Low-field magnetoresistive properties of polycrystalline and epitaxial perovskite manganite films

The low-field magnetoresistance (MR) properties of polycrystalline ${\mathrm{La}}_{\mathrm{0.67}}{\mathrm{Sr}}_{\mathrm{0.33}}{\mathrm{MnO}}_{\mathrm{3}}$ and ${\mathrm{La}}_{\mathrm{0.67}}{\mathrm{CaO}}_{\mathrm{33}}{\mathrm{MnO}}_{\mathrm{3}}$ thin films with different grain sizes have been investigated and compared with epitaxial films. MR as high as 15% has been observed in the polycrystalline films at a field of 1500 Oe at low temperatures, whereas the MR of the epitaxial films is less than 0.3% in the same field range. Based on the magnetization dependence of the MR, the current-voltage characteristics, and the temperature dependence of the resistivity, we attribute the low-field MR to spin-dependent scattering of polarized electrons at the grain boundaries which serve as pinning centers for the magnetic domain walls.