We prepared epitaxial ferromagnetic manganite films on 45 • bicrystal substrates by pulsed laser ablation. Their low-and high-field magnetoresistance (MR) was measured as a function of magnetic field, temperature and current. At low temperatures hysteretic changes in resistivity up to 70 % due to switching of magnetic domains at the coercitive field are observed. The strongly non-ohmic behavior of the current-voltage (I-V ) leads to a complete suppression of the MR effect at high bias currents with the identical current dependence at low and high magnetic fields. We discuss the data in view of tunneling and mesoscale magnetic transport models and propose an explicit dependence of the spin polarization on the applied current in the grain boundary region.
By laser ablation we prepared thin films of the colossal magnetoresistive compound La1−xCaxMnO3 with doping levels 0.1 < x < 0.9 on MgO substrates. X-ray diffraction revealed epitaxial growth and a systematic decrease of the lattice constants with doping. The variation of the transport and magnetic properties in this doping series was investigated by SQUID magnetization and electrical transport measurements. For the nonmetallic samples resistances up to 1013 Ω have been measured with an electrometer setup. While the transport data indicate polaronic transport for the metallic samples above the Curie temperature the low doped ferromagnetic insulating samples show a variable range hopping like transport at low temperature.
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