Grain boundary effects on magnetotransport in bi - epitaxial films of La 0 . 7 Sr 0

The low field magnetotransport of La0.7Sr0.3MnO3 (LSMO) films grown on SrTiO3 substrates has been investigated. A high qualtity LSMO film exhibits anisotropic magnetoresistance (AMR) and a peak in the magnetoresistance close to the Curie temperature of LSMO. Bi-epitaxial films prepared using a seed layer of MgO and a buffer layer of CeO2 display a resistance dominated by grain boundaries. One film was prepared with seed and buffer layers intact, while a second sample was prepared as a 2D square array of grain boundaries. These films exhibit i) a low temperature tail in the low field magnetoresistance; ii) a magnetoconductance with a constant high field slope; and iii) a comparably large AMR effect. A model based on a two-step tunneling process, including spin-flip tunneling, is discussed and shown to be consistent with the experimental findings of the bi-epitaxial films. Typeset using REVTEX Current address: Dept. of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.