Abstract:
This thesis describes the results from an experimental study of the magnetic and transport properties of two strongly correlated transition metal oxides. The first material under study is the ferromagnetic half-metal double perovskite, Sr2FeMoO6, in which we have made isoelectronic (Ba2+) and electronic (La3+) substitutions onto the strontium site. Magnetoresistance measurements on Sr2-xBaxFeMoO6 revealed that the low temperature magnetoresistance is dominated by inter-grain transport while the intra-grain contribution is evident when the temperature is close to the ferromagnetic transition temperature. Transport measurements on Sr2-xLaxFeMoO6 clearly showed that the doping dependence of the thermoelectric power is surprisingly similar to the one observed in the superconducting cuprates. In addition, it was found that the electronic doping leads to an increase in the ferromagnetic transition temperature, which supports the band filling model. Substitution on the Fe site was also investigated by partially replacing Fe with the non-magnetic aluminium element (Sr2Fe1-xA1xMoO6). It was found from thermoelectric power measurement that the Fe electronic state is below 3+, which is inconsistent with theoretical models but is in good agreement with Mössbauer measurements. In addition, magnetic measurements showed that the reduction in the ferromagnetic ordering temperature could be explained in terms of a 3D percolation model.
The second compound is the oxygen deficient strontium iron oxide SrFeO3-6. The temperature dependence of the thermoelectric power was measured in this compound for the first time and shown to be reminiscent of the charge-ordering Verwey transition observed in Fe3O4. Magnetic measurements show an increase of a weak ferromagnetic signal versus the oxygen deficiency that could originate from a Dzyaloshinsky-Moriya interaction in the distorted FeO6 octahedra. Finally, we observed a large magnetoresistance near room temperature for compounds close to the orthorhombic SrFeO2 75 phase.
