A nanomagnetic study of phase transition in manganite thin films and ballistic magnetoresistance in magnetic nanocontacts

This work focuses on two largely unexplored phenomena in micromagnetics: the temperature-driven paramagnetic insulator to ferromagnetic (FM) metallic phase transition in perovskite manganite and ballistic magnetoresistance in spin-polarized nanocontacts. To investigate the phase transition, an off-the-shelf commercial scanning force microscope was redesigned for operation at temperatures from 350 K to 100 K. This adaptation is elaborated in this thesis. Using this system, both ferromagnetic and charge-ordered domain structures of (La _1−xPr_x)_0.67Ca_0.33MnO₃ thin film were observed by magnetic force microscopy (MFM) and electric force microscopy (EFM) operated in the vicinity of the peak resistance temperature (T_p). Predominantly in-plane oriented FM domains of sub-micrometer size emerge below T_p and their local magnetic moment increased as the temperature is reduced. Charge-ordered insulating regions show a strong electrostatic interaction with an EFM tip at a few degrees above T_p and the interaction correlates well with the temperature dependence of resistivity of the film. Cross-correlation analysis between topography and magnetic structure on several substrates indicates FM domains form on the flat regions of the surface, while charge ordering occurs at surface protrusions.; In the investigation of ballistic magnetoresistance, new results on half-metallic ferromagnets formed by atomic or nanometer contacts of CrO₂-CrO ₂ and CrO₂-Ni are presented showing magnetoconductance as high as 400%. Analysis of the magnetoconductance versus conductance data for all materials known to exhibit so-called ballistic magnetoresistance strongly suggests that magnetoconductance of nanocontacts follows a universal mechanism. If the maximum magnetoconductance is normalized to unity and the conductance is scaled with the resistivity of the material, then all data points fall onto a universal curve independent of the contact material and the transport mechanism. The analysis has been applied to all available magnetoconductance data of magnetic nanocontacts in the literature. The results are in agreement with a theory that takes into account only the spin-scattering within a magnetic domain wall and are independent of whether the transport is ballistic or diffusive.