The role of grain boundaries in the magnetoresistance ͑MR͒ properties of the manganites has been investigated by comparing the properties of epitaxial and polycrystalline films of La 0.67 D 0.33 MnO 3Ϫ␦ (DϭCa, Sr, or vacancies͒. While the MR in the epitaxial films is strongly peaked near the ferromagnetic transition temperature and is very small at low temperatures, the polycrystalline films show large MR over a wide temperature range down to 5 K. The results are explained in terms of switching of magnetic domains in the grains and disorder-induced canting of Mn spins in the grain-boundary region. ͓S0163-1829͑96͒51746-7͔

The low-field magnetoresistance (MR) properties of polycrystalline La0.67Sr0.33MnO3 and La0.67CaO33MnO3 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.

We have used a self-aligned lithographic process to fabricate magnetic tunnel junctions of La 0.67 Sr 0.33 MnO 3 down to a few micrometers in size. We have obtained a magnetoresistance ratio as large as 83% at low magnetic fields of a few tens of Oe, which correspond to the coercivities of the magnetic layers. Transmission-electron-microscopy analysis has revealed the heteroepitaxial growth of the trilayer junction structure, La 0.67 Sr 0.33 MnO 3 /SrTiO 3 /La 0.67 Sr 0.33 MnO 3 . We have observed current-voltage characteristics typical of electron tunneling across an insulating barrier. The large magnetoresistance is likely due to the nearly half-metallic electronic structure of the manganites.