Magnetism and Transport Properties of Transition Metal Oxides and Nanoparticles

This dissertation is devoted to the study of the properties of transition metal oxides in both thin film and nanocrystalline forms.

The first section is devoted to the transport properties of manganese oxide thin film samples. The colossal magnetoresistance in these materials is usually explained using double-exchange, but this explanation is only partially correct. Recent theoretical and experimental work has shown that these compounds have a strong tendency towards phase-separation. The impact of strain on phase separation has been investigated by growing films of La_5/8-0.3Pr_0.3Ca_3/8MnO₃ on a variety of substrates. Very small changes in the induced strain cause large changes in the magnetic and magnetotransport properties of the film.

The second section is devoted to scanning tunneling microscopy studies of these same films at room temperature. In this case, small localized holes are observed at the Mn⁴⁺ ion sites. The distribution of holes is non-random and shows strong short range correlation.

The final section of this thesis is devoted to small particles of binary transition metal oxides. These oxides are antiferromagnetic in bulk samples, but when the dimensions of the particle are small (<50nm), they show an enhanced magnetic moment. Nanoparticles of a-Fe₂O₃ have been prepared and their magnetic properties have been measured, and an increase in particle moment versus temperature has been observed using standard analysis techniques. More advanced analysis techniques indicate that this increase is a result of a transition specific to the a-Fe₂O₃ system, and not the result of thermoinduced magnetization