Current dependence of grain boundary magnetoresistance in La0.67Ca0.33MnO3films

We prepared epitaxial ferromagnetic manganite films on 45 degrees bicrystal substrates by pulsed laser ablation. Their low- and high-field magnetoresi stance (MR) was measured as a function of magnetic field, temperature and c urrent. At low temperatures hysteretic changes in resistivity up to 70% due to switching of magnetic domains at the coercitive field are observed. The strongly nonohmic behavior of the current-voltage leads to a complete supp ression of the MR effect at high bias currents with the identical current d ependence at low and high magnetic fields. We discuss the data in view of t unneling and mesoscale magnetic transport models and propose an explicit de pendence of the spin polarization on the applied current in the grain bound ary region. (C) 1999 American Institute of Physics. [S0021-8979(99)00316-3] .