Magnetic Properties of Mesoporous and Nano-particulate Metal Oxides
Hill, Adrian H
The magnetic properties of the first row transition metal oxides are wide and varied and have been studied extensively since the 1930’s. Observations that the magnetic properties of these material types change with the dimension of the sample have stimulated many theoretical and experimental studies of the systems involved. As sample sizes decrease towards the nanoscale long range crystallographic order is no longer possible. However, the application of mesoporous silica samples as hard exo-templates to direct the formation of mesoporous metal oxides has provided a new opportunity to explore the influence of scale of crystallographic order further. These types of samples have pore systems running through the material on the mesoscale (diameter between 2nm to 50nm) with pore walls truly in the nanoscale region (7nm to 9nm thick) crystallographically ordered over large scale distances. The work presented in this thesis presents magnetic and crystallographic studies of a variety of the first row transition metal oxides from chromium to nickel in three dimensional mesoporous forms predominantly using SQUID magnetometry and neutron powder diffraction. Rietveld refinements of diffraction data from hematite and eskolaite (®-Fe2O3 and Cr2O3) show that the samples have space groups identical to their bulk counterparts, however slight differences in lattice parameters are observed. Refinement of magnetic properties has also been performed and compared to magnetic property measurements. Of particular interest are results from a mesoporous hematite which show suppression of a well defined first-order magnetic phase transition (the Morin transition). This suppression has been studied extensively with neutron powder diffraction and preliminary inelastic neutron spectroscopic measurements. Comparisons with hematite nanoparticles which also show the suppression of the Morin transition can be drawn. Parametric neutron powder diffraction studies on Co3O4 and NiO samples shows that the Néel ordering temperatures are lowered as the mesoporous structure is imposed. This too was observed in eskolaite. Other studies have been carried out on mesoporous alpha-MnO2 (magnetometry) and nanoscale Li1+xMn2–xO4 (X-ray photo electron spectroscopy) with comparisons to their bulk counterparts and finally nanoparticulate hausmannite Mn3O4 (magnetometry and muon spin relaxation) which exhibits spin-glass type behaviour.