presented to aid interpretation of this data.
In the ferrite spinels, NPD fails to detect magnetic SRO above 150K. However, a theoretical justification for the observation of SRO by EELS above this temperature has been presented (McComb et al, 2003). This justification involves an analysis of the interaction times of electrons and neutrons with the sample in EELS and NPD respectively. This suggests SRO detection may have a dependence upon the velocity of the incident species. To investigate this further, the NPD patterns from the three spinels investigated have been split into two sets – those generated from long wavelength ‘slow’ neutrons and those generated from short wavelength ‘fast’ neutrons – with the aim of observing a difference in SRO peak detection. These results are inconclusive for the chromite spinels, but suggest SRO is detected to higher temperatures in zinc ferrite using short wavelength ‘fast’ neutrons. This would support the argument made for the observation of SRO in the ferrite spinel ELNES.
Finally, an experimental investigation into the effect of magnetic ordering type on ELNES is presented. A manganese perovskite (Pr0.5Sr0.5MnO 3) which undergoes a ferromagnetic to antiferromagnetic transition at temperatures attainable in the electron microscope has been identified. The phase change in the crystal structure associated with this transition has been identified using bright field imaging and electron diffraction. However, experimental ELNES spectra recorded in both the ferromagnetic and antiferromagnetic regimes showed no significant differences. Theoretical calculations using FEFF8.2 have been carried out to aid the interpretation of the experimental data.