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Point contact investigations of film and interface magnetoresistance of La$_{0.7}$Sr$_{0.3}$MnO$_3$ heterostructures on Nb:SrTiO$_{3}$

STM based magnetotransport measurements of epitaxial La$_{0.7}$Sr$_{0.3}$MnO$_3$ 32 nm thick films with and without an internal LaMnO$_3$ layer (0-8 nm thick) grown on Nb doped SrTiO$_3$ are presented. The measurements reveal two types of low field magnetoresistance (LFMR) with a magnitude of $\sim 0.1-1.5\%$. One LFMR contribution is identified as a conventional grain boundary/domain wall scattering through the symmetric I-V characteristics, high dependence on tip placements and insensitivity to introduction of LaMnO$_3$ layers. The other contribution originates from the reverse biased Nb doped SrTiO$_3$ interface and the interface layer of La$_{0.7}$Sr$_{0.3}$MnO$_3$. Both LFMR contributions display a field dependence indicative of a higher coercivity ($\sim$200 Oe) than the bulk film. LaMnO$_3$ layers are found to reduce the rectifying properties of the junctions, and sub micron lateral patterning by electron beam lithography enhances the diodic properties, in accordance with a proposed transport model based on the locality of the injected current.